Which statement correctly describes the effect of a high positive likelihood ratio (LR+) on post-test probability when the test is positive?

Which statement correctly describes the effect of a high positive likelihood ratio (LR+) on post-test probability when the test is positive?

• A. No effect
• B. Increases probability toward disease
• C. Decreases probability
• D. Depends on pretest probability

Answer: (B)

A positive result is interpreted through likelihood ratios to update your estimate of disease. A high positive likelihood ratio means the test is very good at distinguishing those with disease from those without, so a positive result substantially increases the odds that the patient has the disease. In practice, you convert the pretest probability to pretest odds, multiply by the LR+, and convert back to a probability: post-test odds = pretest odds × LR+, post-test probability = post-test odds / (1 + post-test odds). Since LR+ > 1 raises post-test odds, the post-test probability moves upward toward disease when the test is positive. The bigger the LR+, the larger the increase. For example, starting with a pretest probability of 30% and an LR+ of 10, the post-test probability rises to about 81%. The exact amount depends on the starting pretest probability, but the direction is always toward higher probability with a strong LR+ after a positive test.

A positive result is interpreted through likelihood ratios to update your estimate of disease. A high positive likelihood ratio means the test is very good at distinguishing those with disease from those without, so a positive result substantially increases the odds that the patient has the disease.

In practice, you convert the pretest probability to pretest odds, multiply by the LR+, and convert back to a probability: post-test odds = pretest odds × LR+, post-test probability = post-test odds / (1 + post-test odds). Since LR+ > 1 raises post-test odds, the post-test probability moves upward toward disease when the test is positive. The bigger the LR+, the larger the increase. For example, starting with a pretest probability of 30% and an LR+ of 10, the post-test probability rises to about 81%. The exact amount depends on the starting pretest probability, but the direction is always toward higher probability with a strong LR+ after a positive test.