Aspirin for the prevention of cardiovascular disease: calculating benefit and harm in the individual patient.

AIMS: To estimate the absolute reduction in the risk of cardiovascular events and absolute increase in gastrointestinal haemorrhage associated with aspirin for individuals with different baseline risks.
METHODS: Calculation of absolute treatment effects from estimates of: (i) baseline risks for cardiovascular event and gastrointestinal haemorrhage; and (ii) relative risks of these events with treatment. Baseline cardiovascular risks were derived from existing risk scores, and baseline risk of gastrointestinal haemorrhage from an observational cohort study. Changes in relative risks were obtained from clinical trial data. The effects of aspirin treatment were calculated in examples of two individuals with very different baseline risks.
RESULTS: Treatment of a healthy 74-year-old man (blood pressure 144/88 mm Hg and no history of gastrointestinal disorder) would reduce his annual risk of a cardiovascular event from 2% to 1.74% (absolute risk reduction 0.26%, number needed to treat 385), but increase the gastrointestinal haemorrhage risk from 0.3% to 0.51% (absolute risk increase 0.21%, number needed to harm 476). In a 66-year-old obese man, following a transient ischaemic attack, and with a history of hospital treatment for a peptic ulcer, the annual risk of a cardiovascular event would be reduced from 5% to 4.35% (absolute risk reduction 0.65%, number needed to treat 153), but the risk of gastrointestinal haemorrhage would increase from 1.08% to 1.83% (absolute risk increase 0.75%, number needed to harm 133).
CONCLUSIONS: Estimating benefit and harm by taking into account the baseline risks in each individual allows patients and doctors to judge for themselves the magnitude of the trade-offs involved in taking aspirin.