Evidence-appraisal glossary
Pre-test probability
The probability that a person has the condition before their test result is known, estimated from the prevalence in similar people plus the individual's own symptoms and risk factors.
Also called: prior probability, pretest probability.
Pre-test probability is the starting point for interpreting any diagnostic test: you combine it with the test's likelihood ratio to get the post-test probability. The same test result carries very different meaning at different pre-test probabilities, which is why a positive result in a low-risk person is often a false alarm. A common mistake is treating a published sensitivity or specificity as if it tells you the chance of disease directly; it does not, because that chance depends on the pre-test probability.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.