In patients with cardiovascular disease, the use of aspirin has been shown to substantially reduce ischemic vascular events through its antiplatelet activity. In a recent meta-analysis of more than 200,000 patients, aspirin was shown to reduce ischemic vascular events by 22 percent. However, in certain persons, aspirin therapy does not provide an adequate antiplatelet activity. These patients have aspirin resistance, which is defined by platelet function testing and presumed clinical unresponsiveness. This resistance has been reported in the literature, but little information links the laboratory documentation of aspirin resistance with clinical outcomes. Gum and colleagues attempted to determine if aspirin resistance is associated with clinical events.

The researchers conducted a prospective trial of consecutive patients who presented to an outpatient clinic between January 1997 and September 1999. To be included in the study, patients had to have a history of cardiovascular disease, such as myocardial infarction, stroke, documented coronary stenosis on cardiac catheterization, or invasive cardiovascular revascularization procedure. Patients who were at least 21 years of age were given 325 mg of aspirin as their only antiplatelet medication during the study. Aspirin sensitivity was assessed by optical platelet aggregation. The primary outcome measures included myocardial infarction, cerebrovascular accident, and mortality. Mean follow-up time was 679 days.

Of the 326 patients enrolled in the study, 17 patients (5.2 percent) were found to be aspirin resistant based on laboratory evaluation using optical aggregation. Patients with aspirin resistance had a significantly increased risk of myocardial infarction, cerebrovascular accident, and mortality when compared with those who did not have aspirin resistance. A stratified multivariate analysis also identified aspirin resistance as independently associated with major adverse long-term outcomes.

The authors conclude that aspirin resistance in a stable population resulted in a greater than threefold increase in the risk of major adverse events. They add that aspirin resistance is an important and real clinical diagnosis, and that with safe and effective alternative antiplatelet agents available, screening for aspirin resistance in cardiovascular patients is a useful tool.