Define negative predictive value (NPV) and its interpretation.

• A. NPV is the probability that a positive test indicates disease.
• B. NPV is the probability that a test is positive when the person is diseased.
• C. NPV is the probability that a negative test result indicates absence of disease.
• D. NPV is the prevalence of disease.

Answer: (C)

A negative predictive value tells you how confident you can be that a person who tests negative truly does not have the disease. It is the proportion of true negatives among all negative test results (true negatives divided by the sum of true negatives and false negatives).

In interpretation terms, a high NPV means a negative result is reliable for ruling out disease in that setting. NPV depends on disease prevalence as well as test accuracy: when disease is rare, most negatives are true negatives and NPV is high; when disease is common, more false negatives can occur and NPV decreases.

For example, if 100 people test negative and 95 truly do not have the disease, the NPV is 95%, meaning a negative result correctly indicates absence of disease 95% of the time.