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If a patient has no history of vascular disease and a low risk of vascular events, should they be taking a daily dose of aspirin? A recent meta-analysis from the Antithrombotic Trialists’ (ATT) collaboration, published in the Lancet,¹ should help to clarify this confusing issue.

Six primary prevention trials, totalling over 95,000 people, were included in this latest meta-analysis, along with 16 secondary prevention trials (around 17,000 individuals). As the ATT investigators analysed individual participant data, this allowed them to estimate the magnitude of risk factors—such as age, male sex, diabetes mellitus, and high blood pressure—for selected outcomes. Therefore, they were able to identify the first time an individual experienced any of the following: a serious vascular event, a major coronary event, stroke, death from any cause, and any extracranial bleeds (defined in the studies as those requiring a transfusion or leading to death).

Results from the primary prevention trials showed that aspirin reduced the per-year risk of a serious vascular event from 0.57% to 0.51% (relative risk [RR] 0.88; 95% confidence interval [CI] 0.82 to 0.94, p=0.0001). Further analysis showed that aspirin did not significantly reduce the risk of stroke or death from vascular causes, but it did significantly reduce the per-year risk of non-fatal myocardial infarction (0.18% vs 0.23%; RR 0.77 [99% CI 0.67 to 0.89], p<0.0001). In the secondary prevention trial, aspirin was also found to significantly reduce the per-year risk of a serious vascular event (6.7% vs 8.2%; RR 0.81 [95% CI 0.75 to 0.87], p<0.0001).

The primary prevention results also showed that aspirin allocation increased the per-year risk of major extracranial bleeding, and this information has attracted the most attention in the popular press under headlines such as ‘Aspirin: more harm than good?’² The data showed a significant increase in major extracranial bleeding between the aspirin and control groups (0.10% vs 0.07% per year; RR 1.54 [95% CI 1.30 to 1.82], p<0.0001); mostly through an increase in non-fatal bleeds. Although there was also an increase in major extracranial bleeds associated with aspirin in the secondary prevention trials, the investigators noted that this data was incompletely reported and that the meta-analysis might be unreliable.

The investigators’ interpretation of the data is that, in primary prevention without previous disease, the value of aspirin in the reduction of occlusive events is uncertain, and needs to be weighed against an increase in major bleeds. They concluded that ‘Although the currently available trial results could well help inform personally appropriate judgements by individuals about their own use of long-term aspirin, they do not seem to justify general guidelines advocating the routine use of aspirin in all apparently healthy individuals above a moderate level of risk of coronary heart disease.’

These latest ATT results have therefore added to the debate on the blanket usage of aspirin in people without vascular disease. Further analysis of the data, and results from other trials currently under way, are awaited with interest.

Julia Morris, Editor
julia.morris@mgp.ltd.ukG

References

1. Antithrombotic Trialists’ Collaboration. Aspirin in the primary and secondary prevention of vascular disease: collaborative meta-analysis of individual participant data from randomised trials. Lancet 2009; 373 (9678): 1849–1860.
2. The Times online website. Aspirin: more harm than good? www.timesonline.co.uk/tol/life_and_style/health/article986148.ece (accessed 4 June 2009).G