Abstract T P386: The Clinical Implications of Assessing Platelet Responsiveness to Aspirin Using Whole Blood Platelet Aggregation versus Urinary 11-Dehydrothromboxane Testing

Abstract

Background: Whole blood aggregometry (WBA) is a direct measure of the effects of aspirin (ASA) on platelet aggregation while the measurement of urinary thromboxane B2 (TXB2) quantitates the metabolite produced by the ASA inhibited COX-1 enzyme. The objectives of this study were to a) compare indirect urine testing to a direct measure of platelet activity, b) determine the impact on clinical decision making if only TXB2 were measured, and c) compare time to obtain results from both procedures. Methods: Patients ages 18 to 90 who were on a stable dose of ASA for at least 14 days and had a confirmed stroke/TIA in the past 90 days were included. Patients were excluded if they used any medication that altered platelet function. Once consented, as per standard of care, blood was collected for WBA as well as a urine sample for a salicylate test for compliance. As per protocol, urine was sent out to have TXB2 levels measured in pg/mg of CR. Non-responsiveness was defined as aggregation to collagen 1 ug/mL >10 ohms of impedance, ohms ratio of collagen 1 to 5 ug/mL >.5, or arachidonic acid (AA) >6 ohms. Borderline patients were classified as either responsive or non-responsive for analysis according to the treatment decisions that were made based on clinical judgment. ABCD 2 scores were calculated and TXB2 test turnaround time was noted. Results: In 58 patients TXB2 reported only 11 as non-responsive to ASA while WBA found 18 as non-responsive and 4 as borderline. Chi square analysis found significantly different proportions of participants to be responsive or non-responsive to ASA (p=.024) through each test. ASA therapy was adjusted based on WBA results typically the next day while mean turnaround time for TXB2 was 7.29 days. ABCD 2 scores based on risk factors predicted 10.5 ischemic events while only two actually occurred. Conclusions: WBA testing reported significantly more patients to be ASA non-responsive, allowing therapy changes to be made immediately. TBX2 testing failed to identify many patients that were ASA non-responsive and results did not become available for 1 to 2 weeks.