Predictive Value Calculator (PPV/NPV)

About this calculator

This calculator finds the positive predictive value (PPV) and negative predictive value (NPV) of a diagnostic test from a 2×2 confusion matrix of true positives, false positives, false negatives, and true negatives: PPV = TP/(TP+FP) is the probability that a positive result is truly positive, and NPV = TN/(TN+FN) is the probability that a negative result is truly negative.

Unlike sensitivity and specificity, which are fixed properties of a test, PPV and NPV depend heavily on disease prevalence in the population being tested — the same test performs very differently screening a low-risk general population versus a high-risk clinical group, which is why a positive result on a rare-disease screening test is often less reliable than intuition suggests.

Clinicians and diagnosticians interpreting lab and imaging test results, epidemiologists designing screening programs, and medical students learning Bayesian test interpretation all use PPV and NPV to translate a raw test result into a real probability of disease.