Predictive Values (PPV, NPV) Calculator

Positive Predictive Value (probability that a positive test result is truly positive):

$$PPV = \frac{\text{Sens} \times \text{Prev}}{\text{Sens} \times \text{Prev} + (1 - \text{Spec}) \times (1 - \text{Prev})}$$

Negative Predictive Value (probability that a negative result is truly negative):

$$NPV = \frac{\text{Spec} \times (1 - \text{Prev})}{\text{Spec} \times (1 - \text{Prev}) + (1 - \text{Sens}) \times \text{Prev}}$$

Positive Likelihood Ratio — how much a positive result increases the odds of disease:

$$LR^+ = \frac{\text{Sensitivity}}{1 - \text{Specificity}}$$

Negative Likelihood Ratio — how much a negative result decreases the odds of disease:

$$LR^- = \frac{1 - \text{Sensitivity}}{\text{Specificity}}$$

Likelihood ratios are used with the Fagan nomogram: multiply pre-test odds by LR to get post-test odds. LR+ > 10 and LR− < 0.1 indicate a very useful diagnostic test.

PPV answers: "If I test positive, what's the chance I'm actually sick?" Low PPV in low-prevalence settings is the false positive paradox.

NPV answers: "If I test negative, what's the chance I'm truly healthy?" NPV is typically high when prevalence is low.

LR+ > 10: A positive result is strong evidence for the disease. LR+ of 2–5 is moderate, < 2 is weak.

LR− < 0.1: A negative result effectively rules out the disease. LR− of 0.2–0.5 is moderate, > 0.5 is weak.