Chapter e42: Evaluation of Kidney Function

Chronic kidney disease (CKD) is an increasingly alarming worldwide health concern, with nearly 2 million people in the United States estimated to require hemodialysis or kidney transplantation by 2030.¹ In response to this widespread problem, standardized approaches are now used for the identification of individuals with CKD and their subsequent stratification into risk categories for the development of end-stage kidney disease (ESKD or ESRD) (see Chapter 44).^1,2 These efforts have heightened the awareness of the need for early identification of patients with CKD and the importance of monitoring the progression of kidney disease.

Assessment of kidney function using both qualitative and quantitative methods is an important part of the evaluation of patients and an essential characterization of individuals who participate in clinical research investigations. Estimation of creatinine clearance (CL_cr) has been considered the clinical standard for assessment of kidney function for nearly 50 years, and continues to be used as the primary method of stratifying kidney function in drug pharmacokinetic studies submitted to the United States Food and Drug Administration (FDA).^3,4 New equations to estimate glomerular filtration rate (GFR) are now used in many clinical settings to identify patients with CKD, and in large epidemiology studies to evaluate risks of mortality and progression to stage 5 CKD, that is, ESKD.^5,6 Other tests, such as urinalysis, radiographic procedures, and biopsy, are also valuable tools in the assessment of kidney disease, and these qualitative assessments are useful for determining the pathology and etiology of kidney disease. Urinalysis, for example, may give clues to the primary location, such as glomerular or tubular, of the renal disease. Follow-up studies, such as imaging procedures or kidney biopsy, may then further differentiate the specific cause, thereby guiding the selection of the optimal therapeutic intervention.

Quantitative indices of GFR or CL_cr are considered the most useful diagnostic tools for identifying the presence and monitoring the progression of CKD. These measures can also be used to quantify changes in function that may occur as a result of disease progression, therapeutic intervention, or a toxic insult.⁷ The measurement or estimation of CL_cr, however, remains the most commonly used index for individualizing medication dosage regimens in patients with acute and CKD. Furthermore, CL_cr has been the predominant index used to stratify patients in pharmacokinetic studies which serve as the basis for the design of renal dosing algorithms in FDA-approved package inserts and tertiary drug information sources.⁴ It is important to note that the term kidney function includes the combined processes of glomerular filtration, tubular secretion, and reabsorption, as well as endocrine and metabolic functions. Alterations in any or all of these functions, whether declining or improving, are associated primarily with GFR. This chapter critically evaluates the various methods that can be used for the quantitative assessment of kidney function in individuals with normal kidney function, as well as in those with CKD and acute kidney injury (AKI) (Table e42-1). Where appropriate, discussion regarding the qualitative assessment of the kidney function is also presented, including the role of imaging procedures and invasive tests such as kidney biopsy.

The kidney is largely responsible for the maintenance of body homeostasis via its role in regulating urinary excretion of water, electrolytes, endogenous substances such as urea, medications, and environmental toxins. It accomplishes this through the combined processes of glomerular filtration, tubular secretion, and reabsorption.

Glomerular Filtration

Glomerular filtration is a passive process by which water and small-molecular-weight (less than 5-10 kDa) ions and molecules diffuse across the glomerular–capillary membrane into the Bowman capsule and then enter the proximal tubule (Fig. e42-1). Most proteins are too large (greater than 60 kDa) to be substantially filtered, and their filtration is impeded by the electronegative charge on the epithelial surface of the glomerulus. Thus, compounds presented to the glomerulus in the protein-bound state are usually not significantly filtered and remain in the peritubular circulation.

Secretion

Secretion is an active process that predominantly takes place in the proximal tubule and facilitates the elimination of compounds from the renal circulation into the tubular lumen. Several highly efficient transport pathways exist for a wide range of endogenous and exogenous substances, resulting in renal clearances of these actively secreted entities that often greatly exceed GFR and in some cases approximate renal blood flow. These transporters are typically found among the solute-linked carrier (SLC) and ATB-binding cassette (ABC) super families. The primary SLC transporters include the organic anion transporting polypeptides (OATPs), organic cation transporters (OCTs), organic anion transporters (OATs), and multidrug and toxin extrusion proteins (MATEs) (Fig. e42-2; Table e42-2). The key ABC transporters include multidrug resistance protein (MDR1 or P-glycoprotein [P-GP]), MRP1 and MRP2. Both group of transporters are expressed in a variety of different epithelial membranes throughout the body and play a role in intestinal absorption, blood-brain barrier penetration, and excretion into the bile and urine. Recent research has identified their localized role in drug uptake and efflux in kidney tissues, where the OCT (1-3) and OAT (1-3) are involved in basolateral influx of substrates, and P-GP, MRP, and MATE are apical efflux transporters.^8,9,10,11 Their presence in liver and kidney contributes to the hepatic and renal elimination of many drugs. For example, P-GP, which is located on the apical membrane of the proximal tubule, plays an important role in the renal elimination of a wide range of drugs, such as cimetidine, digoxin, and procainamide. Studies designed to elucidate the mechanisms of drug-induced renal disease have also revealed the important role of drug transporters. For example, cobicisat, a CYP3A inhibitor that is used to enhance the response of several human immunodeficiency virus (HIV) regimens, has been associated with elevations in serum creatinine. The mechanism is attributed to combined inhibition of creatinine uptake (OCT2, OCT3) and efflux (MATE1).¹⁰ The coordination of multiple drug transporters working together can result in a high degree of urinary excretion. For example, pramipexole undergoes OCT2-mediated uptake along with MATE-mediated efflux, resulting in extensive tubular secretion and renal clearance values of 500 to 800 mL/min.⁹ Overall, the net process of tubular secretion for drugs is likely a result of multiple secretory pathways acting simultaneously.

Reabsorption

Reabsorption of water and solutes occurs throughout the nephron, whereas the reabsorption of most medications occurs predominantly along the distal tubule and collecting duct. Urine flow rate and physicochemical characteristics of the molecule influence these processes: highly ionized compounds are not reabsorbed unless pH changes within the urine increase the fraction unionized, so that reabsorption may be facilitated.

Intact Nephron Hypothesis

The “intact nephron hypothesis” described by Bricker,¹² more than 40 years ago, proposes that “kidney function” of patients with renal disease is the net result of a reduced number of appropriately functioning nephrons. As the number of nephrons is reduced from the initial complement of 2 million, those that are unaffected compensate; that is, they hyperfunction. The cornerstone of this hypothesis is that glomerulotubular balance is maintained, such that those nephrons capable of functioning will continue to perform in an appropriate fashion. Extensive studies have indeed shown that single-nephron GFR increases in the unaffected nephrons; thus, the whole-kidney GFR, which represents the sum of the single-nephron GFRs of the remaining functional nephrons, may remain close to normal until there is extensive injury. Based on this, one would presume that a measure of one component of nephron function could be used as an estimate of all renal functions. This, indeed, has been and remains our clinical approach. We estimate GFR and assume secretion and reabsorption remain proportionally intact.

Glomerular Filtration Capacity

GFR is dependent on numerous factors, one of which is protein load. Bosch¹³ suggested that an appropriate comprehensive evaluation of kidney function should include the measurement of “filtration capacity.” Recently, the concept of renal function reserve (RFR) has been defined as the capacity of the kidney to increase GFR in response to physiological or pathologic conditions.¹⁴ This is similar in context to a cardiac stress test. The patient may have no hypoxic symptoms, for example, angina while resting, but it may become quite evident when the patient begins to exercise. Subjects with normal renal function administered an oral or intravenous (IV) protein load prior to measurement of GFR have been noted to increase their GFR by as much as 50%. As renal function declines, the kidneys usually compensate by increasing the single-nephron GFR. The RFR will be reduced in those individuals whose kidneys are already functioning at higher-than-normal levels because of preexisting kidney injury or subclinical loss of kidney mass (Fig. e42-3) Thus RFR may be a complementary, insightful index of renal function for many individuals with as yet unidentified CKD.

Quantification of renal function is not only an important component of a diagnostic evaluation, but it also serves as an important parameter for monitoring therapy directed at the etiology of the diminished function itself, thereby allowing for objective measurement of the success of treatment. Measurement of renal function also serves as a useful indicator of the ability of the kidneys to eliminate drugs from the body. Furthermore, alterations of drug distribution and metabolism have been associated with the degree of renal function. A discussion of pharmacokinetic changes in patients with renal disease is extensively reviewed in Chapter 48. Although several indices have been used for the quantification of GFR in the research setting, estimation of CL_cr and GFR are the primary approaches used in the clinical arena.

The kidney synthesizes and secretes many hormones involved in maintaining fluid and electrolyte homeostasis. Secretion of renin by the cells of the juxtaglomerular apparatus and production and metabolism of prostaglandins and kinins are among the kidney’s endocrine functions.¹⁵ In addition in response to decreased oxygen tension in the blood, which is sensed by the kidney, erythropoietin is produced and secreted by peritubular fibroblasts. Because these functions are related to renal mass, decreased endocrine activity is associated with the loss of viable kidney cells. In patients with stages 3 to 5 CKD and those with moderate-to-severe AKIs, secretion of erythropoietin is impaired leading to reduced red blood cell formation; normocytic anemia and symptoms of reduced oxygen delivery to tissues such as fatigue, dyspnea, and angina (see Chapter 44). Indeed, anemia-induced renal hypoxia results indirectly in erythropoietin gene activation, tubular necrosis, and apoptosis, thereby contributing to further renal cell injury.¹⁶ This cyclic relationship between kidney disease, suppression of erythropoietin secretion, and cardiovascular disease is also referred to as the cardio-renal anemia syndrome.¹⁷

The kidneys perform a wide variety of metabolic functions, including the activation of vitamin D, gluconeogenesis, and metabolism of endogenous compounds such as insulin, steroids, and xenobiotics. It is common for patients with diabetes and stages 4 to 5 CKD to have reduced requirements for exogenous insulin,¹⁸ and require supplemental therapy with activated vitamin D₃ (calcitriol) or other vitamin D analogs (paricalcitol, doxercalciferol) to avert the bone loss and pain associated with CKD-associated metabolic bone disease.¹⁹ Cytochrome P450 (CYP), N-acetyltransferase, glutathione transferase, renal peptidases, and other enzymes responsible for the degradation and activation of selected endogenous and exogenous substances have been identified in the kidney. The CYP enzymes in the kidneys are as active as those in the liver, when corrected for organ mass. In vitro and in vivo studies have shown that CYP-mediated metabolism is impaired in the presence of renal failure or uremia. In clinical studies using CYP3A probes in ESRD patients receiving hemodialysis, hepatic CYP3A activity was reported to be reduced by 28% from values observed in age-matched controls; partial correction was noted following the hemodialysis procedure.^20,21 Impaired nonrenal clearance in the presence of renal failure has been documented for a number of drugs with a fraction eliminated renally unchanged of less than 30%, including those that undergo extensive CYP metabolism by CYP1A2, CYP2D6, CYP3A4, and CYP2C9, such as duloxetine, rosuvastatin, and telithromycin.²²

Qualitative and Semiquantitative Indices of Kidney Function

Patients who develop CKD remain relatively asymptomatic until they reach stage 4 to 5 and systemic manifestations and/or secondary complications become evident (Table e42-3). As renal function declines, patients may develop de novo or experience an exacerbation of hypertension, edema, electrolyte abnormalities, anemia, or other complications (see Chapter 29). The Kidney Disease Outcomes Quality Initiative (KDOQI) workgroup currently recommends that all patients with CKD, and those at increased risk for CKD, undergo at least yearly a comprehensive laboratory assessment that comprises (a) S_cr to estimate GFR; (b) albumin-to-creatinine ratio in a spot urine specimen; (c) examination of urine sediment for red blood cell and white blood cell counts; (d) renal ultrasonography; (e) serum electrolytes, including sodium, potassium, chloride, and bicarbonate; (f) urine pH; and (g) urine specific gravity.²³ The role of each of these indices in the identification and monitoring of CKD is discussed in detail below.

Laboratory Procedures to Detect the Presence of Kidney Disease

Urinalysis is an important tool for detecting and differentiating various aspects of kidney disease, which often goes unnoticed as the result of its asymptomatic presentation. Urinalysis can be used to detect and monitor the progression of diseases such as diabetes mellitus, glomerulonephritis, and chronic urinary tract infections.²⁴ A typical urinalysis provides information about physical and chemical composition, most of which can be completed quickly and inexpensively by visual observation (volume and color) and dipstick testing.

Chemical Analysis of Urine

pH

The normal urine pH typically ranges from 4.5 to 7.8, and an elevation above this may suggest the presence of urea-splitting bacteria. In patients with renal tubular acidosis, urine pH is usually more than 5.5 because of impaired hydrogen ion secretion in the distal tubule or collecting duct.

Glucose

Glucose is usually not present in the urine because the kidney normally completely reabsorbs all the glucose filtered at the glomerulus. When a patient’s blood glucose concentration exceeds the maximum threshold for glucose reabsorption (~180 mg/dL [~10.0 mmol/L]), glucosuria will be present. Routine assessment of glucosuria to monitor diabetics has been replaced by newer methods of direct blood glucose measurements. Urine glucose testing is now predominantly used as a screening tool for the detection of diabetes.

Ketones

Acetoacetate and acetone are not normally found in the urine; they are however excreted in patients with diabetic ketoacidosis. They are also present under conditions of fasting or starvation. Typically, values of acetoacetate excretion are reported as small (less than 20 mg/dL [less than 2 mmol/L]), moderate (30-40 mg/dL [3-4 mmol/L]), and large (greater than 80 mg/dL [greater than 8 mmol/L]).

Nitrite

Nitrite is not usually present in urine. The presence of nitrite is most commonly the result of conversion from urinary nitrate by bacteria in the urine. The presence of nitrite thus suggests that the patient has a urinary tract infection, commonly caused by gram-negative rods such as Escherichia coli. Although false-positive results are very rare, false-negative results are more common and may be caused by lack of dietary nitrate, reduced urine nitrate concentration as a consequence of diuresis, or infections caused by bacteria such as enterococci and Acinetobacter, which do not reduce nitrate, and pseudomonads, which convert nitrate to nitrogen gas.

Leukocyte Esterase

Leukocyte esterase is released from lysed granulocytes in the urine; its presence is suggestive of urinary tract infection. False-positive tests can result from delayed processing of the urine sample, contamination of the sample with vaginal secretions (such as blood or heavy mucus discharge), or by Trichomonas infection (such as trichomoniasis). False-negative tests can be produced by the presence of high levels of protein or ascorbic acid.

Heme

The heme test indicates the presence of hemoglobin or myoglobin in the urine. A positive test without the presence of red blood cells suggests either red cell hemolysis or rhabdomyolysis.

Protein or Albumin

Evaluation of urinary protein or albumin is now a standard tool to characterize the severity of CKD and to monitor the rate of disease progression or regression.²³ Persistent proteinuria or albuminuria, that is, present on at least three occasions over a period of 3 to 6 months, is now considered a principal marker of kidney damage. Under normal conditions, plasma proteins remain in the glomerular capillaries and thus do not cross the glomerular basement membrane or enter the urinary space. Some of these proteins, such as albumin and globulins are not filtered by the glomerulus as a result of charge and size selectivity (greater than 40 kDa). Smaller proteins (less than 20 kDa) pass across the glomerular basement membrane but are usually readily reabsorbed in the proximal tubule. Most healthy individuals excrete between 30 and 150 mg/day of total protein consisting of approximately 30 mg/day of albumin. The other proteins that may be found in the urine are secreted by the tubules (Tamm-Horsfall, immunoglobulin A, and urokinase) or composed of smaller proteins such as β₂-microglobulin, apoproteins, enzymes, or peptide hormones. Increased renal excretion of these low-molecular-weight proteins is considered a sensitive marker of tubulointerstitial disease.

Historically, the sulfosalicylic acid test was used as a crude measure of proteinuria. This test is performed by adding sulfosalicylic acid to urine and then visually comparing this admixture with a tube of untreated urine; the degree of turbidity is then qualitatively graded (0-4 plus) as the index of the presence of proteinuria. Dipstick tests are now the most common means to determine in a semiquantitative fashion a patient’s urinary protein or albumin excretion. False-positive results can occur in the presence of alkaline urine (pH greater than 7.5), when the dipstick is immersed too long, in those with highly concentrated urine, in the presence of drugs such as penicillin, sulfonamides, or tolbutamide, as well as blood, pus, semen, or vaginal secretions. False-negative results occur with dilute urine (specific gravity less than 1.015) and when proteinuria is caused by nonalbumin or low-molecular-weight proteins such as heavy or light chains or Bence Jones proteins. The results of these dipstick tests are graded as negative (less than 10 mg/dL [less than 100 mg/L]), trace (10-20 mg/dL [100-200 mg/L]), 1+ (30 mg/dL [300 mg/L]), 2+ (100 mg/dL [1,000 mg/L]), 3+ (300 mg/dL [3,000 mg/L]), or 4+ (greater than 1,000 mg/dL [greater than 10,000 mg/L]). Benchtop portable analyzers designed for point-of-care testing, such as the Chemstrip 101 Urine Analyzer (Roche Diagnostics) and Clinitek 50 Urine Chemistry Analyzer (Bayer Corporation) are increasingly used as an alternative to visual urinalysis test-strip evaluation, and provide rapid results for urinary albumin-to-creatinine ratio.²⁵

Dipstick test strips that are specific for low levels of albuminuria (30-300 mg/day) should be employed when one is screening individuals at risk for CKD. For example, the Microalbumin 2-1 Combo Strip (Teco Diagnostics) is a semiquantitative test for both albumin and creatinine in a spot urine sample. When dipped into a urine sample, colors range from pale green to aqua blue (for albumin) and shades of purple-brown (for creatinine), depending on the amounts present in the sample.²⁶ The presence of hemoglobin (≥5 mg/dL [≥50 mg/L] or visibly bloody urine) or bilirubin (≥15 mg/dL [≥257 μmol/L] or visibly dark brown color urine) can interfere with test results from these strips, but no interferences have been observed with ascorbic acid. In patients with a positive protein or albumin dipstick test, a 24-hour urine collection with measurement of albumin excretion can be used to quantitate precisely the degree of albuminuria. However, this method requires a high degree of patient compliance and is being replaced by a similarly accurate but less cumbersome technique: calculation of the ratio of protein or albumin (in milligram) to creatinine (in gram [or mmol]) in an untimed (spot) urine specimen. The normal ratio, KDIGO category A1, is less than 30 mg albumin or less than 200 mg protein per gram of creatinine (less than 3.4 mg albumin or less than 22.6 mg protein per mmol of creatinine), with values between 30 and 300 mg of albumin per gram of creatinine (3.4-34.0 mg of albumin per mmol of creatinine) classified as albuminuria that is KDIGO category A2.²³ Positive test results should be repeated, particularly in patients without an underlying cause for CKD, such as diabetes or hypertension. Monitoring of the degree of glomerular injury in CKD patients should use the albumin-to-creatinine ratio, whereas for patients with clinical proteinuria, KDIGO category A3, that is, excretion of more than 500 mg per day or an albumin-to-creatinine ratio more than 300 mg/g (greater than 30mg/nmol), the protein-to-creatinine ratio can be used.

The albumin-to-creatinine ratio has also been incorporated into a new CKD classification system as recommended by a Work Group of the Kidney Disease Improving Global Outcomes (KDIGO).² This new standardized “CGA” scoring system requires evaluation of both estimated GFR (G1 to G5) and albuminuria (A1-A3) in a given patient. This approach is expected to provide a more accurate evaluation of a patient’s kidney function than reliance on eGFR alone. A detailed description of how to apply this CGA classification system in the monitoring plan for CKD patients is provided in Chapter 44.

Specific Gravity

Specific gravity is a measure of urine weight relative to water (1.00) that is performed using a refractometer. Thus, specific gravity is dependent on water intake and urine-concentrating ability. Normal values range from 1.003 to 1.030. Osmolality, which is a measure of the number of solute particles in the urine, is a more accurate measure of the kidney’s ability to make a concentrated urine. Generally the two values correlate; however, when large quantities of heavier molecules, such as glucose, are in the urine, the specific gravity may be elevated relative to the osmolality. These tests are used in the assessment of urine-concentrating ability and are most informative when interpreted along with the hydration status of the patient and plasma osmolality.²⁷

Microscopic Analysis of Urine

Microscopic examination is a critical aspect of urinalysis. Formed elements that may be detected in the urine include erythrocytes and leukocytes, casts, and crystals. An important consideration in the assessment of hematuria is whether the cells are of renal origin. More than two cells per high-power field is abnormal, and the presence of dysmorphic cells suggest renal parenchymal origin either because they were damaged as they passed through the glomerulus or due to exposure to the varying osmotic environments of the tubular lumen. White blood cells may be present in the urine in association with infection or inflammatory conditions, such as interstitial nephritis. More than one cell per high-power field is usually considered abnormal. Contamination of the sample should also be considered when there are many cells and may be a result of the presence of menses or of inadequate sample collection. Casts are cylindrical forms composed of protein, with or without cells that take the shape of the collecting tubules, where they are formed. Casts without cells are labeled hyaline casts and consist of the Tamm-Horsfall mucoprotein, secreted by the renal tubules. They are nonspecific and may appear in concentrated urine. In the presence of red or white blood cells, casts may be formed that include the cells, indicating that the cells were of renal origin. Solubility of the Tamm-Horsfall protein is increased as urine pH rises; therefore, sample collection for casts should occur with the first morning void when the urine is most acidic. Otherwise, casts may dissolve and elude detection.

A variety of crystals may be visualized in the urinary sediment in healthy individuals as well as those with kidney diseases. The most common crystals are those composed of uric acid, calcium oxalate, calcium phosphate, calcium magnesium ammonium pyrophosphate, and cystine. Many of these have a unique crystalline form, which permits them to be identified with microscopy.²⁸ Images of common types of urinary casts and crystals can be found at publically available Web sites such as http://www.medical-labs.net/example-of-microscopic-urine-sediment-exam-results-794/.

Serum or Blood Urea Nitrogen

Amino acids metabolized to ammonia are subsequently converted in the liver to urea, the production of which is dependent on protein availability (diet) and hepatic function. Urea undergoes glomerular filtration followed by reabsorption of up to 50% of the filtered load in the proximal tubule. The nitrogen component of urea in serum (SUN) or blood (BUN) is considered normal if it is in the range of 5 to 20 mg/dL (1.8-7.1 mmol/L).²⁹ The reabsorption rate of urea is predominantly dependent on the reabsorption of water. The excretion of urea may, therefore, be decreased under conditions that necessitate water conservation such as dehydration although the GFR may be normal or only slightly reduced. This condition is evident when a patient exhibits prerenal azotemia, or an increase of the BUN to a greater extent than the S_cr. The normal BUN-to-creatinine ratio is 10 to 15:1 using conventional units (or 40-60:1 when both are expressed in identical molar units), and an elevated ratio is suggestive of a decreased effective circulating volume, which stimulates increased water, and hence, urea reabsorption. Creatinine is not reabsorbed to any significant extent by the kidneys. Despite these limitations, the BUN is usually used in combination with the S_cr as a simple screening test for the detection of renal dysfunction.

Serum and Urine Creatinine

Creatinine remains the most important endogenous biomarker used for the detection of kidney disease. The concentration of creatinine in serum is a function of creatinine production and renal excretion. Creatinine is a product of creatine metabolism from muscle; therefore, its production is directly dependent on muscle mass. At steady state, the “normal” S_cr range is generally reported as 0.5 to 1.5 mg/dL (44-133 μmol/L) for males and females.²⁹ Creatinine is eliminated primarily by glomerular filtration, and as GFR declines, the S_cr rises (Fig. e42-4).

The third National Health and Nutrition Examination Survey (NHANES III) revealed a mean S_cr in Caucasian women and men of 0.96 mg/dL (85 μmol/L) and 1.16 mg/dL (103 μmol/L), respectively.³⁰ Values were lower among Mexican Americans, and higher among African Americans. For all groups, the S_cr increased with age. The report also estimated that among US community-dwelling adults, 10.9 million have a S_cr greater than 1.5 mg/dL (133 μmol/L); 3.0 million have a S_cr greater than 1.7 mg/dL (150 μmol/L); and 0.8 million have a S_cr greater than 2.0 mg/dL (177 μmol/L). Although the S_cr alone is not an optimal measure of kidney function, it is often used as a marker for referral to a nephrologist. There is presently no accepted single standard for an “abnormal” S_cr, as it is dependent on gender, race, age, and lean body mass.

Several methods have been used for the determination of the S_cr. The kinetic Jaffe reaction remains the cornerstone of routine quantification of creatinine in most clinical laboratories. This colorimetric method is based on the reaction of creatinine with alkaline picrate. This nonspecific method also reacts with noncreatinine chromogens in the serum, including bilirubin and proteins such as albumin, and hemoglobin, which may result in a falsely increased S_cr.²⁴ The noncreatinine chromogens are not present in the urine in sufficient quantities to interfere with the urinary creatinine measurement, because the concentration of creatinine in urine is several-fold greater than the serum concentration.

This “normal” interference results in an increase in the S_cr of approximately 10% and thereby the measured CL_cr (mCL_cr) would underestimate the true GFR by 10%. In subjects with normal renal function, this tends to counterbalance the effect of the contribution of tubular secretion of creatinine, which increases urine creatinine by nearly 10%. Thus, a mCL_cr has been proposed to serve as a good measure of GFR in subjects with normal renal function. However, this false increase in S_cr becomes less noticeable as the true creatinine concentration rises, due to the increasing contribution of tubular secretion to the renal clearance of creatinine.³¹ This becomes of major importance when kidney function is reduced to less than 50% of normal.

Diabetic ketoacidosis may produce increased concentrations of acetoacetate, which serves as a chromophore in the Jaffe reaction, thereby increasing the S_cr. Other substances that also react with this procedure in the serum include glucose, protein, pyruvate, fructose, uric acid, and ascorbic acid (Table e42-4). In addition, some antibiotics are associated with a false increase in the S_cr, including cephalothin, cefazolin, cephalexin, cefoxitin, cefaclor, cephradine, and clavulonic acid,^32,33 whereas other agents, such as the fluoroquinolones (ciprofloxacin, fleroxacin, lomefloxacin, ofloxacin, levofloxacin, sparfloxacin, and temafloxacin), do not produce a false elevation in S_cr.³⁴ The degree of interference is dependent on the serum concentration of the antibiotic, so blood samples for creatinine should be obtained when the antibiotic concentration is lowest (at the end of a dosing interval). These interferences are not observed when the S_cr is measured using an enzymatic technique employing creatinine iminohydrolase, creatininase, creatinase, or sarcosine oxidase.

Endogenous and drug interferences continue to plague creatinine measurements for both the Jaffe and enzymatic methods. Recent modifications to the Jaffe method have been employed to address some nonspecific aspects of the assay.³⁵ For example, the “compensated Jaffe” method is used on the Roche Diagnostics, and Siemens analyzers. Here, a fixed concentration is subtracted from all results in order to adjust for nonspecific protein influence. The “uncompensated Jaffe” method used by Abbott, Beckman, and Siemens has been reported to be less susceptible to interferences by bilirubin when compared to other Jaffe methods.³⁶

Standardized S_cr assays are now used in most hospital clinical laboratories.^37,38 The method involves calibration of creatinine values based on standardized reagents that are traceable to an isotope dilution-mass spectrometry (IDMS) method. This approach has significantly reduced interlaboratory variability in S_cr values, and has resulted in a downward shift of the “normal range” for S_cr values by 0.1 to 0.2 mg/dL (9-18 μmol/L) when compared to noncalibrated ranges. For example, the normal reference range reported by the UNC Hospitals laboratory for healthy males was reduced from 0.8 to 1.4 mg/dL (71-124 μmol/L; pre-IDMS) to 0.7 to 1.3 mg/dL (62-115 μmol/L; post-IDMS).³⁸ Other recent modifications include reporting S_cr values in mg/dL to two decimal places (eg, 0.93 mg/dL), and values in μmol/L to the nearest whole number (eg, 82 μmol/L). This practice is aimed at reducing rounding errors that in the past contributed to bias between creatinine-based GFR or CL_cr estimates, but it does not improve GFR estimation in those without CKD.

Despite alterations in creatinine assay methods, many drugs still interfere with these assays resulting in false elevations or reductions in S_cr values. The antifungal agent 5-flucytosine cross-reacts with creatinine when measured using the Ektachem enzymatic system, but does not interact with the Jaffe method.³⁹ Daly et al.⁴⁰ reported a false-negative effect of dobutamine and dopamine on the S_cr value when measured using the Ektachem system. The interference is concentration-dependent and results in a 10% to 100% decrease of the S_cr. The authors hypothesized that both drugs compete with the chromogenic dye for oxidation by hydrogen peroxide in a concentration-dependent manner. The problem is most evident when blood samples are contaminated with residual IV solution containing the interfering drug. Thus, standardization of procedures within the clinic setting and awareness of the creatinine assay method used at the institution are critical to properly interpret each patient’s S_cr value.

Other compounds are known to truly alter the S_cr by inhibition of the active tubular secretion of creatinine. Among these are cimetidine and trimethoprim, which compete for creatinine secretion at the cationic transport system in a dose-dependent fashion. Cimetidine, given as a single 400-mg dose can result in a reduction of the CL_cr-to-inulin clearance ratio from 1.30 to 1.03, without a change in inulin clearance. Ranitidine, an H₂-receptor antagonist similar to cimetidine, however, does not have a similar effect on creatinine secretion following single doses of 300 to 1,200 mg.⁴¹ Other drugs known to interfere with the active tubular secretion of creatinine include rilpivirine, dolutegravir, cobicistat, pyrimethamine and amiodarone. These interactions typically result in serum creatinine elevations of 0.1 to 0.4 mg/dL (0.9-35 μmol/L) with apparent reductions in CL_cr of up to 30 mL/min/1.73 m² (0.29 mL/s/m²).⁴²

The S_cr is dependent on the “input” function, or formation rate, and “output” function, or elimination rate. Its formation rate depends on the zero-order production from creatine metabolism, as well as input from other sources, such as dietary intake. Over 95% of creatine stores are found in skeletal muscle, with creatine metabolism being directly proportional to muscle mass. Thus, individuals with more muscle mass have a higher S_cr at any given degree of kidney function than those with less muscle mass. Strenuous exercise is associated with an increase of approximately 10% in the S_cr. In contrast, cachectic patients, as the result of minimal muscle mass, will have very low S_cr, as do those with spinal cord injuries.⁴³ Elderly patients and those with poor nutrition may also have low S_cr (less than 1.0 mg/dL [88 μmol/L]) secondary to decreased muscle mass.

Creatine or methyl guanidine acetic acid is found in many commonly ingested food sources such as fish and red meat. During the cooking of meat, some creatine is converted to creatinine, which is rapidly absorbed following ingestion. S_cr may rise as much as 50% within 2 hours of a meat meal and remain elevated for as long as 8 to 24 hours.⁴⁴ Ingestion of creatine as an ergogenic dietary supplement is currently popular, as a means to increase skeletal muscle stores of phosphocreatine leading to adenosine triphosphate (ATP) resynthesis of adenosine diphosphate (ADP). There are conflicting reports as to the effect of creatine ingestion on the S_cr. Poortsmans et al.⁴⁵ evaluated a short-term regimen of 20 g creatine per day for 5 days in healthy subjects, and reported no significant change in the S_cr, creatinine excretion rate, or CL_cr. Robinson et al.⁴⁶ and Jagim et al.⁴⁷ reported a 25% to 40% increase in the S_cr after ingestion of 20 g creatine per day for either 5 days or 8 weeks. The renal excretion rate of creatinine was not measured. Spillane et al.⁴⁸ reported that creatine ethyl ester intake at a dose of 20 g/day for 6 days had a nearly threefold greater increase in S_cr than creatine monohydrate, indicating that different forms of creatine likely have differing effects on S_cr values.

The issue of whether, or not, creatine ingestion adversely affects kidney function was studied by Edmonds et al.⁴⁹ They noted that creatine supplementation led to an increase in renal disease progression in a rat model for renal cystic disease, suggesting that creatine supplementation may be a risk factor in patients with preexisting renal disease. Thus it is important to question ambulatory patients regarding their dietary intake for the 24 hours preceding the measurement of S_cr or CL_cr.

Diurnal variation in S_cr may also affect the accuracy of the CL_cr determination. Although the fluctuation is minimal, the observed peak plasma creatinine concentration generally occurs at approximately 7:00 PM, whereas the nadir is in the morning. The impact of diurnal variation is minimized by using S_cr measurements that are drawn at similar times during longitudinal evaluations, or using 24-hour urine collections for CL_cr measurements.

Serum and Urine Cystatin C

Cystatin C is a 132-amino-acid (13.3-kDa) cysteine protease inhibitor produced by all nucleated cells of the body that is considered a biomarker of renal function. It is freely filtered at the glomerulus and undergoes both reabsorption and catabolism in the proximal tubule. The renal handling of this biomarker is distinctly different from creatinine and exogenous GFR markers such as inulin, iohexol, and iothalamate. Originally introduced in Europe, it was recommended as a biomarker of kidney function because of findings that serum concentrations significantly correlated with GFR as well as S_cr. It was hypothesized that since cystatin C production was not affected by muscle mass, it would provide a more reliable estimate of renal function than S_cr. It is known that serum cystatin C (cysC) concentrations can be altered by many factors other than kidney function, such as age, nutritional status, gender, weight, height, cigarette smoking, serum C-reactive protein levels, steroid therapy, and rheumatoid arthritis.^50,51 Recent studies have reported conflicting results about the sensitivity of serum cysC compared to creatinine in predicting various forms of AKI.^52,53,54,55 cysC concentrations in serum has been shown to perform better than S_cr for predicting contrast-induced kidney injury, resulting in a new definition of contrast-induced acute kidney injury (CIAKI), consisting of a more than 10% increase in cysC within 24 hours after exposure to contrast media.^52,53 Serum cysC also detected AKI up to 2 days earlier than serum creatinine in critically ill patients,⁵⁴ and cysC concentration was a better predictor of AKI in pediatric cardiac surgery patients.⁵⁵ However, three prior studies did not show a clear advantage for serum cysC concentration to predict AKI in cardiopulmonary bypass and cardiothoracic surgery patients.^56,57,58 In a recent multicenter study of 1,150 adults followed after cardiac surgery, cysC was noted to be a less sensitive biomarker than creatinine for detecting AKI in the postoperative period.⁵⁸ These differential findings suggest that the role of cysC in assisting with the diagnosis of AKI is yet to be fully determined.

The recent development of automated immunoassay techniques for cysC determination is based on a latex-enhanced immunoturbidimetric assay. cysC in the sample (serum or urine) binds to anti-cysC antibody, which is coated on latex particles, resulting in agglutination. The degree of the turbidity caused by agglutination is measured optically (at 540 nm) and is proportional to the amount of cysC in the sample. The range of serum values is 0.13 mg/L to 8.0 mg/L (10-599 nmol/L), with normal values of 0.55 to 1.18 mg/L (41-88 nmol/L) for women and 0.60 to 1.11 mg/L (45-83 nmol/L) for men based on data from the NHANES study.⁵⁹

The utility of cysC as a quantitative measure of GFR is gaining acceptance in clinical and research settings. In a 4-year longitudinal study in 30 Pima Indians without CKD, the reciprocal cysC index (100/cysC) was shown to be a better predictor of declining measured GFR (mGFR) than estimated GFR (eGFR), or CL_cr.⁶⁰ However, in patients being treated for malignant disease, Page et al.⁶¹ reported cysC concentrations increased independent of CL_cr. cysC concentrations were also noted to be independent of GFR and weakly associated with mCL_cr, in pediatric and adult renal transplant recipients, possibly because of the formation of cystatin–immunoglobulin complexes or reduced tubular catabolism.^62,63 Two additional studies have shown a strong association between serum cysC and cardiovascular disease in elderly and non-CKD patients, suggesting that it may provide useful prognostic information in some populations.^64,65 A recent study by Peralta et al. in nearly 25,000 adults enrolled in the REGARDS and NHANES III studies, showed that GFR based on cysC values identified a 14% higher incidence of CKD when compared to creatinine-based eGFR.⁶⁶ This suggests that serum cysC may be a more sensitive index of early kidney damage compared to serum creatinine, although further research is needed to identify conditions and patient populations in whom nonrenal factors may influence serum cysC concentrations.

Alternative Methodologies to Identify Kidney Injury

Urinary biomarkers, such as kidney injury molecule-1 (KIM-1), neutrophil gelatinase-associated lipocalin (NGAL), and cysC have shown promise in detecting AKI⁶⁷ (see Chapter 43). Because it is normally reabsorbed and metabolized in the renal proximal tubule, it is hypothesized that tubular damage would lead to increased urinary excretion of cysC. Koyner et al.⁶⁸ showed that postop cardiac surgery patients with elevated urinary cysC-to-creatinine ratios had the highest prevalence of developing AKI. S_cr did not peak until 48 hours after intensive care unit (ICU) admission, suggesting that urinary biomarkers such as cysC may be beneficial in early diagnosis of AKI. Use of serum cysC as a quantitative measure of GFR requires further evaluation, and may yield useful information for comprehensive evaluations of health and cardiovascular status, including detection of acute and chronic changes in kidney function.

β-Trace protein (BTP) has recently been proposed as a new alternate endogenous marker of GFR.^69,70 Similar to cysC, it is a low-molecular-weight glycoprotein (168 amino acids) that is filtered through the glomerular basement membrane and reabsorbed in the renal proximal tubule. Elevated concentrations of BTP in serum have been reported in patients with CKD,⁷¹ kidney transplant recipients,⁷² and children.⁷³ Equations to estimate GFR that incorporate BTP concentrations in adults and pediatric populations are currently being evaluated.⁷⁴ Other markers of early AKI include urinary tissue inhibitor of metalloproteinase 2 (TIMP-2) and urinary insulin-like growth factor binding protein 7 (IGFBP-7). These two biomarkers are important because they are involved in G1 cell cycle arrest of renal tubular cells during the early period of cell injury. Combining these markers into a duplex “TIMP-2 × IGFBP-7” biomarker algorithm in over 500 patients accurately predicted AKI following cardiac surgery in high risk patients.^75,76 These results supported the September 2014 FDA approval of NephroCheck^® (Astute Medical, Inc; http://www.astutemedical.com/us/products/nephrocheck-test/) as a urinary diagnostic device indicated for early detection of AKI.

The gold standard quantitative index of kidney function is a mGFR. A variety of methods may be used to measure and estimate kidney function in the acute care and ambulatory settings. Measurement of GFR is important for early recognition and monitoring of patients with CKD and as a guide for drug-dose adjustment.

It is important to recognize conditions that may alter renal function independent of underlying renal pathology. For example, protein intake, such as oral protein loading or an infusion of amino acid solution, may increase GFR.¹³ As a result, inter- and intrasubject variability must be considered when it is used as a longitudinal marker of renal function. Dietary protein intake has been demonstrated to correlate with GFR in healthy subjects. Brändle et al.⁷⁷ evaluated renal function in four groups of healthy volunteers, each ingesting a diet controlled for protein over a 4-month period. The GFR was nonlinearly related to the urine nitrogen excretion, with an observed maximum of 181.7 mL/min (3.03 mL/s) at a urinary nitrogen excretion rate of 20 g/day (1.43 mol/day), or 125 g/day protein intake. Subjects who are vegetarian have a lower GFR because of their reduced dietary protein intake relative to individuals who consume a similar caloric but normal-protein-content diet. When challenged with a protein load, the vegetarian subjects are able to increase their GFR to the “normal” range.¹³ Findings from the Nurses’ Health Study⁷⁸ indicate that longitudinal changes in GFR are independent of the source of protein (nondairy animal, dairy, or vegetable) in women with normal renal function. However, women with mild renal insufficiency (GFR 71 ± 7 mL/min [1.18 ± 0.12 mL/s]) who consumed the highest amount of protein (93 g/day) had a threefold greater risk of a more than or equal to 5 mL/min (≥0.08 mL/s) decline in GFR compared to the lowest protein group (60 g/day); rates of decline were highest in those consuming nondairy animal protein. The increased GFR following a protein load is the result of renal vasodilation accompanied by an increased renal plasma flow. The exact mechanism of the renal response to protein is unknown, but may be related to extrarenal factors such as glucagon, prostaglandins, and angiotensin II, or intrarenal mechanisms, such as alterations in tubular transport and tubuloglomerular feedback.^79,80 Despite the evidence of a “renal reserve,” standardized evaluation techniques have not been developed. Therefore, assessment of a mGFR must consider the dietary protein status of the patient at the time of the study.

Measurement of Glomerular Filtration Rate

A mGFR remains the single best index of kidney function. As renal mass declines in the presence of age-related loss of nephrons or disease states such as hypertension or diabetes, there is a progressive decline in GFR. The rate of decline in GFR can be used to predict the time to onset of stage 5 CKD, as well as the risk of complications of CKD. Accurate measurement of GFR in clinical practice is a critical variable for the individualization of the dosage regimens of renally excreted medications so that one can maximize their therapeutic efficacy and avoid potential toxicity.

The GFR is expressed as the volume of plasma filtered across the glomerulus per unit time, based on total renal blood flow and capillary hemodynamics. The normal values for GFR are 127 ± 20 mL/min/1.73 m² (1.22 ± 0.19 mL/s/m²) and 118 ± 20 mL/min/1.73 m² (1.14 ± 0.19 mL/s/m²) in healthy men and women, respectively. These measured values closely approximate what one would predict if the normal renal blood flow were approximately 1.0 L/min/1.73 m² (0.01 mL/s/m²), plasma volume was 60% of blood volume, and filtration fraction across the glomerulus was 20%. In that situation the normal GFR would be expected to be approximately 120 mL/min/1.73 m² (1.16 mL/s/m²).

Optimal clinical measurement of GFR involves determining the renal clearance of a substance that is freely filtered without additional clearance because of tubular secretion or reduction as the result of reabsorption. Additionally, the substance should not be susceptible to metabolism within renal tissues and should not alter renal function. Given these conditions, the mGFR is equivalent to the renal clearance of the solute marker:

where renal CL is renal clearance of the marker, A_e is the amount of marker excreted in the urine in a specified period of time, t, and AUC_0-t is the area under the plasma-concentration-versus-time curve of the marker.

Under steady-state conditions, for example during a continuous infusion of the marker, the expression simplifies to

where C_ss is the steady-state plasma concentration of the marker achieved during continuous infusion. The continuous infusion method can also be employed without urine collection, where plasma clearance is calculated as CL = infusion rate/C_ss. This method is dependent on the attainment of steady-state plasma concentrations and accurate measurement of infusate concentrations. Plasma clearance can also be determined following a single-dose IV injection with the collection of multiple blood samples to estimate area under the curve (AUC_0-∞). Here, clearance is calculated as CL = dose/AUC. These plasma clearance methods commonly yield clearance values 10% to 15% higher than GFR measured by urine collection methods.^81,82

Several markers have been used for the measurement of GFR and include both exogenous and endogenous compounds. Those administered as exogenous agents, such as inulin, sinistrin, iothalamate, iohexol, and radioisotopes, require specialized administration techniques and detection methods for the quantification of concentrations in serum and urine, but generally provide an accurate measure of GFR. Methods that employ endogenous compounds, such as creatinine or cysC, require less technical expertise, but produce results with greater variability. The GFR marker of choice depends on the purpose and cost of the compound which ranges from $2,000 per vial for radioactive for ¹²⁵I-iothalamate (Glofil-125, QOL Medical) to $6 per vial for nonradiolabeled iothalamate (Conray-60, Mallincrodt) or iohexol (Omnipaque-300, GE Medical) (Table e42-5).

Inulin and Sinistrin Clearance

Inulin is a large fructose polysaccharide (5,200 Da), obtained from the Jerusalem artichoke, dahlia, and chicory plants. It is not bound to plasma proteins, is freely filtered at the glomerulus, is not secreted or reabsorbed, and is not metabolized by the kidney. The volume of distribution of inulin approximates extracellular volume, or 20% of ideal body weight. Because it is eliminated by glomerular filtration, its elimination half-life is dependent on renal function and is approximately 1.3 hours in subjects with normal renal function. Measurement of plasma and urine inulin concentrations can be performed using high-performance liquid chromatography.⁸³ Sinistrin, another polyfructosan, has similar characteristics to inulin; it is filtered at the glomerulus and not secreted or reabsorbed to any significant extent. It is a naturally occurring substance derived from the root of the North African vegetable red squill, Urginea maritime, which has a much higher degree of water solubility than inulin. Assay methods for sinistrin have been described using enzymatic procedures, as well as high-performance liquid chromatography with electrochemical detection.⁸⁴ Alternatives have been sought for inulin as a marker for GFR because of the problems of availability, high cost, sample preparation, and assay variability.

Iothalamate Clearance

Iothalamate is an iodine-containing radiocontrast agent that is available in both radiolabeled (¹²⁵I) and nonradiolabeled forms. This agent is handled in a manner similar to that of inulin; it is freely filtered at the glomerulus and does not undergo substantial tubular secretion or reabsorption. The nonradiolabeled form is most widely used to measure GFR in ambulatory and research settings, and can safely be administered by IV bolus, continuous infusion, or subcutaneous injection.⁸² Plasma and urine iothalamate concentrations can be measured using high-performance liquid chromatography.^85,86 Plasma clearance methods that do not require urine collections have been shown to be highly correlated with renal clearance, making them particularly well-suited for longitudinal evaluations of renal function.^82,87 These plasma clearance methods require two-compartment modeling approaches because accuracy is dependent on duration of sampling. For example, Agarwal et al.⁸⁷ demonstrated that short sampling intervals can overestimate GFR, particularly in patients with severely reduced GFR. In individuals with GFR more than 30 mL/min/1.73 m² (greater than 0.29 mL/s/m²), a 2-hour sampling strategy yielded GFR values that were 54% higher compared with 10-hour sampling, whereas the 5-hour sampling was 17% higher. In individuals with GFR less than 30 mL/min/1.73 m² (less than 0.29 mL/s/m²), the 5-hour GFR was 36% higher and 2-hour GFR was 126% higher than the 10-hour measurement. The authors proposed a 5- to 7-hour sampling time period with eight plasma samples to be the most appropriate and feasible approach for most GFR evaluations.

Iohexol

Iohexol, a nonionic, low osmolar, iodinated contrast agent, has also been used for the determination of GFR. It is eliminated almost entirely by glomerular filtration, and plasma and renal clearance values are similar to observations with other marker agents: Strong correlations of 0.90 or greater and significant relationships with iothalamate have been reported.^88,89,90 These data support iohexol as a suitable alternative marker for the measurement of GFR. A reported advantage of this agent is that a limited number of plasma samples (as few as two collected at 120 and 300 minutes after injection) can be used to quantify iohexol plasma clearance.⁹¹ For patients with a reduced GFR more time must be allotted—more than 24 hours if the eGFR is less than 20 mL/min (0.33 mL/s).

Radiolabeled Markers

The GFR has also been quantified using radiolabeled markers, such as ¹²⁵I-iothalamate (614 Da, radioactive half-life of 60 days), ^99mTc-DPTA (393 Da, radioactive half-life of 6.03 hours), and ⁵¹Cr-ethylenediaminetetraacetic acid (⁵¹Cr-EDTA; 292 Da, radioactive half-life of 27 days).⁹² These relatively small molecules are minimally bound to plasma proteins and do not undergo tubular secretion or reabsorption to any significant degree. ¹²⁵I-iothalamate and ^99mTc-DPTA are used in the United States, whereas ⁵¹Cr-EDTA is used extensively in Europe. The use of radiolabeled markers allows one to determine the individual contribution of each kidney to total renal function.⁹³ Various protocols exist for the administration of these markers and subsequent measurement of GFR using either plasma or renal clearance calculation methods. The nonrenal clearance of these agents appears to be low (3-8 mL/min [0.05-0.13 mL/s]), suggesting that plasma clearance is an acceptable technique except in patients with severe renal insufficiency (GFR less than 30 mL/min [less than 0.50 mL/s]). Indeed, highly significant correlations between renal clearance among radiolabeled markers has been demonstrated.⁹⁴ Although total radioactive exposure to patients is usually minimal, use of one of these agents does require compliance with radiation safety committees and appropriate biohazard waste disposal.

Optical Real-Time Glomerular Filtration Rate Markers

A clinically applicable technique to rapidly measure GFR, particularly in critically ill patients with unstable kidney function, is highly desirable. The currently available GFR measurement approaches, as outlined above, are technically demanding, time-consuming, and often cost-prohibitive. Research is underway to develop rapid, accurate, safe, and inexpensive techniques to address this need.^95,96 For example, small, nontoxic, exogenously administered fluorescent tracers are being investigated for “real-time” GFR measurement, and at least two have entered early phase clinical trials. Both methods involve administration of optically active compounds, with continuous detection of the fluorescence signal using fiber optic or photonics technologies. A system involving injection of fluorescent dextran molecules with blood sampling and rapid (bedside) detection over 120 minutes is being developed by FAST BioMedical (Indianapolis, IN), and an NIH-funded clinical trial in healthy volunteers and patients with AKI/CKD was completed in 2014 (NCT01978314, ClinicalTrials.gov). Another method being developed by MediBeacon, LLC (St. Louis, MO), involves injection of a hydrophilic pegylated pyrazine dye (MB-102), with continuous detection using a transdermal photonics system. The results of preclinical safety and early Phase 1 testing indicate that the lead compound (MB-102) yields comparable estimates of GFR to those obtained with iohexol administration in healthy subjects.⁹⁷ The current challenge of these approaches will be to translate fluorescent signals into quantitative in vivo measurements of GFR.

Creatinine

Measured Creatinine Clearance

Although the measured (24-hour) CL_cr has been used as an approximation of GFR for decades, it has limited clinical utility for a multiplicity of reasons. Short-duration witnessed mCL_cr correlates well with mGFR based on iothalamate clearance performed using the single-injection technique. In a multicenter study⁹⁸ of 136 patients with type 1 diabetic nephropathy, the correlations of simultaneous mCL_cr, and 24-hour CL_cr (compared to CL_iothalamate) were 0.81 and 0.49, respectively, indicating increased variability with the 24-hour clearance determination. In a selected group of 110 patients, measurement of a 4-hour CL_cr during water diuresis provided the best estimate of the GFR as determined by the CL_iothalamate. Furthermore, the ratio of CL_cr to CL_iothalamate did not appear to increase as the GFR decreased. These data suggest that a short collection period with a water diuresis may be the best CL_cr method for estimation of GFR.

A limitation of using creatinine as a filtration marker is that it undergoes tubular secretion. Tubular secretion augments the filtered creatinine by approximately 10% in subjects with normal kidney function. If the nonspecific Jaffe reaction is used, which overestimates the S_cr by approximately 10% because of the noncreatinine chromogens, then the measurement of CL_cr is a very good measure of GFR in patients with normal kidney function. Tubular secretion, however, increases to as much as 100% in patients with kidney disease, resulting in mCL_cr values that markedly overestimate GFR. For example, Bauer et al.³¹ reported that the CL_cr-to-CL_inulin ratio in subjects with mild impairment was 1.20; for those with moderate impairment, it was 1.87; and in those with severe impairment, it was 2.32. Thus, a mCL_cr is a poor indicator of GFR in patients with moderate to severe renal insufficiency, that is, stages 3 to 5 CKD.

Because cimetidine blocks the tubular secretion of creatinine the potential role of several oral cimetidine regimens to improve the accuracy and precision of mCL_cr as an indicator of GFR has been evaluated. The CL_cr-to-CL_DPTA ratio declined from 1.33 with placebo to 1.07 when 400 mg of cimetidine was administered four times a day for 2 days prior to and during the clearance determination.⁹⁹ Similar results were observed when a single 800-mg dose of cimetidine was given 1 hour prior to the simultaneous determination of CL_cr and CL_iothalamate; the ratio of CL_cr to CL_iothalamate was reduced from a mean of 1.53 to 1.12.¹⁰⁰ Thus a single oral dose of 800 mg of cimetidine should provide adequate blockade of creatinine secretion to improve the accuracy of a CL_cr measurement as an estimate GFR in patients with stages 3 to 5 CKD.

To minimize the impact of diurnal variations in S_cr on CL_cr, the test is usually performed over a 24-hour period with the plasma creatinine obtained in the morning, as long as the patient has stable kidney function. Collection of urine remains a limiting factor in the 24-hour CL_cr because of incomplete collections, and interconversion between creatinine and creatine that can occur if the urine is not maintained at a pH less than 6.

Estimation of Glomerular Filtration Rate

Because of the invasive nature and technical difficulties of directly measuring GFR in clinical settings, many equations for estimating GFR have been proposed over the past 10 years (Table e42-6). A series of related GFR estimating equations have been developed for the primary purpose of identifying and classifying CKD in many patient populations.^{101,102,103,104,105,106,107,108} The initial equation was derived from multiple regression analysis of data obtained from the 1,628 patients enrolled in the Modification of Diet in Renal Disease Study (MDRD) where GFR was measured using the renal clearance of ¹²⁵I-iothalamate methodology. A four-variable version of the original MDRD equation (MDRD4), based on plasma creatinine, age, sex, and race, was shown to provide a similar estimate of GFR results when compared to a six-variable equation predecessor.¹⁰² However, this equation was shown to be inaccurate at GFR more than 60 mL/min/1.73m² (less than 0.58 mL/s/m²), for reasons not associated with standardization of S_cr (IDMS) assay results.¹⁰³ The MDRD4–IDMS equation is still included in the recommendations of the National Kidney Foundation (NKF) and the National Kidney Disease Education Program (NKDEP) for calculating the eGFR in patients with a history of CKD risk factors and a GFR less than 60 mL/min/1.73 m² (less than 0.58 mL/s/m²). A recent study conducted by the FDA compared the eGFR estimated by the MDRD4 equation to the CL_cr estimated by the Cockcroft-Gault (CG) equation in 973 subjects enrolled in pharmacokinetic studies conducted for new chemical entities submitted to the FDA from 1998 to 2010.¹⁰⁷ The MDRD4 eGFR results consistently overestimated the CL_cr calculated by the CG method. The FDA investigators concluded that “For patients with advanced age, low weight, and modestly elevated serum creatinine concentration values, further work is needed before the MDRD equations can replace the CG equation for dose adjustment in approved product information labeling.”

A single eGFR equation may not be best suited for all populations, and choice of equation has been shown to impact CKD prevalence estimates.¹⁰⁸ This has led to a revitalized interest in the development of new equations to estimate GFR. The newest equations to be proposed for the estimation of GFR have been derived from wider CKD populations than the MDRD study, and include the CKD-EPI^109,110 and the Berlin Initiative Study (BIS).¹¹¹ The CKD-EPI equation was developed from pooled study data involving 5,500 patients (including the original MDRD population), with mean GFR values of 68 ± 40 mL/min/1.73 m² (0.65 ± 0.39 mL/s/m²) (range 2-190 mL/min/1.73 m² [0.02-1.83 mL/s/m²]). It has been reported that the CKD-EPI equation is less biased (2.5 vs 5.5 mL/min/1.73 m² [0.024 vs 0.053 mL/s/m²]) but similarly imprecise compared to MDRD4.¹¹⁰

Chronic Kidney Disease–EPI Equation

The CKD-EPI study equation was compared to the MDRD equation using pooled data from patients enrolled in research or clinical outcomes studies, where GFR was measured by any exogenous tracer.¹¹⁰ The results of the study indicated that the bias of CKD-EPI equation was 61% to 75% lower than the MDRD equation for patients with eGFR of 60 to 119 mL/min/1.73 m² (0.58-1.15 mL/s/m²). Based on these findings, the CKD-EPI equation is most appropriate for estimating GFR in individuals with eGFR values more than 60 mL/min/1.73 m² (greater than 0.58 mL/s/m²). Both the KDOQI and the Australasian Creatinine Consensus Working Groups now recommend that clinical laboratories switch from the MDRD4 to CKD-EPI for routine automated reporting.^23,112 If one’s clinical lab does not automatically calculate eGFR using the CKD-EPI, it becomes a bit of a challenge since the equation requires a more complex algorithm than the MDRD equation.

Limitations of the pooled analysis approach used to develop the MDRD and CKD-EPI equations include the use of different GFR markers between studies (iothalamate, ⁵¹CR-EDTA, ^99mTc-DTPA), different methods of administration of the GFR markers (subcutaneous and IV), and different clearance calculations (renal clearance vs plasma disappearance). These limitations may partly explain the reduced accuracy observed with the MDRD4 equation at GFR values more than 60 mL/min/1.73 m² (greater than 0.58 mL/s/m²). Additionally, a recent inspection of the MDRD GFR study data showed that large intrasubject variability in GFR measures was a likely contributor to the inaccuracy of the gold standard method ([¹²⁵I] iothalamate urinary clearance) that was used to create the MDRD equation.¹¹³

Numerous studies have reported that the MDRD4 and CKD-EPI eGFR equation derived values overestimate CG equation values for eCL_cr.^114,115 For example, Wargo et al.¹¹⁴ evaluated 409 patients with CKD admitted to a tertiary care clinic. The CKD-EPI equation values significantly overestimated the CG equation derived eCLcr determinations (39.9 mL/min vs 34.8 mL/min [0.67 mL/s vs 0.58 mL/s], respectively; p less than 0.001), with 95% of cases ranging from –5.1 mL/min to +15.3 mL/min (–0.09 mL/s to +0.26 mL/s). In the largest retrospective study comparing the eCLcr by CG and eGFR by MDRD4, Melloni et al.¹¹⁵ reported that eGFR (expressed in mL/min) resulted in a failure to make manufacturer-recommended dose reductions for enoxaparin and the glycoprotein IIb/IIIa inhibitor (GPI) eptifibatide in up to 50% of their cohort of more than 49,000 patients. The excessive eGFR-derived doses were also correlated with major bleeding episodes (odds ratio 1.57 [95% CI 1.35-1.84]). Thus, the MDRD4 equation for eGFR should not be used in lieu of eCLcr by CG for renal dose adjustments for these drugs. The results of these studies highlight the need to understand that eGFR equations such as MDRD4 and CKD-EPI were developed for the purpose of identifying and stratifying CKD based on large multicenter epidemiologic studies. Extension of their use for individualized drug dosing has not been fully evaluated and automatic substitution of MDRD4 or CKD-EPI in place of estimated or mCL_cr for drug dose calculations should be avoided (Fig. e42-5).

Cystatin C-Based Equations

Addition of serum cysC as a covariate in equations to estimate GFR has been employed as a means to improve creatinine-based estimations of GFR that historically were limited to the following variables; lean body mass (LM), age, sex, race, and S_cr.^{116,117,118,119,120,121}

A significant limitation of serum cysC as a renal biomarker is the influence of body mass on serum concentrations. MacDonald et al.¹¹⁶ and Vupputuri et al.¹¹⁹ reported that fat-free mass is a significant covariate in GFR determination using cysC. Using GFR measured as plasma inulin clearance, LM accounted for at least 16.3% of the variance in GFR values obtained using serum cysC (p less than 0.001). When using a serum cysC-based estimate of GFR, which incorporates the serum cysC, age, race, and sex, a higher prevalence of CKD was reported in obese patients when compared to the MDRD4 equation.¹¹⁹ In a recent retrospective analysis of over 1,000 elderly individuals (mean age 85 years) enrolled in the Cardiovascular Health Study, GFR was estimated using the CKD-EPI and CKD-EPI-cysC equation, specifically equation 9 in Table e42-6.¹²¹ In this population, all-cause mortality rates were significantly different between equations. The CKD-EPI equation yielded a U-shaped association, whereas the CKD-EPI-cysC equation yielded a linear relationship at eGFR values less than 60 mL/min/1.73 m² (less than 0.58 mL/s/m²), suggesting that cysC does not accurately predict mortality risk in patients with low S_cr, reduced muscle mass, and malnutrition. The combined use of serum cysC and creatinine in modified CKD-EPI equations has recently been reported. The CKD-EPI_{creatinine_cystatin C,} equation 10 in Table e42-6 is now recommended for use in patients where unreliable serum creatinine values are anticipated, such as extremes in body mass, diet, or creatinine assay interferences.¹²⁰

The BIS, another eGFR equation includes cysC for CKD classification.¹¹¹ An analysis conducted in a cross-sectional cohort of 570 Caucasians average ages 78.5 years and mGFR 60 mL/min/1.73m² (range: 17-116 mL/min/1.73m² [mGFR 0.58 mL/s/m² and range of 0.16-1.12 mL/s/m²]) by iohexol plasma clearance compared the eGFR calculated by BIS1 (creatinine) and BIS2 (creatinine + cysC), to eCLcr by CG with eGFR by multiple CKD-EPI equations. The BIS2 equation yielded the smallest bias followed by the BIS1 and Cockcroft–Gault equations. The total error rate for misclassification of subjects with GFR less than 60 mL/min/1.73m² (less than 0.58 mL/s/m²) was smallest for the BIS2 equation (11.6%), followed by the CKD-EPI_{cystatin C} equation (15.1%). Among the creatinine-based equations, BIS1 had the smallest misclassification rate (17.2%), followed by the CKD-EPI (20.4%). Further evaluation of the BIS equations in patients with varying ethnicities, chronic diseases, and GFR values is warranted before its use can be broadly recommended.

Additional research on the utility of these new eGFR equations in diverse ethnic groups has resulted in modifications or “correction factors,” such as those for Japanese¹²⁴ and Chinese¹²⁵ populations. Studies to determine the impact of adding cysC into these equations are yet to be reported.

A variety of online resources provide eGFR calculators such as the NKDEP Web site,¹²⁶ which provides eGFR calculators based on the MDRD4–IDMS, CKD-EPI, and CKD-EPIcreatine_cystatin C equations. The NKDEP continues to recommend that laboratories report eGFR values greater than or equal to 60 as “greater than 60 mL/min/1.73 m² (greater than 0.58 mL/s/m²), not as an exact number,” due to inaccuracies of eGFR equations at higher levels of GFR. It should be noted that one must verify that a given equation is appropriate for the institutional creatinine reporting method.

Estimation of Creatinine Clearance

Many equations describing the mathematical relationships between various patient factors and mCL_cr, the most widely recognized surrogate for GFR in clinical settings. Most equations incorporate factors such as age, gender, weight, and S_cr, without the need for urine collection. The most widely used of these estimators is the CG equation,¹²⁷ which identified age and body mass as factors, which significantly contribute to the estimate of CL_cr. This relationship was based on observations from 249 male patients with stable kidney function in whom the creatinine production rates were estimated. Estimated creatinine clearance (eCL_cr), using the CG equation, is one of the methods endorsed by the FDA for stratifying patients in drug development pharmacokinetic studies, and has been reported most often in FDA-approved package inserts for new drug entities since the 1990s.^4,105

One of the key considerations with the use of this equation is whether or not a modified weight index should replace actual body weight. Several modified weight indices have been proposed and this remains a controversial issue. For obese individuals, defined as those with a body mass index (BMI) greater than or equal to 30 kg/m² but less than 40 kg/m,² it is generally recommended that total or actual body weight be used. This is based on a recent analysis by the FDA indicating that the CG equation had less than 10% bias in nearly 600 normal weight, overweight, and obese individuals enrolled in drug pharmacokinetic studies.¹²⁸ In morbidly obese individuals (BMI ≥40 kg/m,² obesity class III) an alternate measure of body weight such as lean body weight (LBW) was shown to significantly reduce bias in the CG equation, where LBW is calculated as:

and BMI is calculated as:

Regardless of the approach used to estimate renal function in obese patients, it is imperative that drug therapy outcomes be monitored closely in this population.

Luke et al.¹²⁹ evaluated the ability of the CG method and four other methods to determine eCL_cr, with inulin clearance being considered the standard measure of GFR. The simultaneously determined inulin and measured creatinine clearances correlated best, r² = 0.85, and the mCL_cr overestimated CL_inulin by approximately 15% due to tubular secretion of creatinine. Of the five eCL_cr, the ones calculated by the CG and Mawer et al. methods¹³⁰ correlated the best with GFR. Other methods, such as Jelliffe¹³¹ and Hull et al.¹³² consistently underestimated the mCL_cr (Table e42-7).

Patients undergoing screening for participation in the African American Study of Kidney Disease (AASK) were evaluated for kidney function based on eCL_cr, simultaneous ¹²⁵I-iothalamate and measured 24-hour CL_cr.¹³³ The simultaneous mCL_cr provided the best estimate of GFR. The CG method was the preferred method for eGFR, based on performance and ease of use. This method was noted to underestimate the GFR by 9%, perhaps because of the increased excretion rate of creatinine by black patients.¹³⁴

Administration of cimetidine has also resulted in improved performance of the CG equation as means to calculate eGFR. Ixkes et al.¹³⁵ gave patients three 800-mg doses of cimetidine in 24 hours, and measured creatinine plasma levels from 3 to 7 hours following the final dose. During this 4-hour period, the CL_iothalamate was determined as the measure of GFR. The CG calculations were performed with the plasma creatinine measurement 3 hours after the last dose of cimetidine. The ratio of the CG eCL_cr-to-CL_iothalamate decreased from 1.28 ± 0.21 to 0.98 ± 0.11 in the presence of cimetidine. This cimetidine dosing schedule also improved the accuracy of CG eCL_cr relative to GFR in renal transplant patients with GFR values ranging from 20 to 80 mL/min/1.73 m² (0.19-0.77 mL/s/m²).^136,137

Liver Disease

Evaluation of renal hemodynamics is particularly complicated in patients with liver disease and cirrhosis, where filtration fraction is associated with the degree of ascites, renal artery vasoconstriction, and vascular resistance.¹³⁸ The estimation of CL_cr or GFR can be problematic in patients with preexisting liver disease and renal impairment. Lower-than-expected S_cr values may result from reduced muscle mass, protein-poor diet, diminished hepatic synthesis of creatine (a precursor of creatinine), and fluid overload can lead to significant overestimation of CL_cr. Orlando et al.¹³⁹ evaluated 10 healthy subjects, 10 patients with mild liver disease, and 10 with severe liver disease, and observed a mCL_cr-to-CL_inulin ratio of 1.05, 1.03, and 1.04 for each group, respectively. When the CL_cr of patients with severe liver disease was estimated using the CG equation, the resultant ratio (eCL_cr-to-CL_inulin) was 1.23. Lam et al.¹⁴⁰ likewise noted an overestimation by CG of the mCL_cr in patients with severe disease, by 40% to 100%.

Studies of renal function in patients with severe hepatic disease confirm the earlier observations of Hull et al.¹³² and Caregaro et al.¹⁴¹ who reported that mCL_cr overestimated GFR by up to 50% in hepatic patients with a GFR of 56 ± 19 mL/min/1.73 m² (0.54 ± 0.18 mL/s/m²) because of increased tubular secretion of creatinine. The effect of cimetidine administration on mCL_cr was evaluated in a small study by Sansoe et al.¹⁴² In 12 patients with compensated cirrhosis, S_cr values increased from 0.68 ± 0.11 to 0.94 ± 0.14 mg/dL (60 ± 10 μmol/L-83 ± 12 μmol/L) during coadministration of cimetidine (1,000 mg given as 400 mg × 1 then 200 mg every 3 hours) during a 9-hour clearance period. The mCL_cr declined from 138 ± 20 prior to cimetidine administration to 89 ± 13 mL/min (2.30 ± 0.33-1.49 ± 0.22 mL/s), with no change in mGFR.

Evaluations of new eGFR equations for use in patients with liver disease have yielded mixed results. In cirrhotic patients being evaluated for liver transplant (mGFR 58 ± 5.1 mL/min/1.73 m² [0.56 ± 0.049 mL/s/m²]) the eGFR by the MDRD4 and the eCL_cr by CG significantly overestimated mGFR by 30% to 50%, and both were considered unacceptable methods for kidney function assessment in liver transplant patients.^143,144 Gerhart et al.¹⁴⁵ evaluated the performance of the CKD-EPI and MDRD4–IDMS equations in patients with liver disease following transplantation (group 1, n = 59) and those with cirrhosis (group 2; n = 44). When compared to mGFR, both equations yielded slightly positively biased estimates of GFR (4-9 mL/min/1.73 m² [0.04-0.09 mL/s/m²]) in transplanted patients. However, in patients with hepatic cirrhosis, both equations were significantly positively biased (40-42 mL/min/1.73 m² [0.39-0.40 mL/s/m²]), with low precision (21-26 mL/min/1.73 m² [0.20-0.25 mL/s/m²]) and low accuracy with only 7% of patients having eGFR values within 30% of the mGFR. Incorporation of cysC into eGFR equations has recently shown promise in patients with liver disease. In 72 cirrhotic patients, Mindikoglu et al.¹⁴⁶ reported that the CKD-EPI_{creatinine-cystatin C} equation was significantly more precise than the CG, CKD-EPI, or CKD-EPI_{cystatin C} when compared to iothalamate-mGFR. The accuracy of the CKD-EPI_{creatinine-cystatin C} equation, measured as percentage of eGFR that differed by more than 30% with respect to mGFR, was significantly less than mCL_cr (p = 0.024), CG (p = 0.0001), MDRD (p = 0.027), and CKD-EPI_creatinine (p = 0.012) equations. In summary, renal function assessment in patients with hepatic disease should be performed by measuring glomerular filtration, and GFR estimation equations that combine creatinine and cysC are preferred.

Other Special Populations

Davis and Chandler¹⁴⁷ confirmed the accuracy of the CG eCL_cr method in trauma patients with stable kidney function, and Thakur et al.⁴³ demonstrated its acceptable performance in 42 paraplegic patients. Kidney transplant recipients are frequently monitored for renal function, as numerous complications may occur during the life of the allograft. Ruiz-Esteban et al.¹⁴⁸ evaluated the bias and precision of the MDRD4 and CKD-EPI relative to CG in 153 postrenal transplant patients. Here, the mean bias for MDRD4 was –10.6 ± 12.7 compared to –9.8 ± 11.3 mL/min/1.73 m² (–0.10 ± 0.12 compared to –0.09 ± 0.11 mL/s/m²) for CKD-EPI (p = 0.006), with the CKD-EPI having a higher percentage of patients within 30% of the CG value than the MDRD equation (86.9% vs 81.7%, p less than 0.001). Huang et al.¹⁴⁹ reported the inability of several CL_cr equations to predict renal function in hospitalized patients with advanced HIV disease. All of the prediction methods overestimated the measured 24-hour CL_cr. The reasons for the poor predictability of these methods are unclear, although 24-hour collection methods result in increased variability, often because of inadequate collection of urine. Another recently discovered problem with estimating CL_cr in HIV patients relates to drug-creatinine interactions. Nucleoside reverse transcriptase inhibitors (dolutegravir, rilpivirina) and cobicistat can block OCT2 and MATE1-mediated tubular secretion of creatinine, leading to elevations in serum creatinine (0.1-0.2 mg/dL[0.9-1.8 μmol/L]) and reductions in CL_cr by up to 5 to 15 mL/min (0.08-0.25 mL/s).^42,150

Kidney function assessment during pregnancy is usually performed using a 24-hour CL_cr determination, and estimation equations have been shown to perform poorly particularly in the preeclampsia population. For example, Alper et al.¹⁵¹ recently evaluated the CG, MDRD4 and CKD-EPI equations in 543 women, aged 16 to 49 years, with preeclampsia after the 20th week of gestation. When compared to 24-hour mCL_cr (mean 133 ± 43 mL/min [2.22 ± 0.72 mL/s]), the CG equation was positively biased (36 ± 2 mL/min [0.60 ± 0.03 mL/s]) whereas both the MDRD4 and CKD-EPI were negatively biased (–20 ± 1.5 mL/min [–0.33 ± 0.025 mL/s]).Thus, kidney function estimating equations should not be used during pregnancy.

Unstable Kidney Function

Patients with unstable kidney function or AKI present a unique situation because S_cr values are changing, and steady state cannot be assumed, which is one of the assumptions of all the above-mentioned eCL_cr methods. It is now widely accepted that a change in the S_cr of more than 50% over a period of 7 days, or an increase in S_cr by at least 0.3 mg/dL (≥27 μmol/L) over a 24- to 48-hour period indicates the presence of AKI.¹⁵² Methods to measure GFR in this population, such as ¹²⁵I-iothalamate clearance, are cumbersome and costly especially in the acute care setting. Although several equations have been proposed the calculate eCL_cr in AKI patients or those with rapidly progressive renal disease,^153,154,155 a rigorous evaluation of the accuracy and precision of each of these proposed methods is lacking and none of them is currently recommended for clinical use. Use of semiquantitative approaches is preferred for the purpose of estimating severity of disease using RIFLE or AKIN criteria (see Chapter 43 for more details).

Use of abbreviated CL_cr measurements (less than 24 h) may be valuable for detecting early evidence of AKI in critically ill patients. Pickering et al.¹⁵⁶ recently reported that 4-hour CL_cr measurements were significantly better at predicting AKI events than S_cr alone in 484 patients. However, the lack of true mGFR in these patients prevented assessment of the accuracy and precision of the CL_cr values. Hoste et al.¹⁵⁷ used 1-hour CL_cr measurements to evaluate the CG and MDRD equations in critically ill patients within 1 week of ICU admission. Both equations were poorly correlated with CL_cr and were similarly imprecise with Bland-Altman 95% confidence intervals ranging from –77 to 64 mL/min (–1.29 to 1.07 mL/s) for CG and –77 to 58 mL/min/1.73 m² (–0.74 to 0.56 mL/s/m²) for MDRD.

It is thus, ultimately, most important to recognize that renal function in patients with AKI is generally markedly lower than one would estimate using steady-state methods, and dose adjustments should be made if necessary to avoid drug toxicity (see Chapters 28 and 33).

Kidney Function in Children

Kidney function in the neonate is difficult to assess because of difficulty in urine and blood collection, the frequent presence of a non–steady-state S_cr, and apparent disparity between development of glomerular and tubular function. Preterm infants demonstrate significantly reduced GFR prior to 34 weeks, which rapidly increases and becomes similar to term infants within the first week of life.¹⁵⁸ Evaluation of GFR in preterm infants on day 3 of life, using an inulin infusion, failed to identify a relationship between patient weight and GFR. Gestational age, which ranged from 23.4 to 36.9 weeks (mean: 30.2 weeks), however, correlated with both GFR and reciprocal of S_cr. The inulin clearance increased from 0.67 to 0.85 mL/min (0.011-0.014 mL/s) in those with gestational age less than 28 weeks versus those of 32 to 37 weeks of age, while S_cr decreased from 1.05 to 0.73 mg/dL (93-65 μmol/L), respectively. Creatinine was measured using a specific enzymatic method to avoid interference from bilirubin or drugs.¹⁵⁹ CL_cr has also been evaluated in infants younger than 1 week, and values of 17.8 mL/min/1.73 m² (0.171 mL/s/m²) on day 1 increased to 36.4 mL/min/1.73 m² (0.351 mL/s/m²) by day 6.¹⁶⁰ In light of these rapid changes in GFR, estimation of GFR is not recommended for infants younger than 1 week. Kidney function expressed as GFR standardized to body surface area (BSA) increases with age and stabilizes at approximately 1 year. In older children, GFR is best assessed using standard measurement techniques for GFR. Subcutaneous administration of ¹²⁵I-iothalamate has been effectively used to measure GFR in children ranging in age from 1 to 20 years.¹⁶¹ The original equation to estimate GFR as described by Schwartz et al.¹⁶² is dependent on the child’s age and length:

where k is defined by age group: infant (1-52 weeks) = 0.45; child (1-13 years) = 0.55; adolescent male = 0.7; and adolescent female = 0.55. Serum creatinine in μmol/L can be converted to mg/dL by multiplication using 0.0113 as the conversion factor.

A newer version of the Schwartz equation¹⁶³ was developed from a population of 349 children (1-19 years) with mild-to-moderate CKD enrolled in the Chronic Kidney Disease in Children (CKiD) study. This simple equation is commonly referred to as the Schwartz “Bedside” formula:

Lee et al.¹⁶⁴ recently reported that this new Bedside Schwartz equation performed better than the original Schwartz equation for patients with mild-moderate CKD, but was less accurate in patients with mild CKD. Thus, the appropriate use of the Bedside Schwartz equation and accuracy in subpopulations of CKD is yet to be fully determined.

Equations derived from adult populations have also been evaluated in pediatric patients. Peirrat et al.¹⁶⁵ compared the MDRD, Schwartz, and CG equations in children 3 to 19 years. In children younger than 12 years, the Schwartz and MDRD equations were significantly more biased than CG, and CG provided the best prediction of GFR in children older than 12 years. The results of these investigations suggest that further studies will be needed to clarify the value of any of these predictive methods in children. Most recently, an equation for GFR based on beta-trace protein was shown to yield similar values of GFR compared to the Schwartz equation in 387 pediatric patients (10.7 ± 7.1 years) who underwent a ^99mTC-DTPA GFR scan.⁷⁴ The most recent GFR equation evaluated in pediatrics includes use of cysC, BUN, S_cr (in mg/dL) and demographic data derived from over 600 pediatric patients enrolled in the CKiD study¹⁶⁶:

This equation had the lowest root-mean square error (0.147), highest R² (0.863) and frequency of values within 30% of iohexol-mGFR (91.3%) when compared to seven other GFR equations.

Kidney Function in the Elderly

Cross-sectional studies have historically shown that GFR declines as a function of age.^167,168 The largest prospective study conducted in healthy elderly individuals is the Baltimore Longitudinal Study on Aging (BLSA).¹⁶⁸ In an initial analysis of 254 BLSA participants without kidney disease, it was reported that mCL_cr decreases at the rate of approximately 0.75 mL/min/1.73 m²/y (0.0072 mL/s/m²/y) beginning at the fourth decade of life. These subjects were then evaluated prospectively for up to 23 years. Interestingly, approximately one-third of the subjects showed no change in renal function from their baseline value, and a small number showed an increased clearance. These changes may be a result of normal physiologic changes or of subclinical insults to the kidneys initiating the events leading to chronic progressive loss of renal function. Fliser et al.¹⁶⁹ studied renal functional reserve in healthy young (23-32 years) and elderly (61-82 years) volunteers using an amino acid infusion technique. mGFR increased by 16% in young and 17% in elderly subjects following the infusion. Renal functional reserve thus appears to be maintained in healthy elderly individuals.

Interpretation of the S_cr alone is difficult in the elderly patient primarily because of the decreased muscle mass and resultant lower production rate of creatinine. Thus, the S_cr often remains within the normal range despite a reduction in the number of functional nephrons. As renal function declines, the kidneys excrete a larger fraction of creatinine. This perpetuates the “normal” S_cr. Recent recommendations such as the adoption of standardized creatinine assays by clinical laboratories and reporting of S_cr values to two decimal places will likely improve the accuracy of renal function estimation in the elderly population.³⁵

The CG formula¹²⁷ continues to provide a valid estimate of the CL_cr in elderly populations. Smythe et al.¹⁷⁰ estimated CL_cr in 23 patients older than 60 years using seven different methods, and compared the results to a measured 24-hour CL_cr determination. Estimations were performed with the actual S_cr and also with the S_cr corrected, or rounded, up to 1.0 mg/dL (88 μmol/L) if the actual value was less than 1.0 mg/dL (less than 88 μmol/L). Changing the S_cr to 1.0 mg/dL (88 μmol/L) resulted in a significantly lower eCL_cr (–28.8 mL/min [–0.481 mL/s]) compared to the unadjusted S_cr (+2.3 mL/min [+0.038 mL/s]). In patients older than 60 years with S_cr less than 1.0 mg/dL (less than 88 μmol/L), rounding the S_cr value up to 1.0 mg/dL (88 μmol/L) resulted in dose estimates for gentamicin that were significantly lower (–90 ± 67 mg/day) than doses calculated based on the actual S_cr value.¹⁷¹ In an analysis of the BLSA dataset, Dowling et al.¹⁷² evaluated 269 elderly individuals: age 81 ± 6 (mean + SD) years, S_cr 1.1 ± 0.4 mg/dL (97 ± 35 μmol/L), and 24-hour mCL_cr of 53 ± 13 mL/min (0.88 ± 0.22 mL/s). The CG equation yielded the least biased estimate of mClcr, whereas the MDRD4 and CKD-EPI equations significantly overestimated the CG and mClcr values by 30% to 47%. Rounding low serum creatinine values up to an arbitrary value of 1.0 mg/dL (88 μmol/L) resulted in CG values that significantly underestimated mClcr (44 vs 56 mL/minute [0.73 vs 0.93 mL/s], p less than 0.001) and uncorrected CG (p less than 0.001). Taken together, these results strongly suggest that the commonly accepted practice of fixing or rounding S_cr to an arbitrary value in elderly patients should be avoided.

An alternative to the estimation of GFR or a 24-hour mCL_cr is a 4-hour mCL_cr performed during water diuresis. This approach correlated with the inulin clearance as well as with an observed inpatient 24-hour mCL_cr.⁹³ However, one must be aware of the potential risk of hyponatremia in the geriatric patient who is unable to tolerate an oral water load, as well as the need for complete bladder emptying to ensure accurate results. O’Connell et al.¹⁷³ assessed the accuracy of 2- and 8-hour urine collections compared with 24-hour CL_cr determinations in 45 hospitalized patients older than 65 years with indwelling urethral catheters. The 8-hour timed urine collection for CL_cr showed minimal bias (2.2 mL/min [0.037 mL/s]) as compared with the 24-hour value, whereas the 2-hour determination was both positively biased (11 mL/min [0.18 mL/s]) and less precise (25 mL/min [0.42 mL/s]).

Impact on Drug Dosing Recommendations

The automated reporting of eGFR in the clinical setting has led some practitioners to consider substituting eGFR in place of eCL_cr for renal dose adjustments as recommended in regulatory agency approved product labelling. The prime concern with this approach, particularly in the elderly, is that substitution of eGFR values in CL_cr-based dosage adjustment algorithms may result in dosing errors and toxicity especially for drugs with narrow therapeutic indices since eGFR tends to overestimate eCL_cr.^{172,174,175,176,177,178,179,180,181}Roberts et al.¹⁷⁴ reported that the MDRD eGFR values overestimated gentamicin clearance by 29% (p less than 0.001), whereas the Cockcroft and Gault yielded only 10% overestimation (p less than 0.01), and MDRD overestimated renal function more as age increased. Retrospective studies in more than 1,200 patients with renal disease have shown that overestimation of renal function using the MDRD4 with or without IDMS eGFR equation results in up to 30% to 60% higher doses for digoxin, amantadine, and various antimicrobials compared to doses calculated using eCL_cr.^{176,177,178,179,180,181} In contrast Stevens et al.¹⁸² reported that use of an eGFR (MDRD4 equation in mL/min based on a calculated BSA), yields dosage regimens for a subset of drugs that are similar to those calculated using mGFR. The authors of this study concluded that the MDRD4 equation could be used for renal dose adjustments. However, this practice was not widely adopted. Modifications to the CKD-EPI equation, including use of cysC, are now being evaluated in drug pharmacokinetic studies, and may be particularly useful for patients with apparently normal renal function. For example, Frazee et al.¹⁸³ retrospectively evaluated vancomycin use in critically ill patients (n = 170) and found that the CKD-EPI_{creatinine-cystatin C} equation (converted to mL/min) yielded a 2.5-fold improvement in achieving target trough values, when compared to eCLcr–. Of note, the predicted trough values were improved among those with normal GFR (greater than 120 mL/min [greater than 2 mL/s]), where under dosing is often problematic. A more detailed discussion of the utilization of kidney function estimates and renal dosing approaches is provided in Chapter 48.

CKD (see Chapter 44) will eventually lead to ESRD, necessitating dialysis (see Chapter 45) or transplantation for survival (see Chapter 90). The rate of progression can be slowed and in some cases halted through dietary modification, strict blood pressure control, initiation of angiotensin-converting enzyme inhibitor or angiotensin receptor blocker therapy to reduce urinary protein excretion, and improved glucose control in patients with diabetes mellitus (see Chapter 44). The efficacy of these interventions is optimally assessed with the sequential measurement of an accurate and sensitive index of mGFR such as iohexol, iothalamate, or radioisotope clearance.¹⁸⁴

Alternatively, use of newer methods to estimate GFR, based on creatinine, cysC, or a combination of both or traditional methods, such as calculation of the linear decline in the reciprocal of the S_cr as a function of time, can be used to evaluate the rate of progression of kidney disease.^1,30

Clinicians can also use the eGFR or eCL_cr plotted as a function of time as a prognostic tool, to predict when dialysis may be needed (eCL_cr less than 15 mL/min [less than 0.25 mL/s]) or as a marker for evaluating the success of therapeutic interventions to alter the rate of decline in kidney function. Several factors, such as changes in dietary intake of creatinine and decreased muscle mass, which are associated with a reduction in the production of creatinine, may alter the utility of these relationships. Low levels of albuminuria serve as an early marker of kidney disease in patients with diabetic nephropathy¹⁸⁵ along with numerous other conditions, such as hypertension and obesity.^186,187,188 Albuminuria is a more sensitive marker than total protein for monitoring CKD progression, and it is a modifiable risk factor for kidney disease progression and cardiovascular disease.^1,64,88 Thus patients with low levels of albuminuria (30-300 mg/day) on at least two occasions or overt albuminuria (greater than 300 mg/day) should begin to receive pharmacotherapy. For children, albuminuria is considered present if albumin excretion exceeds 0.36 mg/kg/day, and overt albuminuria has been defined as an excretion rate that exceeds 4 mg/kg/day. The urinary albumin-to-creatinine ratio is also an accurate predictor of 24-hour albuminuria. Monitoring guidelines suggest that a urine albumin to creatinine ratio of more than 30 mg/g (greater than 3.4 mg/mmol) places the patient at increased risk of developing diabetic nephropathy, mortality, and ESRD and is an indication for the initiation of pharmacotherapeutic intervention.^5,24 Based on the established link between albuminuria and adverse clinical outcomes in more than 1.5 million individuals with varying levels of eGFR, the albumin-to-creatinine ratio is included with eGFR as part of an updated approach for staging CKD recommended by KDIGO.²

Measurement of renal plasma and blood flow is rarely, if ever, determined in the clinical setting; and it is only occasionally used in research settings to evaluate hemodynamic changes related to disease or drug therapy. The kidneys receive approximately 20% of cardiac output and representative values of renal blood flow in men and women of about 1,200 ± 250 and 1,000 ± 180 mL/min/1.73 m² (11.6 ± 2.4 and 9.6 ± 1.7 mL/s/m²) have been reported, respectively.¹⁸⁹ Renal plasma flow (RPF) is estimated to be 60% of blood flow if it is assumed that the average hematocrit (HCT) is 40% and that it can be measured by the use of model compounds that are eliminated from the plasma compartment on a single pass through the kidneys. Because only 20% of the plasma is filtered at the glomerulus, the compound must undergo active tubular secretion and minimal to no reabsorption to be completely eliminated. To accurately reflect RPF, the extraction through the kidney must be nearly 100%. Para-aminohippuric acid (PAH) is an organic anion that has been used extensively for the quantification of RPF. PAH is approximately 17% bound to plasma proteins and is eliminated extensively by active tubular secretion. Because PAH elimination is active, saturation of the transport processes should be anticipated, and concentrations of PAH in plasma should not exceed 10 mg/L. Furthermore, PAH is also metabolized, possibly within the kidney, to N-acetyl-PAH, and it is important for the analytical method to differentiate the parent compound from the metabolite.^86,190 The extraction ratio (ER) for PAH is 70% to 90% at plasma concentrations of 10 to 20 mg/L; hence, the term “effective” renal plasma flow (ERPF) has been used when the clearance of PAH is not corrected for the ER or if it is assumed to be 1. Normal values are about 650 ± 160 mL/min (10.9 ± 2.7 mL/s) for men and 600 ± 150 mL/min (10.0 ± 2.5 mL/s) for women.¹⁸⁹ Children will reach normalized adult values by 3 years, and ERPF will begin to decline as a function of age after 30 years, reaching about one-half of its peak value by 90 years. The method for calculation of ERPF is based on the relationship between organ clearance, ER, and flow:

Effective renal blood flow (ERBF) can be estimated from ERPF by assuming the extraction ratio is 1 and correcting for the red blood cell volume of the blood (HCT):

ERPF can also be measured using the radioisotopes¹³² I-orthoiodohippurate (¹³¹I-OIH) or ^99mTc-mercaptoacetyltriglycine (^99mTc-MAG3).¹⁹¹ One important advantage of this method is its ability to measure ERPF in total or for each kidney independently, as well as its ability to produce renal images. Russell and Dubovsky,¹⁹² using a single-injection technique, compared clearance methods with and without urine collection and showed similar results with each method.

Although GFR is the best overall indicator of renal function, it may not provide an accurate measure of tubular function, either secretory capacity or cellular function, suitable for use in the research environment.¹⁹³ Tubular secretory function can be assessed by measuring PAH transport as the prototype marker of the organic anion secretory system. N¹-methylnicotinamide (NMN) and tetraethylammonium are prototype compounds secreted by the cationic transport system and may be used as markers of cationic secretory capacity. Edwards et al.¹⁹⁴ demonstrated delayed recovery of NMN clearance among patients with psoriasis treated with low-dose cyclosporine, as compared with the recovery of GFR and renal blood flow Dowling et al.¹⁹⁰ explored the usefulness of famotidine as a marker for dose-dependent cationic transport, but was unable to demonstrate saturation at doses up to ten times clinically indicated values, likely due to the contribution from other parallel secretory pathways such as MDR1/P-GP.¹⁹⁵

Furthermore, Karyekar et al.¹⁹⁶ demonstrated that itraconazole, an inhibitor of MDR1, significantly reduced the renal tubular secretion of cimetidine in healthy individuals, suggesting that cimetidine may be used as an in vivo probe of renal P-GP function. It should also be recognized that these transport systems are not necessarily mutually exclusive. Indeed, probenecid, which is secreted by the anionic pathway, inhibits the secretion of cationic compounds. Quantitative measures of tubular transport capacity are currently limited primarily to the research setting.

Other measures of tubular function are less specific and are regarded primarily as indices of damage within the nephron. Low-molecular weight proteins located in the proximal tubule, such as β₂-microglobulin, can be used as urinary biomarkers to detect early tubular toxicity of drugs such as carboplatin, ifosfamide, and etoposide.¹⁹⁷ Other low-molecular-weight proteins used as markers of tubular function include retinol-binding protein (21 kDa), protein HC (also known as α₁-microglobulin, 27 kDa), KIM-1, NGAL, interleukin-18, and fatty-acid binding proteins (FABPs).^67,198,199 These proteins are normally freely filtered at the glomerulus and then completely reabsorbed by the proximal tubule. Increases in their excretion are thus suggestive of tubular dysfunction but are not diagnostic. In each case, the maximal reabsorptive capacity may be exceeded, leading to net excretion of the protein.

Numerous urinary enzymes such as N-acetylglucosaminidase, alanine aminopeptidase, alkaline phosphatase, γ-glutamyltransferase, pyruvate kinase, glutathione transferase, lysozyme, and pancreatic ribonuclease have been used as diagnostic markers for renal disease. Jung et al.²⁰⁰ compared the ability of five enzymes (N-acetylglucosaminidase, alanine aminopeptidase, alkaline phosphatase, γ-glutamyltransferase, and lysozyme) to detect early rejection episodes in kidney transplant patients. Only N-acetylglucosaminidase and alanine aminopeptidase were early predictors of rejection. N-acetylglucosaminidase is an enzyme contained within the lysosome of the tubular cell and is released when the lysosome is damaged, whereas alanine aminopeptidase is an enzyme of the brush border. Both markers were increased approximately 2 days earlier than S_cr in patients with transplant rejection. Recently, biomarkers associated with fibrosis and collagen degradation in the kidney such as matrix metalloproteinases (ProMMP9), have been shown to be associated with acute allograft rejection, interstitial fibrosis and tubular atrophy.²⁰¹

Radiologic Studies

The etiology of kidney disease can be evaluated using several qualitative diagnostic techniques, including radiography, ultrasonography, magnetic resonance imaging, and biopsy. The standard radiograph of the kidneys, ureters, and bladder (the KUB) provides a gross estimate of kidney size and identifies the presence of calcifications.⁹² Although easy to perform, the value of the information is minimal, and more detailed evaluations are often necessary. The IV urogram (formerly known as IV pyelogram) involves the administration of a contrast agent to facilitate visualization of the urinary collecting system. It is primarily used in the assessment of structural changes that may be associated with hematuria, pyuria, or flank pain, resulting from recurrent urinary tract infections, obstruction, or stone formation. For patients with low GFRs, retrograde administration of dye into the ureters may be performed to facilitate visualization of the collecting system. Contrast agents are also employed during renal angiography for the assessment of renovascular disease. The captopril (angiotensin-converting enzyme inhibitor) test is also a useful adjunct. Under conditions of unilateral renal artery stenosis, the affected kidney produces large quantities of angiotensin II, which constricts the efferent arteriole to maintain GFR. The administration of captopril results in reduced uptake of the contrast agent because the efferent arteriole is dilated, thereby decreasing the perfusion pressure of the affected kidney. For patients with bilateral disease, a decrease in uptake is observed in both kidneys.²⁰² Computed tomography is a cross-sectional anatomic imaging procedure. The procedure is frequently performed with contrast to enhance imaging. Spiral, or helical, computed tomography, a more recent technique, provides for three-dimensional visualization of tissues. Computed tomography is performed as a test for the evaluation of obstructive uropathy, malignancy, and infections of the kidney.

Renal Ultrasonography

Ultrasonography uses sound waves to generate a two-dimensional image. The echogenicity of the kidney is compared with that of an adjacent organ—liver on the right and spleen on the left—with an increased echogenicity indicating an abnormal finding. Ultrasonography can distinguish the renal pyramids, medulla, and cortex, and abnormalities in structure, such as occurs with obstruction. Renal ultrasonography is also used as a guide for site localization during percutaneous kidney biopsy.

Magnetic Resonance Imaging

Magnetic resonance imaging is based on aligning hydrogen nuclei in the body with the use of a powerful magnet and applying radiofrequency pulses. The signals emitted by the hydrogen nuclei during realignment on repeated pulses allows for generation of the tissue image. Realignment times can also be altered with the use of contrast agents (gadolinium, gadopentetate), leading to increased signal intensity and improved imaging. Magnetic resonance imaging is useful for the assessment of obstruction, malignancy, and renovascular lesions. The relative advantages and limitations of these procedures are discussed in more detail in recent reviews.^92,203

Biopsy

Renal biopsy is used in several conditions to facilitate diagnosis when clinical, laboratory, and imaging findings prove inconclusive. Proteinuria and hematuria are both associated with renal parenchymal disease. When less-invasive studies are unsuccessful in differentiating the cause and the possible causes have different therapeutic approaches, biopsy may be indicated. Functional status of the kidney is not assessed with biopsy, and severity of disease and progression is best measured using quantitative tests discussed above. Contraindications to renal biopsy include a solitary kidney, severe hypertension, bleeding disorder, severe anemia, cystic kidney, and hydronephrosis, among others. Complications resulting from biopsy primarily include hematuria, which may last for several days, and perirenal hematoma.²⁴

Comprehensive approaches to evaluate renal function in CKD patients include the CG equation for estimating CL_cr and drug dosing, the creatinine plus cysC CKD-EPI equation for estimation of GFR and staging of CKD, and determination of albumin-to-creatinine ratio as a marker of the integrity of the glomerular basement membrane. Measurement of GFR using exogenous administration of iothalamate, iohexol, or radioisotope techniques such as ^99mTc-DPTA is increasingly being employed to assess progression of disease or acceptability of an individual to be a kidney donor. Other qualitative assessments of kidney function, such as radiography, computed tomography, magnetic resonance imaging, sonography, and biopsy, are most useful to identify the underlying cause of kidney disease.

1. The glomerulus is primarily responsible for ______ of unbound drug in the kidney:

 A. Filtration

 B. Reabsorption

 C. Secretion

 D. Endocytosis

2. Active drug secretion occurs most often in which of the following nephron segments:

 A. Glomerulus

 B. Proximal tubule

 C. Loop of Henle

 D. Distal tubule

3. Which of the following is/are involved in drug efflux at the basolateral membrane of the proximal tubule:

 A. MRP1

 B. ENT1

 C. OAT4

 D. Both A and B

4. According to the intact nephron hypothesis, reabsorption ______ and single nephron glomerular filtration rate (GFR) ______ in the surviving nephrons:

 A. Increases, increases

 B. Decreases, decreases

 C. Increases, decreases

 D. Decreases, increases

5. The kidney is responsible for synthesizing each of the following hormones, EXCEPT:

 A. Erythropoietin

 B. Prostaglandin

 C. Parathyroid hormone

 D. Renin

6. The decreased serum creatinine values observed during dobutamine therapy are likely due to:

 A. Analytical interference

 B. Increased tubular secretion of creatinine

 C. Increased GFR caused by dobutamine

 D. Increased muscle breakdown

7. Which of the following renal function indices is least influenced by changes in fluid or volume status:

 A. Serum creatinine

 B. Blood urea nitrogen

 C. Urine specific gravity

 D. Urine sodium

8. Which of the following renal function indices is least affected by dietary protein intake:

 A. Serum creatinine

 B. Blood urea nitrogen

 C. Creatinine clearance

 D. Urine sodium

9. The most appropriate method for initial testing of proteinuria in a patient with chronic kidney disease (CKD) risk factors is:

 A. Urine protein:albumin ratio

 B. Urine albumin:creatinine ratio

 C. 24-hour urine protein excretion

 D. Urine dipstick for total protein

10. Each of the following provides an accurate measure of GFR, except:

 A. Iohexol clearance

 B. Iothalamate clearance

 C. Inulin clearance

 D. Probenecid clearance

11. Which of the following equations is most appropriate for estimating a patient's GFR for the purpose of determining their CKD category/stage?

 A. 6-variable MDRD

 B. CKD-EPI equation

 C. CKD_cysC equation

 D. BIS equation

12. The authoritative source that provides the FDA-approved recommendations for drug dosage recommendations in renal impairment is:

 A. ePocrates

 B. Drugdex/Micromedex

 C. AHFS Drug Information

 D. Approved Product labelling (package insert)

13. When using the Cockcroft-Gault (CG) equation to estimate creatinine clearance in obese patients, it is recommended that lean body weight be used in patients with:

 A. BMI ≥ 40 kg/m²

 B. BMI 30 to 39 kg/m²

 C. BMI 25 to 29 kg/m²

 D. None of the above

14. J.S. is a 70-year-old African American male (5′8″ [173 cm], 85 kg) with a history of hypertension and CKD. His serum creatinine today is 1.50 mg/dL (133 μmol/L) (using the IDMS calibrated assay). What is his estimated creatinine clearance?

 A. 55.0 mL/min (0.92 mL/s)

 B. 43.5 mL/min (0.72 mL/s)

 C. 37.2 mL/min (0.62 mL/s)

 D. 29.4 mL/min (0.49 mL/s)

15. What is J.S.'s estimated GFR (in mL/min/1.73 m²)?

 A. 56.0 mL/min/1.73 m²

 B. 49.4 mL/min/1.73 m²

 C. 35.0 mL/min/1.73 m²

 D. 30.2 mL/min/1.73 m²

16. What is J.S.'s estimated GFR when expressed in mL/min?

 A. 38.9 mL/min

 B. 45.6 mL/min

 C. 65.2 mL/min

 D. 72.1 mL/min

17. J.R. is a 68-year-old Caucasian man (60 kg, 5′7″ [170 cm]) with a history of hypertension, cerebral stroke, and benign prostatic hypertrophy. He presents to the ambulatory care clinic today for evaluation of a viral infection to be treated with acyclovir. His serum creatinine value today is 0.63 mg/dL (56 μmol/L). Which one of the following approaches should be used to assess this patient's renal function for the purpose of renal dose adjustment for acyclovir?

 A. Measure a chromium-labeled ethylenediaminetetraacetic acid GFR

 B. Estimate creatinine clearance using the CG equation

 C. Estimate GFR using the MDRD equation

 D. Conduct a timed 24-hour urine collection

18. An appropriate clinical monitoring plan to evaluate renal protective therapy in patients with CKD should include each of the following items EXCEPT:

 A. Estimated creatinine clearance

 B. Urinary albumin:creatinine

 C. Urinary cystatin C concentration

 D. Estimated GFR

19. The elevation in serum creatinine observed during cobicistat therapy is most likely attributed to:

 A. Tubular apoptosis

 B. Acute interstitial injury

 C. Afferent arteriole vasoconstriction

 D. Inhibition of OCT2 and MAT1- mediated tubular secretion of creatinine

20. In the clinical setting, the renal clearance of PAH is considered an index of _______.

 A. Fractional excretion of sodium

 B. Renal plasma or blood flow

 C. Glomerular filtration rate

 D. Renal tubular reabsorption

1. A

2. B

3. D

4. D

5. C

6. A

7. A

8. D

9. B

10. D

11. B

12. D

13. A

14. A

15. D

16. A

17. B

18. C

19. D

20. B