Aspirin and clopidogrel response variability: review of the published literature.

Antiplatelet resistance has been proposed as a possible mechanism to explain recurrent cardiovascular events in patients who have coronary artery disease and who are undergoing dual antiplatelet therapy. A comprehensive search on PubMed was conducted for literature that was printed in the English language between January 1996 and November 2007 on aspirin and clopidogrel resistance. Significant traits for aspirin hyporesponsiveness were female sex, older age, and lower levels of hemoglobin. Diabetes mellitus and elevated body mass index showed trends toward a higher incidence of resistance in some aspirin trials but did not reach statistical significance. Clopidogrel studies suggested that patients with type-2 diabetes mellitus are more likely to manifest inadequate response to the medication. Although 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors were initially suspected to decrease response to clopidogrel, later studies refuted this possibility. Patients with a suboptimal response to aspirin or clopidogrel seem to be at increased risk of recurrent cardiovascular events. Large clinical trials with standardized laboratory methods and well-defined protocols are needed to determine whether common features exist in patients with suspected hyporesponsiveness to antiplatelet therapy, and to validate the clinical relevance of response variability. A concise nonarbitrary definition of physiologic "resistance" is needed, and investigators should identify patients as having a variable response to antiplatelet therapy.