Positive & Negative Predictive Value Calculator

Sensitivity and Specificity describe how well a test distinguishes between people with and without a condition. However, interpreting an individual's test result also requires consideration of the population base rate of that condition (prevalence). This calculator combines sensitivity, specificity, and base rate information to estimate positive predictive value (PPV) and negative predictive value (NPV), providing a more clinically meaningful interpretation of test performance. See our learning resource on sensitivity and specificity for more details.

Parameters

Sensitivity %

True positive rate - proportion of affected individuals the test correctly flags.

Specificity %

True negative rate - proportion of unaffected individuals the test correctly clears.

Base rate %

Prevalence of the condition in the population being tested (not in a validation sample).

Population size (N)

Positive predictive value

PPV = TP / (TP + FP)

Negative predictive value

NPV = TN / (TN + FN)

Sample of 100 patients — shaded by outcome

True positive True negative False positive False negative

Outcome	Count	Per 1,000
True positive	–	–
False positive	–	–
False negative	–	–
True negative	–	–

Computed from Bayes' theorem: PPV = (Se·p) / (Se·p + (1−Sp)·(1−p)) and NPV = (Sp·(1−p)) / (Sp·(1−p) + (1−Se)·p), where Se is sensitivity, Sp is specificity, and p is base rate. This tool is for educational and clinical-reasoning purposes; it does not replace validated normative data for a specific instrument or population.