Platelet aggregation in patients with atrial fibrillation taking aspirin or warfarin

Although warfarin and perhaps aspirin may be effective in preventing thromboembolism in patients with nonvalvular atrial fibrillation, some patients develop cerebral infarction despite these therapies. The purpose of this study was to determine inhibition of platelet aggregation in patients on aspirin and platelet reactivity in those on warfarin in the Stroke Prevention in Atrial Fibrillation study.

Twenty-four patients in the Stroke Prevention in Atrial Fibrillation study at the University of Illinois at Chicago, 17 on enteric-coated aspirin 325 mg/d and 7 on warfarin to produce an International Normalized Ratio of 2.0 to 4.5, had platelet aggregation studies performed during a 10-month period and interpreted by an investigator blinded to therapy. Epinephrine, adenosine diphosphate, collagen, and arachidonic acid were used as aggregating agents. Compliance was determined by pill count for those patients on aspirin.

Seven patients taking aspirin had partial and 10 had complete inhibition of platelet aggregation. Three of seven patients on warfarin had hyperaggregable platelets. Compliance was 80% or greater for those patients taking aspirin. One patient on warfarin had partial inhibition of platelet aggregation.

Some patients in the Stroke Prevention in Atrial Fibrillation trial on aspirin 325 mg/d did not achieve complete inhibition of platelet aggregation. Others had hyperaggregable platelets. These findings suggest platelet-dependent mechanisms for aspirin and warfarin failure to prevent stroke in these patients.