Comparison of Different Laboratory Tests to Identify "Aspirin Resistance" and Risk of Vascular Events among Ischaemic Stroke Patients: A Double-Blind Study

"Aspirin resistance" (AR) is associated with increased risk of vascular events. We aimed to compare different platelet function tests used in identifying AR and assess their implications on clinical outcome. We performed platelet aggregation studies on non-cardioembolic ischaemic stroke patients taking aspirin 100 mg/day and 30 non-stroke controls. Data were collected on demographics, vascular risk factors, and concomitant medications. Cut-offs for AR were (1) light transmission aggregometry (LTA) of >= 20% using arachidonic acid (AA), >= 70% using ADP, or >= 60% using collagen; and (2) VerifyNow (R) assay >= 550 ARU. Telephone follow-ups were conducted by study staff blinded to AR status to ascertain the occurrence of vascular outcomes (stroke, myocardial infarction, amputation, death). A total of 113 patients were recruited, mean age 65 +/- 8 years, 47% women, 45 +/- 15 days from index stroke. 50 (44.3%, 95% CI 34.9-53.9) had AR on at least 1 test. Frequency of AR varied from 0% to 39% depending on method used and first vs. recurrent stroke. There were strong correlations between LTA AA, VerifyNow (R) and Multiplate (R) ASPItest (r = 0.7457-0.8893), but fair to poor correlation between LTA collagen and Multiplate (R) COLtest (r = 0.5887) and between LTA ADP and Multiplate (R) ADPtest (r = 0.0899). Of 103 patients with a mean follow up of 801 +/- 249 days, 10 (9.7%) had vascular outcomes, of which six had AR by LTA-ADP. AR by LTA-ADP is associated with increased risk of vascular outcome (p = 0.034). Identification of AR is not consistent across different platelet function tests. LTA of >= 70% using 10 mu M ADP in post-stroke patients taking aspirin is associated with increased risk of vascular outcome.