A clinical trial reports an Odds Ratio of 0.45 with p = 0.03 for a new drug reducing stroke. The absolute risk in the control group is 20%. The approximate Number Needed to Treat (NNT) is closest to:

• A 6
• B 9
• C 15
• D 11 ✓

Explanation

OR ≈ RR when the outcome is rare; however with 20% baseline risk, we must use the formula: Risk in treatment group ≈ OR × baseline risk / [1 – baseline risk × (1 – OR)] = 0.45 × 0.20 / [0.80 + 0.45 × 0.20] = 0.09 / [0.80 + 0.09] = 0.09/0.89 ≈ 0.101. ARR = 0.20 – 0.101 = 0.099 ≈ 0.09. NNT = 1/0.09 ≈ 11. The NNT rounded to the nearest whole number is 11.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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