Prescribing Advice for GPs

An NHS Prescribing Advisers' Blog

Aspirin ineffective in those with low ABI

The Journal of the American Medical Association (JAMA) has published the results of a study that aimed to assess the efficacy of aspirin in preventing primary cardiovascular events in patients with a low ankle brachial index (ABI).

ABI is the ratio of systolic blood pressure at the ankle and arm. ABI is used to diagnose peripheral vascular disease and is associated with an elevated risk of coronary events.

3,350 men and women aged 50 to 75 were recruited to the study. None had clinical cardiovascular disease but all had ABI less than or equal to 0.95. Follow up was for a mean of 8.2 years for a primary composite outcome of fatal or nonfatal coronary event, stroke or revascularisation. Participants were randomly assigned to treatment with 100mg aspirin daily or matching placebo.

The study found no significant difference in the rate of the primary outcome between the two study groups (Hazard Ratio 1.03, 95% confidence interval 0.84 - 1.27). Additionally, there were no differences in the two secondary outcomes (a composite of the primary outcome and angina, intermittent claudication or transient ischaemic attack or all-cause mortality). The study also assessed the rate of major haemorrhage requiring a hospital admission. This was higher in the patients treated with aspirin but the difference was not significant (HR 1.71, 95% CI 0.99 - 2.97).

The authors conclude that among this population "the administration of aspirin compared to placebo did not result in a significant reduction in vascular events". The authors also suggest that ABI assessment is unlikely to be a useful screening tool in primary care settings.

The results of the study may be limited by low levels of medication compliance with the treatments taken for 60% of the trial person-years. Also, the study was designed and powered to detect a 25% relative reduction in events. Recent analyses have indicated that aspirin may only produce a 12% reduction and perhaps this study was underpowered.

Action: This study adds some more weight to the conclusions reached by the Drug and Therapeutics Bulletin that the use of aspirin in the primary prevention of cardiovascular events is unjustified.

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