PPV/NPV Calculator

What a Result Actually Tells You

Sensitivity and specificity describe a test in the abstract, but they don't directly answer the question a patient actually has: "I tested positive — what's the chance I really have this?" That's what predictive values calculate. They flip the confusion matrix around, looking at outcomes from the test result's perspective rather than the true condition's perspective.

The Formula

PPV is the probability that a positive result is a true positive. NPV is the probability that a negative result is a true negative.

Why Predictive Values Shift With Prevalence

Unlike sensitivity and specificity, which are largely properties of the test itself, PPV and NPV depend heavily on how common the condition is in the population being tested. The same test applied to a high-prevalence group (say, a specialist clinic) and a low-prevalence group (a general screening population) will produce very different PPV values, even though sensitivity and specificity stay the same — a low-prevalence setting drives more false positives relative to true positives.

Where This Distinction Matters

• Interpreting your own test result — PPV and NPV are the numbers that answer "how much should I trust this result," as opposed to describing the test in general.
• Population screening programs — a test with excellent sensitivity and specificity can still have a surprisingly low PPV when screening a low-prevalence general population.
• Choosing where to deploy a test — the same test can be more or less useful depending on the baseline prevalence of the group it's applied to.

How to Use This Calculator

1. Enter the number of True Positives (TP).
2. Enter the number of False Positives (FP).
3. Enter the number of True Negatives (TN).
4. Enter the number of False Negatives (FN).
5. Select Calculate to see PPV and NPV as percentages.

Related Calculations

Start from raw counts with the Sensitivity/Specificity Calculator, or compare test performance with the Likelihood Ratio Calculator.