Nonmicrobiologic Laboratory Studies

Common nonmicrobiological laboratory tests include the white blood cell count (WBC) and differential, erythrocyte sedimentation rate (ESR), and determination of the C-reactive protein level. In most cases, the WBC count is elevated in response to infection, but it may be decreased owing to overwhelming or long-standing infection. The differential is the percentage of each type of WBC (Table 69-1). In response to infection, neutrophils leave the bloodstream and enter the tissue to "fight" against the offending pathogens (i.e., leukocytosis). During an infection, immature neutrophils (e.g., bands) are released at an increased rate to help fight infection, leading to what is known as a bandemia or left shift. Therefore, a WBC count differential is key to determining if an infection is present. It is important to note that some patients may present with a normal total WBC with a left shift (e.g., the elderly). ESR and C-re-active protein (CRP) are nonspecific markers of inflammation. They increase as a result of the acute-phase reactant response, which is a response to inflammatory stimuli such as infection or tissue injury. These tests may be used as markers of infectious disease response because they are elevated when the disease is acutely active and usually fall in response to successful treatment. Clinicians may use these tests to monitor a patient's response to therapy in osteomyelitis and infective endocarditis. These tests should not be used to diagnose infection because they may be elevated in nonin-fectious inflammatory conditions (e.g., rheumatoid arthritis, polymyalgia rheumatica, and temporal arteritis).

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