Many tests can be conducted to determine whether a patient has an infection. Often, no single test can prove that a patient is infected, but when used in combination with other tests and clinical findings, the clinician can reliably make a definitive diagnosis of infection. Because many tests are nonspecific, there are factors other than infection that can cause a test to be reported as positive when no infection exists. Therefore, the importance of careful interpretation and sound clinical judgment cannot be overemphasized.
White Blood Cell Count and Differential
Understanding the role of the white blood cell (WBC) in fighting infection is important in the diagnosis of infection, the selection of drug therapy, and the monitoring of patient progress. The major role of the WBC is to defend the body against invading organisms such as bacteria, viruses, and fungi. The typical normal range of the WBC is 4,500 to 11,000 cells/mm3 (4.5 × 109 to 11 × 109/L).2 This range will vary between laboratories and patients, as it is dependent on patient age, gender, comorbidity status (WBC, especially neutrophils, increase naturally during pregnancy). WBCs usually are elevated in response to infection, but many other noninfectious conditions can increase the WBC, including stress, inflammatory conditions such as rheumatoid arthritis, and leukemia or in response to certain drugs (e.g., corticosteroids).
WBCs are divided into two groups: the granulocytes, which have prominent cytoplasmic granules, and the agranulocytes, which lack granules. Polymorphonuclear (PMN) granulocytes are made up of neutrophils, basophils, and eosinophils. The two other classes of WBCs are the monocytes and lymphocytes. Neutrophils are the most common type of WBCs in the blood, comprising approximately 70% of the total WBC count. In response to infection, they leave the bloodstream and enter the tissue to interact with and phagocytize offending pathogens. Mature neutrophils sometimes are referred to as segs because of their segmented nucleus, which usually consists of two to five lobes. Immature neutrophils lack this segmented feature and are referred to as bands. During an acute infection, immature neutrophils, such as bands (single-lobed nucleus), are released from the bone marrow into the bloodstream at an increased rate, and the percentage of bands (usually 5%) can increase in relationship to mature cells. The change in the ratio of mature to immature cells is often referred to as a “shift to the left” because of the way the cells were counted by hand with a microscope and charted from immature to mature cells.
Leukocytosis, an increase in WBCs, is a normal host response to infection. Unfortunately, bacterial infection is a common complication of neutropenia from cancer chemotherapy. Neutropenia occurs when the bone marrow does not produce enough WBCs to fight infection. Patients who are neutropenic are incapable of increasing their WBCs in response to infection. In fact, susceptibility to infection in these patients is highly dependent on their WBC status. Patients with absolute neutrophil counts of less than 500 cells/mm3 (0.5 × 109/L) are at high risk for the development of bacterial or fungal infections. The absence of leukocytosis also frequently can occur in the elderly and in severe cases of sepsis.2,3
Lymphocytes comprise 15% to 40% of all WBCs and are of central importance to the immune system. Two functional types of lymphocytes are the T cell, which is involved in cell-mediated immunity, and the B cell, which produces antibodies involved in humoral immunity. Lymphocytosis is frequently associated with acute viral infections such as Epstein–Barr virus infection (mononucleosis) and Cytomegalovirus (CMV) infection and rarely with unusual bacterial infections (i.e., Brucella species infections).
T lymphocytes are characterized on the basis of function (i.e., T-helper cells, TH1 and TH2) and on the basis of surface protein. Most type 1 and type 2 T cells carry a T4 (CD4) marker that recognizes class II major histocompatibility complex (MHC) antigens, and most cytoxic T cells carry a T8 (CD8) marker that recognizes class I MHC antigens. A severe deficiency of CD4 cells is associated with human immunodeficiency virus (HIV) infection and opportunistic infections.4 Malignancies also can adversely affect cellular immunity. Patients with Hodgkin’s disease and other types of lymphoma exhibit defective cell-mediated immunity that predisposes them to a variety of infections, notably fungal diseases and infections by the Listeria species. Drug treatment with cytotoxic chemotherapy and corticosteroids also can have profound deleterious effects on cell-mediated immunity.5 Defects in cell-mediated immune function can be demonstrated by a variety of simple laboratory tests, including quantification of lymphocytes on a routine complete blood cell count and skin testing for anergy. A more detailed investigation includes quantitative measurements of CD4+ and CD8+ cells. Monocytosis is correlated less frequently with acute bacterial infection, although its presence has been associated with the response of certain infections (e.g., tuberculosis) to chemotherapy.6 Eosinophilia can result from parasitic infection. eFigure 25-1 describes a number of cell types and their biologic function.
Various cell types and their biologic functions.
Some nonspecific laboratory tests are useful to support the diagnosis of infection. The inflammatory process initiated by an infection sets up a complex host response that includes. Activation of the nuclear factor-κB (NF-κB) transcription factors plays an important role in the regulation of the immune system. NF-κB is activated by bacterial and viral antigens, which eventually leads to the production of proinflammatory cytokines and chemokines. The rapid detection of activated NF-κB can be measured by transcription factor enzyme-linked immunoassay (TF-ELISA) during a systemic inflammatory response syndrome (SIRS) and is considered to be crucial for the treatment of patients with septicemia. Acute-phase reactants, such as the erythrocyte sedimentation rate (ESR) and the C-reactive protein concentration, are elevated in the presence of an inflammatory process but do not confirm the presence of infection because they are often elevated in noninfectious conditions, such as collagen-vascular diseases and arthritis. However, large elevations in ESR are associated with infections such as endocarditis, osteomyelitis, and intraabdominal infections.7,8
Procalcitonin (PCT) is another acute-phase reactant that is released in response to various cytokines. PCT appears to be a more specific marker for bacterial infections than either C-reactive protein (CRP) or ESR. Controlled clinical trials have show that it can be a valuable tool for the clinician to help assess mortality risks of patients with infections and also can help to determine when to initiate antibacterial therapy in respiratory tract infections.9
Changes in endothelial membranes and the presence of a foreign pathogen and its endotoxins cause inflammatory cytokines, such as interleukin 1 (IL-1), IL-6, and IL-8 and tumor necrosis factor-α (TNF-α), to be produced by macrophages or lymphocytes. Fluctuations in cytokine levels occur during the course of an infection, which can be useful in staging and monitoring the response to therapy. Although abnormally high levels of TNF have been associated with a variety of noninfectious causes, spiked elevations in TNF are found in patients with serious infections, such as sepsis. Studies of the relationship of circulating mediators to patient outcome have determined the value of endotoxin and cytokine measurements in patients with sepsis. Although the combination of elevations in endotoxin and individual cytokines has correlated well with the mortality rate, measurement of IL-6 was by far the best individual cytokine that predicted patient outcome.7 Understanding the balance between these proinflammatory and antiinflammatory processes likely will lead to interventions that can have a direct impact on the outcome of patients with sepsis.10