Positive predictive value is defined as the proportion of true positives among those who test positive. Which formula correctly expresses PPV?

• A. A/(A+B)
• B. A/(A+C)
• C. (A+B)/A
• D. B/(A+B)

Answer: (A)

Positive predictive value measures the probability that someone who tests positive actually has the disease. When you consider all positive test results, some are true positives (they truly have the disease) and some are false positives (they do not). PPV is the proportion of true positives among all positives, so it’s true positives divided by the sum of true positives and false positives. This is why the correct expression is the number of true positives divided by (true positives plus false positives). PPV differs from sensitivity and specificity, and it is affected by disease prevalence in the population.