Evidence-appraisal glossary
Post-test probability
The updated probability that a person has the condition after their test result is taken into account, obtained by combining the pre-test probability with the test's likelihood ratio.
Also called: posterior probability, posttest probability.
Post-test probability is what a clinician actually wants after ordering a test, and it is calculated by converting the pre-test probability to odds, multiplying by the likelihood ratio, and converting back. The positive predictive value is simply the post-test probability after a positive result, computed at one particular pre-test probability, namely the prevalence in the studied group. Because of that dependence, quoting a single post-test probability without stating the assumed pre-test probability can mislead.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.