Predictive values

PPV and NPV depend on disease prevalence in the population being tested — not just test sensitivity and specificity. Adjust the sliders to see how they shift.

Test and disease

Test name

Disease name

Test performance

Sensitivity (%) 99%

Specificity (%) 99%

Population prevalence

Prevalence (%) 0.10%

Results

About PPV and prevalence

PPV = TP / (TP + FP). When prevalence is very low, even a highly specific test produces many false positives relative to true positives, because the number of disease-free individuals vastly outnumbers those with disease. This is the basis of debates around mass screening programmes. The formula is: PPV = (Sens × Prev) / (Sens × Prev + (1 − Spec) × (1 − Prev)).

Likelihood ratios (LR+, LR−) are prevalence-independent and often more useful for communicating test performance across settings.