What is pretest probability?

• A. Estimated likelihood of disease after testing
• B. Estimated likelihood of disease before testing
• C. Probability after test
• D. Proportion of diseased among positive tests

Answer: (B)

Pretest probability is the estimated likelihood that a patient has the disease before any diagnostic test results are known. It comes from your clinical assessment—signs, symptoms, risk factors, exposure—and how common the disease is in the population. This prior probability is what you update when a test result is available, using the test’s characteristics, to arrive at a post-test probability. If the pretest probability is high, a positive test strengthens the diagnosis; if it’s low, a negative test can effectively rule out disease. The other ideas describe probabilities after testing or the likelihood of disease given a positive test (positive predictive value), which are not pretest probability.