What does specificity measure in diagnostic testing?

• A. The ability of a test to correctly identify individuals who do NOT have the disease.
• B. The ability of a test to correctly identify individuals who DO have the disease.
• C. The proportion of positive test results that are true positives.
• D. The rate of false negatives among diseased individuals.

Answer: (A)

Specificity measures how well a diagnostic test identifies people who do not have the disease. It is the proportion of true negatives among those without the disease, calculated as true negatives divided by the sum of true negatives plus false positives. In practical terms, a highly specific test yields few false positives, so a negative result for someone without the disease is reliable, and a positive result is more trustworthy for confirming disease when combined with other information. This is different from sensitivity, which is about correctly identifying those who do have the disease, and from the proportion of positive results that are true positives (which is the positive predictive value and depends on prevalence). For example, if many people tested do not have the disease and the test has high specificity, most of those non-diseased individuals will test negative, with only a small number of false positives.