A patient has a pre-test probability of disease of 10 percent. A test with a positive likelihood ratio of 16 is positive. What is the approximate post-test probability?
Answer and explanation
Correct answer: A. 64 percent
- Step 1: Convert probability to odds, multiply by the likelihood ratio, then convert back. Step 2. Pre-test odds are 0.10/0.90 = 0.111. Post-test odds are 0.111 x 16 = 1.78. Probability = 1.78/(1 + 1.78) = 0.64. Answer: 64 percent. Why not the others: 16 percent multiplies the probability directly by the ratio in error. 80 percent confuses the answer with sensitivity. 90 percent ignores the low pre-test odds.
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