What happens to PPV when disease prevalence increases?

What happens to PPV when disease prevalence increases?

• A. PPV increases
• B. PPV decreases
• C. No change
• D. It depends on specificity

Answer: (A)

PPV is the probability that a positive test result really means you have the disease. When disease prevalence in the population rises, the pool of true disease cases among those tested grows, while the impact of false positives (from imperfect specificity) doesn’t rise as quickly. With the same test accuracy (sensitivity and specificity fixed), increasing prevalence makes a larger share of positives true positives, so PPV goes up. In very low-prevalence settings, many positives are false positives, lowering PPV; as prevalence increases, more positives reflect real disease, raising PPV. So PPV increases with higher prevalence.

PPV is the probability that a positive test result really means you have the disease. When disease prevalence in the population rises, the pool of true disease cases among those tested grows, while the impact of false positives (from imperfect specificity) doesn’t rise as quickly. With the same test accuracy (sensitivity and specificity fixed), increasing prevalence makes a larger share of positives true positives, so PPV goes up. In very low-prevalence settings, many positives are false positives, lowering PPV; as prevalence increases, more positives reflect real disease, raising PPV. So PPV increases with higher prevalence.