When the disease prevalence in a population increases, what happens to the positive predictive value, and why?

• A. It decreases because false negatives increase
• B. It increases because number of false positives approaches zero
• C. It remains the same because predictive values do not depend on prevalence
• D. It increases because sensitivity improves with prevalence

Answer: (B)

Positive predictive value rises as disease prevalence increases because a larger share of the positive test results are true disease cases. With the test’s sensitivity and specificity held constant, higher prevalence boosts the number of true positives while the number of false positives shrinks (since the portion of the population without disease grows smaller). In formula terms, PPV depends on prevalence through the balance of true positives and false positives; as prevalence grows, true positives increase and false positives decline, so the probability that a positive result truly indicates disease goes up. This is why a higher prevalence makes a positive result more trustworthy.