Define PPV and NPV in the context of a clinical test.

• A. PPV is the probability that a person with a positive test truly has the disorder; NPV is the probability that a person with a negative test truly does not have the disorder
• B. PPV measures sensitivity; NPV measures specificity
• C. PPV equals NPV
• D. NPV is unrelated to diagnosis

Answer: (A)

PPV and NPV are about how a test result translates into real disease status for an individual. Positive predictive value is the probability that a person actually has the disorder given a positive test result (P(Disease | Test+)). Negative predictive value is the probability that a person does not have the disorder given a negative test result (P(No Disease | Test−)). These reflect how reliable a positive or negative result is in a specific patient, not just how the test performs in general.

Keep in mind that PPV and NPV depend on how common the disorder is in the population being tested. When the disorder is common, PPV tends to be higher and NPV lower; when the disorder is rare, PPV tends to be lower and NPV higher. This is why sensitivity and specificity describe the test itself, while PPV and NPV describe the practical meaning of a test result for a given patient.