Preanesthetic Evaluation : False-Positive Tests

Definition

A test is useful only to the extent that clinicians (and patients) can understand the implications of a positive or negative result. Few, if any, tests always correctly identify the presence or absence of disease in all patients. Clinicians can decide what to do with a “positive” or “negative” test result only when they have a clear knowledge of the test’s characteristics and its statistical predictive value when applied to a specific patient population. Such “medical decision analysis” directly affects the clinical care of patients.

Some tests provide a qualitative binary positive or negative result. Most test results, however, are quantitative and define a range of “normal” values around a central mean. Therefore members of any population will have an “abnormal” test result but do not actually have a disease. This means that a test result may be misleading owing to natural variability in the general population.

The test can give an incorrect result due to (1) inaccuracy, (2) imprecision, or (3) incorrect performance. The accuracy of a test is the difference between the mean value of test results and the true result, as measured by a gold-standard test. The precision of a test is the reproducibility of results between instruments or persons performing the test. An incorrectly performed test may completely invalidate any result.