Chapter e53

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.

The ever-increasing number and scope of clinical laboratory tests provides the clinician with a powerful set of tools but poses the challenge of appropriate selection of clinical laboratory tests in the most judicious and cost-effective way to deliver effective patient care.

### Diagnosis of Disease

One of the most frequent reasons for performing clinical laboratory tests is to support, confirm, or refute a diagnosis of disease that is suspected based on other sources such as the patient's history, physical examination findings, and imaging studies. The questions that need to be considered are which clinical laboratory tests could be of value in supporting, confirming, or excluding the clinical impression? What is the most efficient test-ordering strategy? If a test result is positive, will that confirm the clinical impression or even formally establish the diagnosis? If negative, does that disprove the clinical suspicion, and what further testing or approach is needed? What are the known sources of false-positive and false-negative results, and how does one recognize these?

### Screening for Disease

Another reason for ordering clinical laboratory tests is screening for disease in asymptomatic individuals (Chap. 4). Perhaps the most common examples of this are the newborn screening programs now being 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. Screening of adults for the presence of diabetes mellitus, renal disease, prostate cancer [by testing serum prostate-specific antigen (PSA) levels], and colorectal cancer (by testing for occult blood in stool) are examples of widely used clinical laboratory screening procedures that are applied to apparently healthy individuals on the basis that early diagnosis and intervention in patients with these diseases leads to improved long-term outcomes.

#### Differences between Screening Tests and Confirmatory Tests

It is important to distinguish between clinical laboratory tests that can be used for screening for disease and those that offer a confirmatory result. Screening tests are generally less expensive ...

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