Which metric is most directly affected by disease prevalence?

• A. Sensitivity
• B. Specificity
• C. Positive Predictive Value
• D. False negative rate

Answer: (C)

Predictive values change with how common the disease is in the population. Positive Predictive Value is the probability that someone actually has the disease given a positive test result. This value is directly influenced by disease prevalence because when the disease is rare, a large portion of positives are false positives, while as prevalence increases, more of the positives are true positives, raising the PPV. For example, with sensitivity around 90% and specificity around 95%, a prevalence of 1% yields a relatively low PPV (a substantial share of positives are false positives), whereas a prevalence of 10% yields a much higher PPV because a larger fraction of positives reflect true disease. Sensitivity and specificity are intrinsic to the test and do not change with prevalence, and the false negative rate is determined by sensitivity, not by how common the disease is. Therefore, the metric most directly affected by prevalence is Positive Predictive Value.