If disease prevalence increases while test sensitivity and specificity remain constant, the positive predictive value will:

• A. Increase
• B. Decrease
• C. Stay the same
• D. Random

Answer: (A)

Positive predictive value rises as disease prevalence increases when sensitivity and specificity remain fixed. PPV is the probability that someone who tests positive actually has the disease. With fixed test characteristics, PPV depends on how common the disease is in the tested population. The formula PPV = [sensitivity × prevalence] / [sensitivity × prevalence + (1 − specificity) × (1 − prevalence)] shows that as prevalence increases, the true-positive portion grows while the false-positive portion shrinks (since fewer people are without disease). This shifts the balance toward true positives among those who test positive, so PPV increases. For example, with reasonable sensitivity and specificity, raising prevalence moves PPV from a low percentage toward a higher one, reflecting that a positive test becomes more likely to indicate actual disease.