High specificity followed by a positive test is most useful for which purpose?

• A. Rule out the disease.
• B. Rule in the disease.
• C. Screen the general population.
• D. Estimate disease prevalence.

Answer: (B)

High specificity means the test produces few false positives. When the result is positive on a test with high specificity, you can be quite confident that the person actually has the disease. That makes it best suited for confirming a diagnosis—i.e., ruling in the disease—especially after an initial screening has flagged someone as potentially affected.

In practice, a sensitive test is used first to catch as many true cases as possible, and a highly specific, positive confirmatory test is then used to verify the disease and avoid false positives. This approach isn’t aimed at screening the general population (which relies on sensitivity to avoid missing cases) or at estimating disease prevalence (which uses population data rather than a single diagnostic test).