How does disease prevalence affect PPV and NPV?

• A. Prevalence does not affect PPV or NPV.
• B. Only PPV changes with prevalence.
• C. Only NPV changes with prevalence.
• D. Both PPV and NPV change with disease prevalence.

Answer: (D)

Predictive values depend on how common the disease is in the population. A test’s sensitivity and specificity are inherent to the test itself and don’t change with prevalence, but how likely a positive or negative result is to reflect reality does change as the disease becomes more or less common.

Positive predictive value is the chance that someone truly has the disease given that their test result is positive. When the disease is more prevalent, a positive result is more often a true case, so the PPV rises. When the disease is rare, many positives come from people without disease, lowering the PPV.

Negative predictive value is the chance that someone truly does not have the disease given that their test result is negative. As prevalence increases, there are more people with the disease who could be missed by the test, so the NPV drops. When the disease is rare, a negative result is highly reassuring, so the NPV is high.

So, as disease prevalence changes, both PPV and NPV change, even though sensitivity and specificity stay the same.