Diagnostic Test Calculator (Sensitivity & Specificity)
Evaluate a diagnostic or screening test. Enter the 2×2 counts against a gold standard to get sensitivity, specificity, PPV, NPV, likelihood ratios and accuracy — each with a 95% confidence interval.
Counts vs. gold standard
For education/research reference — verify against statistical software before publication. Computed locally.
Reading the results
A test with high sensitivity is good for ruling a disease out when negative (SnNout); high specificity is good for ruling it in when positive (SpPin). Remember PPV and NPV shift with prevalence, so a great test can still have a low PPV in a low-prevalence setting.
Frequently asked questions
How are sensitivity and specificity calculated?+
Sensitivity = TP ÷ (TP + FN) — the proportion of diseased correctly identified. Specificity = TN ÷ (TN + FP) — the proportion of healthy correctly excluded. This tool also reports 95% Wilson confidence intervals.
What is the difference between PPV/NPV and sensitivity/specificity?+
Sensitivity and specificity are properties of the test. PPV (TP ÷ (TP+FP)) and NPV (TN ÷ (TN+FN)) depend on disease prevalence in your sample and answer "given this result, how likely is the patient truly positive/negative?".
What are likelihood ratios?+
LR+ = sensitivity ÷ (1 − specificity) and LR− = (1 − sensitivity) ÷ specificity. They combine with pretest probability (Bayes) to give post-test probability and don’t depend on prevalence.
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