What does specificity measure in a screening test?

• A. Probability the test will be negative in individuals who do not have the disease.
• B. Probability the test will be positive in individuals who have the disease.
• C. Proportion of true positives among all positive results.
• D. Probability that the disease is present given a negative result.

Answer: (A)

Specificity measures how well a screening test correctly identifies people who do not have the disease. It is the probability that the test result will be negative among those who are disease-free. In other words, it reflects the true negatives divided by all people without the disease, so a highly specific test yields few false positives.

For example, if a test has high specificity, most individuals without the disease will test negative. This is what you want when you need to rule out disease in healthy people.

The other concepts refer to different relationships: sensitivity is the true positive rate—the probability the test is positive given the person has the disease. The proportion of true positives among all positive results is the positive predictive value. The probability that the disease is present given a negative result is the negative predictive value.