In a randomized controlled trial comparing Drug A vs placebo for prevention of MI, the absolute risk reduction (ARR) is 2% and the relative risk reduction (RRR) is 40%. The Number Needed to Treat (NNT) is:
- A 25
- B 50 ✓
- C 40
- D 100
Correct answer: B. 50
Explanation
NNT = 1/ARR = 1/0.02 = 50. This means 50 patients must be treated with Drug A to prevent one additional MI compared to placebo. The RRR of 40% means the drug reduces relative risk by 40% but the baseline risk determines the absolute benefit. NNT is considered more clinically meaningful than RRR because RRR can be misleadingly high even when ARR and clinical benefit are small. A higher NNT indicates less clinical benefit per patient treated.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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Written and medically reviewed by the StethoPrep medical team.