A patient has a pre-test probability of disease of 20 percent. A test with a positive likelihood ratio of 6 is positive. What is the approximate post-test probability?
Answer and explanation
Correct answer: B. 60 percent
- Step 1: Convert to odds: 20 percent is odds of 0.25.
- Step 2: Multiply by LR: 0.25 times 6 is 1.5. Convert back: 1.5 divided by 2.5 is 60 percent.
Answer
60 percent.
Why not the others: 27 percent: Multiplying the probability by a fraction ignores odds. 80 percent: There is no basis for multiplying the percentage by 4. 120 percent: Multiplying probability by the LR gives an impossible value above 100 percent.
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