Turn a pre-test probability into post-test probabilities with a test’s sensitivity and specificity.
Post-test probability if positive
How to use this tool
Enter the chance before testing, as a percent.
Enter the test’s sensitivity and specificity as percents.
Read the post-test probability after a positive result.
Read the post-test probability after a negative result.
Formula & worked example
With a 10% pre-test probability and a test that is 90% sensitive and 90% specific, a positive result raises the probability to 50%.
Limitations — read this before you rely on it
Assumes the stated sensitivity and specificity apply to your patient and that the pre-test estimate is sound; it does not replace clinical judgement.
Frequently asked questions
Your estimate of the chance of disease before testing, from prevalence, symptoms or clinical judgement.
How many times more likely a positive result is in disease than without it; larger LR+ values shift probability more.
Because false positives matter: with a low pre-test probability, even a good test leaves substantial doubt.
Use LR− the same way; the tool shows the post-test probability if negative alongside the positive case.
Yes, by population, disease stage and test execution; use values measured in a similar setting.