Which metric measures how well a test identifies those without disease?

• A. Sensitivity
• B. Specificity
• C. PPV
• D. NPV

Answer: (B)

Specificity measures the ability of a test to correctly identify people who do not have the disease. It is the proportion of true negatives among all individuals without the disease: specificity = true negatives / (true negatives + false positives). A test with high specificity yields few false positives, so a negative result in someone without disease is reliable for ruling out the condition. By contrast, sensitivity assesses how well the test detects those who do have the disease (true positives). PPV and NPV describe the probability that a given positive or negative result reflects the real disease status and depend on how common the disease is in the population.