Aspirin resistance

OBJECTIVE: To review the literature addressing the problem of aspirin resistance in patients with vascular disease. DATA SOURCES: A MEDLINE search (1966-February 2002) was performed. Key search terms included aspirin, resistance, resistant, failure, tolerance, and nonresponder. English-language studies were identified as well as pertinent references from these articles. DATA SYNTHESIS: Aspirin resistance has been reported in patients with cardiovascular, cerebrovascular, and peripheral vascular disease. Because of differences in the definition of resistance, variations in detection methods, and a lack of controlled trials, the true significance of the problem remains unknown. Multiple mechanisms for resistance have been proposed, including increased reactivity to platelet aggregating factors, genetic polymorphism, and alternate pathways for thromboxane synthesis. The studies to date have failed to demonstrate consistent relationships between aspirin's platelet-inhibiting effects, the impact of dosage escalation, and clinical outcomes. CONCLUSIONS: For many patients, aspirin is an effective antithrombotic agent. However, patients taking aspirin may demonstrate highly variable responses to in vitro tests for platelet aggregation and may experience breakthrough thromboembolic events. Although this phenomenon has been termed aspirin resistance, the lack of a uniform definition or agreement on diagnostic criteria precludes definitive recommendations at this time. In addition, strategies are needed to identify patients at risk for aspirin resistance who might benefit from alternative or combined antiplatelet therapy.