A urinary tract infection (UTI) is defined as microbial infiltration that cannot be accounted for by contamination in an otherwise sterile urinary tract. UTIs encompass infections of the lower urinary tract, upper urinary tract, and kidneys with risk factors consisting of age, sex, pregnancy, diabetes, urinary catheter, and vaginal intercourse. Lower tract UTIs (cystitis) start with periurethral contamination by uropathogens residing in the gut. Upper tract UTIs (pyelonephritis) develop when uropathogens ascend to the kidneys. UTIs are further classified as uncomplicated or complicated. Uncomplicated UTIs are infections occurring in women of childbearing age with structurally and neurologically normal urinary tracts. Complicated UTIs occur in patients with functional or structural abnormalities. Additionally, UTIs in men, pregnant women, children, and patients in health care-associated settings are considered complicated.
Common indicators of cystitis include urinary urgency and frequency, painful voiding, and suprapubic tenderness. Pyelonephritis presents similarly and may also be accompanied by fever, flank pain, nausea, and vomiting. However, certain populations may not present with these typical symptoms. Elderly patients may present with altered mental status, eating habit changes, or gastrointestinal symptoms. Patients with indwelling catheters or neurologic disorders will often lack lower tract symptoms and may present with flank pain, fever, altered mental status, acute hematuria, or pelvic discomfort.
A UTI diagnosis requires symptoms as well as a positive urinalysis and/or urine culture. A urinalysis will identify pyuria, defined as 10^3 or more leukocytes/mm3 in urine, which is a nonspecific marker for UTI. Additionally, the reduction of nitrate to nitrite on urinalysis indicates the presence of certain bacteria (eg Escherichia coli, Klebsiella spp., Enterobacter spp, Proteus spp., Staphylococcus spp., and Pseudomonas spp.). Obtaining a urine culture is not recommended for managing acute uncomplicated cystitis; however, this practice is recommended in complicated UTIs and acute pyelonephritis. Using fresh, midstream urine, clinical confirmation of an UTI by urine culture is defined as 105 or more colony-forming units (CFU)/mL of urine in patients with symptoms.
Asymptomatic bacteriuria (ASB) is defined as patients without indwelling catheters or symptoms who have 1 or more bacterial species grow 105 or more CFU/mL in an appropriately collected urine culture. ASB is a common finding in many women or men with genitourinary tract abnormalities that impair voiding and should not be treated. Populations who should be treated include pregnant women and individuals undergoing invasive genitourinary tract procedures. Additionally, patients with catheters are often bacteriuric with two to five organisms. If asymptomatic, patients also do not to be treated.
Any organism colonizing the urinary tract can cause a UTI, but most are caused by bacteria. The majority of uncomplicated UTIs are caused by gram-negative bacteria, with E. coli being isolated in 70% to 95% of cultures. The most common gram-positive bacteria isolated are S. saprophyticus and enterococci (Table 23-1).
TABLE 23-1Bacterial Causes of UTIs