Free tool · Biostatistics
Pretest/Posttest Probability Calculator
Enter your pretest probability and a test's likelihood ratio to get the posttest probability.
Posttest probability
55.6%
The core idea of diagnostic reasoning
A test result doesn't tell you whether a patient has a disease — it updates your estimate of the probability. This calculator applies that update directly: start with your pretest probability (based on history, exam, and prevalence), apply the test's likelihood ratio, and get a posttest probability.
The formula
Probability has to be converted to odds first, since likelihood ratios operate on odds, not probability directly: pretest odds = pretest probability / (1 − pretest probability). Then: posttest odds = pretest odds × LR, and finally converted back: posttest probability = posttest odds / (1 + posttest odds).
Where the likelihood ratio comes from
If you have a test's sensitivity and specificity but not its likelihood ratio directly, use the likelihood ratio calculator first, then bring that number here.
Your pretest probability estimate matters as much as the math — this tool only does the second half of the reasoning.