Which statement about PPV and NPV is true?

Which statement about PPV and NPV is true?

• A. They are solely determined by test sensitivity and specificity.
• B. They are fixed properties of a test regardless of prevalence.
• C. They are interpreted without considering pretest probability.
• D. They depend on disease prevalence and test characteristics.

Answer: (D)

PPV and NPV are shaped by both how accurate the test is (sensitivity and specificity) and how common the disease is in the population (prevalence). They aren’t fixed properties of a test. If disease prevalence is high, a positive result is more likely to be a true positive, so PPV increases, while NPV tends to decrease. If prevalence is low, PPV falls and NPV rises. This is why the best description is that PPV and NPV depend on disease prevalence and test characteristics. The other ideas—treating PPV/NPV as fixed constants or ignoring pretest probability—don’t fit reality, because the real-world usefulness of a test hinges on both its inherent accuracy and how common the disease is where it’s used.

PPV and NPV are shaped by both how accurate the test is (sensitivity and specificity) and how common the disease is in the population (prevalence). They aren’t fixed properties of a test. If disease prevalence is high, a positive result is more likely to be a true positive, so PPV increases, while NPV tends to decrease. If prevalence is low, PPV falls and NPV rises. This is why the best description is that PPV and NPV depend on disease prevalence and test characteristics. The other ideas—treating PPV/NPV as fixed constants or ignoring pretest probability—don’t fit reality, because the real-world usefulness of a test hinges on both its inherent accuracy and how common the disease is where it’s used.