A systematic review including 20 studies (17 cohort studies, 1 multicentre, descriptive study, 2 case-control studies) with a total of 2930 subjects was abstracted in DARE. The participants in the included studies had a history of stroke, acute coronary syndrome, coronary arterial bypass grafting, percutaneous coronary intervention, stable cardiovascular disease or peripheral vascular disease. Aspirin resistance was assessed, using a variety of platelet function assays. Most studies used aspirin regimens, ranging from 75-325 mg daily, and six studies included adjunct antiplatelet therapy (a loading dose of clopidogrel and/or tirofiban hydrochloride or glycoprotein IIb/IIIa). Compliance was confirmed directly in 14 studies and by telephone or interviews in three.
Overall, 28% of patients were classified as aspirin resistant. Resistance was more prevalent in women (p<0.01) and in patients with renal impairment (p<0.03). 39% of aspirin resistant patients compared with 16% of aspirin sensitive patients had a cardiovascular event (OR 3.85, 95% CI 3.08 to 4.80). There was significant statistical heterogeneity associated with this outcome. Aspirin-resistant patients were at a significantly higher risk of death (OR 5.99, 95% CI 2.28 to 15.72), acute coronary syndrome (OR 4.06, 95% CI 2.96 to 5.56), failure in vascular intervention (OR 4.35, 95% CI 2.26 to 8.37) and new cerebrovascular event (OR 3.78, 95% CI 1.25 to 11.41). Concomitant therapy with clopidogrel or tirofiban provided no benefit to patients identified as aspirin resistant (there was significant statistical heterogeneity).