Understanding diagnostic tests
The reliability of a diagnostic test can be described by a number of measures. It is important that you have an understanding of these measures and what they mean to you as a clinician.
What is a gold standard for diagnostic test?
The gold standard is the best single test or combination of tests that is relevant to the particular diagnosis. Thus, when thinking of exercise tolerance tests as a diagnostic test for coronary artery disease, the gold standard would be angiogram or autopsy. The gold standard is different for different tests, and may be impossible to obtain except under unusual situations (autopsy, for example).
What do the terms sensitivity, specificity, positive predictive value, and negative predictive value mean, and how do they apply to a clinical situation?
Definitions:
Sensitivity is the likelihood that the diagnostic test will indicate the presence of disease when the disease is actually present.
Specificity is the likelihood that the diagnostic disease will indicate the absence of disease when the disease is actually absent.
Positive predictive value is the likelihood that a positive test result actually means that the disease is present.
Negative predictive value is the likelihood that a negative test result actually means that the disease is absent.
Statistics for a test are usually given in percentages. For example, a 70% sensitivity for a particular culture would mean that, on average, the test would read positive for 70% of women who truly have infections.
It is important to understand the clinical meaning of these concepts. Suppose you decide to perform a test for Chlamydia on an asymptomatic female patient. In this setting,
- sensitivity will be the chance that if a woman had a chlamydial infection it would be picked up by the test.
- specificity will be the chance that the test will indicate no infection if, in fact, the woman has no infection.
- positive predictive value is the chance that the positive result that comes back actually represents chlamydial infection.
- negative predictive value is the chance that the negative result that comes back actually represents the absence of a chlamydial infection.
