Specificity is defined as:

• A. Measures the probability that someone with disease tests positive.
• B. Measures the probability that someone without disease tests negative.
• C. Measures the probability that a test is positive in the diseased population.
• D. Measures the rate of false positives.

Answer: (B)

Specificity is the true negative rate: it measures how well a test correctly identifies people who do not have the disease. It answers the question, among those who are disease-free, what fraction tests negative? Mathematically, it’s true negatives divided by the sum of true negatives and false positives. A high specificity means few false positives, so a positive result is strong evidence of disease presence (useful for ruling in disease). The other statements describe sensitivity (probability a diseased person tests positive), the test’s positive rate in the diseased group, or the false positive rate, which is 1 minus specificity.