DEVELOPMENT OF ASPIRIN RESISTANCE IN PERSONS WITH PREVIOUS ISCHEMIC STROKE

Background and Purpose The ex vivo effect of aspirin (ASA) on platelet aggregation, the platelet component of thrombosis, was studied at rep eated intervals in a cohort of patients taking aspirin for recurrent i schemic stroke prevention to define the maintenance of efficacy over t ime. Methods We administered increasing doses of aspirin (from 325 to 1300 mg/d) to patients with previous ischemic stroke and determined th e extent of inhibition of platelet aggregation after 2 weeks and there after at approximately 6-month intervals. Results Over 33 months, 306 patients had platelet aggregation studies performed to define their in itial response to ASA therapy. Of these, 228 had complete and 78 had p artial inhibition of platelet aggregation at initial testing. To date, 119 of those who had complete inhibition and 52 who had partial inhib ition have undergone repeat testing at least once. At repeat testing 3 9 of the 119 (32.7%) with complete inhibition at initial testing had l ost part of the antiplatelet effect of ASA and converted from complete to partial inhibition without change in ASA dosage. Of the 52 with pa rtial inhibition at initial testing, 35 achieved complete inhibition e ither by ASA dosage escalation (in 325 mg/d increments) or fluctuation of response at the same dosage, but 8 of those 35 (22.8%) had reverte d to partial inhibition when tested again. Overall, 8.2% of patients u ltimately exhibited ASA resistance to 1300 mg/d-8 of 52 (15.4%) with p artial inhibition and 6 of 119 (5.0%) with complete inhibition at init ial testing. Conclusions The antiplatelet (and presumably the antithro mbotic) effect of a fixed dose of ASA is not constant over time in all individuals. The mechanisms by which increased dosage requirement or ASA resistance develops and the clinical significance of this developm ent are currently undefined.