A researcher reports that a new diagnostic test has a likelihood ratio positive (LR+) of 15 and a pretest probability of disease of 20%. Using Fagan's nomogram logic, the approximate post-test probability is closest to:
- A 75% ✓
- B 50%
- C 88%
- D 40%
Correct answer: A. 75%
Explanation
Pretest odds = 0.20/0.80 = 0.25. Post-test odds = LR+ × pretest odds = 15 × 0.25 = 3.75. Post-test probability = 3.75/(1+3.75) = 3.75/4.75 ≈ 79%, closest to 75% among the choices. An LR+ of 15 is considered very strong (>10) and substantially increases post-test probability. LR+ of 10 at 20% pretest gives ~71%; LR+ of 15 gives ~79%, making 75% the closest answer. Option C (88%) would require higher pretest probability or even higher LR+.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.