What is posttest probability?

What is posttest probability?

• A. Probability before testing
• B. Probability after test result is applied
• C. Probability of false positive
• D. Proportion of true negatives after testing

Answer: (B)

Posttest probability is the probability that the patient has the condition after the test result is known. It updates the initial estimate you had before testing (the pretest probability) using how accurate the test is (sensitivity and specificity or likelihood ratios). A positive result typically increases the likelihood the person has the disease, while a negative result decreases it, with the amount of change depending on both the test’s accuracy and the starting pretest probability. This concept shows why test results are interpreted in light of what was suspected beforehand, rather than in isolation. It’s not about the probability before testing, nor about a specific error rate like a false positive, nor about the proportion of true negatives after testing.

Posttest probability is the probability that the patient has the condition after the test result is known. It updates the initial estimate you had before testing (the pretest probability) using how accurate the test is (sensitivity and specificity or likelihood ratios). A positive result typically increases the likelihood the person has the disease, while a negative result decreases it, with the amount of change depending on both the test’s accuracy and the starting pretest probability. This concept shows why test results are interpreted in light of what was suspected beforehand, rather than in isolation. It’s not about the probability before testing, nor about a specific error rate like a false positive, nor about the proportion of true negatives after testing.