Screening Versus Diagnostic Tests
Examples of routine population screening currently used in the health care field include Pap smears for women to predict their risk for cervical cancer, mammograms for women to predict their risk for breast cancer, the PPD skin test to predict exposure to tuberculosis mycobacterium (TB) in health care workers, and the prostatic antigen screening (PSA) test for men to predict their risk for prostate cancer.
However, screening tests have limits. A screening test only indicates who, in a given population, is most likely to be at higher risk for developing a disease. As a result, screening tests will have both false positives and false negatives. A false positive occurs when a test misidentifies individuals as being higher risk, when they are actually not at higher risk. A false negative occurs when individuals with a higher risk for the disorder are not identified by the screening test. For this reason, a diagnostic test is done following a positive screening test. A diagnostic test determines with relative certainty whether an individual has a disorder and thus rules out false positives. Diagnostic tests are typically more expensive and/or more invasive and therefore cannot be used as part of the screening process.