What is pretest probability?

• A. Your initial estimate of disease probability before testing
• B. The probability after testing
• C. The probability that a test is positive
• D. The probability of test accuracy

Answer: (A)

Pretest probability is the likelihood that a person has the disease before any diagnostic test is done, based on clinical assessment, risk factors, symptoms, and how common the disease is in the population. It serves as the starting point (the prior) that you update with the test result to arrive at the post-test probability. This helps you decide how strongly to act on the test result and whether further testing is needed. It’s not the probability after testing, not simply the chance a test comes back positive, and not a measure of how accurate the test is (which involves sensitivity, specificity, or overall accuracy).