If disease prevalence increases, which of the following is true about PPV and NPV?

• A. PPV increases and NPV decreases.
• B. Both increase.
• C. Both decrease.
• D. PPV decreases and NPV increases.

Answer: (A)

When disease prevalence rises, the probability that a positive test result is a true disease (PPV) increases, while the probability that a negative test result is truly disease-free (NPV) decreases. This happens because PPV depends on how common the disease is: as more people actually have the disease, a positive result is more likely to be a true positive rather than a false positive. Conversely, NPV depends on how many people do not have the disease: as prevalence goes up, there are more people with disease who could be missed by a negative result, making negatives less trustworthy and reducing NPV.

A quick intuition: with low prevalence, most positives could be false positives, so PPV is low, and negatives are very reassuring, so NPV is high. As prevalence increases, PPV climbs because positives are more likely real, and NPV falls because negatives are more likely to miss disease in a population where the disease is common.