ABPI cannot be used to determine primary prevention benefit from aspirin

A low ankle brachial index (ABI) indicates the presence of atherosclerotic arterial disease, and therefore an increased risk of cardiovascular and cerebrovascular events. Screening for a low ABI may therefore identify a group of asymptomatic individuals who may benefit from preventive treatments. The Aspirin for Asymptomatic Atherosclerosis (AAA) trial was therefore established to investigate the effectiveness of aspirin in preventing cardiovascular events in patients with a low ABI.

Over a ten year period, 28980 men and women between the ages of 50 and 75 years of age were recruited and screened with an ABI test. Of these, 3350 patients with a low ABI (<0.96) were recruited and randomized to treatment with either 100mg aspirin once daily or placebo. After a mean follow-up of 8.2 years, 357 participants had a primary end point event (a composite of initial fatal or nonfatal coronary event or stroke or revascularisation). No statistically significant difference was found between the two treatment groups (13.7 events per 1000 person-years in the aspirin group vs 13.3 in placebo). Similarly, no difference was seen between treatment groups in the secondary outcomes or in all-cause mortality. However, an initial event of major haemorrhage requiring admission to hospital occurred in 34 participants in the aspirin group, compared to 20 in the placebo group (Hazard Ratio, 1.71).
Conclusions:
In an asymptomatic population, the administration of aspirin to patients with a low ABI did not result in a significant reduction in vascular events compared to the administration of placebo.
Fowkes FGR, Price JF, Steward MCW, et al. Aspirin for prevention of cardiovascular events in a general population screened for a low ankle brachial index. A randomized controlled trial. JAMA 2010; 303:841-848. Berger JS. Aspirin as preventive therapy in patients with asymptomatic vascular disease. JAMA 2010; 303:880-882.

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