In a cohort study of 1000 hypertensive patients, 500 received a new antihypertensive and 500 received standard therapy. After 5 years, 40 in the new therapy group and 80 in the standard therapy group suffered a stroke. What is the Number Needed to Treat (NNT) to prevent one stroke?
- A 12.5 ✓
- B 5
- C 25
- D 50
Correct answer: A. 12.5
Explanation
Risk in new therapy group = 40/500 = 0.08; risk in standard group = 80/500 = 0.16. Absolute Risk Reduction (ARR) = 0.16 − 0.08 = 0.08. NNT = 1/ARR = 1/0.08 = 12.5. This means approximately 13 patients need to receive the new therapy for 5 years to prevent one additional stroke compared with standard therapy. NNT is the reciprocal of ARR, not of relative risk reduction.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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