What does likelihood ratio update?

• A. Probability (post-test probability)
• B. Pretest probability
• C. Sensitivity
• D. Specificity

Answer: (A)

Likelihood ratios update the probability that a patient has the disease after seeing a test result. They convert your starting belief (pretest probability) into a revised belief (post-test probability) using odds form. You translate the pretest probability to odds, multiply by the appropriate likelihood ratio (LR+ for a positive result, LR− for a negative result), and then convert back to a probability.

For example, if the pretest probability is 30% and you get a positive test with LR+ of 4, the post-test odds become 0.30/0.70 × 4 ≈ 1.714, which converts to a post-test probability of about 63%. If the result is negative and LR− is 0.25, the post-test odds become 0.4286 × 0.25 ≈ 0.107, giving a post-test probability around 10%.

Remember, sensitivity and specificity determine the likelihood ratios, but what gets updated is your probability estimate after the test.