Number Needed to Treat Calculation
The number needed to treat (NNT) is 33 (Option C), calculated from the absolute risk reduction of 3% (12% mortality in placebo minus 9% mortality in aspirin group).
Calculation Method
The NNT is derived using the following formula:
- Absolute Risk Reduction (ARR) = Control Event Rate - Treatment Event Rate
- ARR = 12% - 9% = 3% = 0.03
- NNT = 1 / ARR = 1 / 0.03 = 33.3 ≈ 33 1, 2
Clinical Context
This calculation is based on the landmark ISIS-2 trial, which demonstrated that 160 mg daily aspirin for one month after acute myocardial infarction reduced 5-week vascular mortality from 12.0% to 9.4% (representing a 23% relative odds reduction) 1, 2.
Key Trial Findings:
- The study enrolled 17,187 patients with suspected acute myocardial infarction across 417 hospitals 1, 2
- Aspirin alone produced highly significant mortality reduction (p < 0.00001) 1
- When combined with streptokinase, the mortality benefit was even greater (8.0% vs 13.2% with neither agent), demonstrating additive effects 1, 2
- Aspirin significantly reduced non-fatal reinfarction (1.0% vs 2.0%) and non-fatal stroke (0.3% vs 0.6%) without significant increase in cerebral hemorrhage 1
Safety Profile:
- Aspirin was not associated with significant increases in bleeds requiring transfusion or cerebral hemorrhage in this post-MI population 1, 2
- The bleeding risk was substantially lower than with streptokinase alone 1
This NNT of 33 means that treating 33 patients with acute myocardial infarction with 160 mg aspirin daily for 5 weeks would prevent one vascular death 1, 2.