Pre-test to Post-test Probability Calculator
Enter pre-test probability, sensitivity, and specificity to compute positive and negative post-test probabilities using likelihood ratios.
Test Parameters
Clinical prevalence or prior probability of disease (0–100)
True-positive rate: how often a positive test is positive when disease is present
True-negative rate: how often a negative test is negative when disease is absent
Likelihood Ratios
LR+
—
Positive LR
LR−
—
Negative LR
Post-test Probabilities
If test is positive
—
Probability of disease given a positive result
If test is negative
—
Probability of disease given a negative result
Interpretation
Summary
Enter pre-test probability, sensitivity, and specificity to compute positive and negative post-test probabilities using likelihood ratios.
How it works
- Enter the pre-test probability (clinical prevalence or prior probability) as a percentage.
- Enter the test sensitivity (true-positive rate) and specificity (true-negative rate) as percentages.
- The calculator converts pre-test probability to pre-test odds using the formula: odds = probability / (1 - probability).
- Positive likelihood ratio (LR+) = sensitivity / (1 - specificity); negative likelihood ratio (LR-) = (1 - sensitivity) / specificity.
- Post-test odds are computed by multiplying pre-test odds by the relevant LR.
- Post-test odds are converted back to probability: probability = odds / (1 + odds).
Use cases
- Determine how much a positive test result shifts the probability of disease.
- Assess whether a negative result is reassuring enough to rule out a condition.
- Compare the diagnostic value of different tests for the same condition.
- Teach or learn Bayesian reasoning in clinical medicine or evidence-based practice.
- Evaluate screening vs. confirmatory tests based on pre-test prevalence.
- Support clinical decision-making in low- or high-prevalence settings.
Frequently Asked Questions
Last updated: 2026-07-24 ·
Reviewed by Nham Vu