Modern medicine relies extensively on the clinical laboratory as a key component of health care. It is estimated that, in current practice, at least 60–70% of all clinical decisions rely to some extent on a laboratory result. For many diseases, the clinical laboratory provides essential diagnostic information. As an example, histopathologic analysis provides basic information about histologic type and classification of tumors and their degree of invasion into adjacent tissues. Microbiologic testing is required to identify infectious organisms and determine antibiotic susceptibility. For many common diseases, expert groups have produced standard guidelines for diagnosis that rely on defined clinical laboratory values, e.g., blood glucose or hemoglobin A1C levels form the basis for diagnosis of diabetes mellitus; the presence of specific serum antibodies is required for diagnosis of many rheumatologic diseases; and serum levels of cardiac markers are a mainstay in diagnosis of acute coronary syndromes.
With their ever-increasing number and scope, clinical laboratory tests provide the clinician with a powerful set of tools but pose challenges in terms of appropriate selection and judicious, cost-effective use to deliver effective patient care.
One of the most frequent reasons for performing clinical laboratory tests is to support, confirm, or refute a diagnosis of disease that is suspected on the basis of other information sources, such as history, physical examination findings, and imaging studies. The following questions need to be considered: Which clinical laboratory tests may be of value in supporting, confirming, or excluding the clinical impression? What is the most efficient test-ordering strategy? Will a positive test result confirm the clinical impression or even definitively establish the diagnosis? Will a negative result disprove the clinical suspicion, and, if so, what further testing or alternative approach will be needed? What are the known sources of false-positive and false-negative results, and how are these misleading results recognized?
Another reason for ordering clinical laboratory tests is to screen for disease in asymptomatic individuals (Chap. 4). Perhaps the most common examples of this application are the newborn screening programs now routinely used in most developed countries. Their purpose is to identify newborns with treatable conditions for which early intervention—even before clinical symptoms develop—is known to be beneficial. In adults, screening tests for diabetes mellitus, renal disease, prostate cancer (measurement of serum prostate-specific antigen [PSA] levels), and colorectal cancer (testing for occult blood in stool), for example, are widely applied to apparently healthy individuals on the grounds that early diagnosis and intervention lead to improved long-term outcomes.