Prevalence of platelet nonresponsiveness to aspirin in patients treated for secondary stroke prophylaxis and in patients with recurrent ischemic events.

To determine the prevalence of platelet nonresponsiveness to aspirin treatment for secondary stroke prophylaxis, the authors studied consecutive patients during a 29-month period. Information regarding their ischemic events, risk factors, and medications was collected. Platelet aggregation in response to collagen and arachidonic acid was used to determine platelet responsiveness to aspirin. A total of 653 patients were evaluated. Of these, 129 patients (20%) were determined to be nonresponsive to aspirin based on continued platelet aggregation in response to collagen, arachidonic acid, or both. A total of 87 (13%) of the 653 patients were clinical aspirin failures (ie, presented with new focal cerebral ischemic symptoms while taking aspirin). Of the patients with new cerebral ischemic symptoms, 57 (66%) were determined to be platelet nonresponsive to aspirin. The odds ratio for platelet nonresponsiveness to aspirin in patients who suffered a recurrent ischemic event while taking aspirin was 14.25 (95% confidence interval: 8.5-23.7; P < .5). Continued platelet aggregation despite aspirin treatment occurred in 20% of ambulatory patients treated for secondary stroke prophylaxis. The prevalence of nonresponsiveness to aspirin was statistically higher in those patients who suffered recurrent cerebral ischemia while taking aspirin (P < .5) compared with patients who remained without new ischemic symptoms.