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Aspirin resistance among long-term aspirin users after carotid endarterectomy and controls: flow cytometric measurement of aspirin-induced platelet inhibition.

Assadian A, Lax J, Meixner-Loicht U, Hagmüller GW, Bayer PM, Hübl W

Department of General and Vascular Surgery, Wilhelminenspital Vienna, Vienna, Austria. afshin_assadian@yahoo.de

BACKGROUND: Numerous studies have indicated that some patient subpopulations do not respond to the antithrombotic effects of aspirin. The objective of this study was to evaluate aspirin-induced inhibition of platelet cyclooxygenase (COX) using a flow cytometric technique in long-term aspirin users after carotid endarterectomy (CEA) and controls with newly diagnosed carotid stenosis not taking aspirin and to compare these results with platelet function analyzer measurements. METHODS: The study included 86 patients with a history of CEA on long-term aspirin therapy (100 mg daily) and 29 age-matched patients with newly diagnosed carotid artery stenosis not taking aspirin. Platelet-rich plasma diluted with phosphate-buffered saline was incubated with arachidonic acid (ARA) at a final concentration of 80 micromol/L. After staining with phycoerythrin-labeled anti-P-selectin (CD62p) antibody, platelet CD62p-antigen expression was measured on a flow cytometer. RESULTS: Flow cytometric measurement of ARA-induced platelet activation showed an inhibition of ARA-induced platelet stimulation in all patients on aspirin therapy, whereas all but two controls (95%) showed expected platelet reactivity. In contrast, results of the platelet function analyzer measurements were normal in 16% of aspirin-treated patients. CONCLUSIONS: Flow cytometric measurement of CD62p expression on platelets after incubation with ARA proved to be a practicable tool to monitor aspirin-induced inhibition of platelet COX. Results in patients on long-term low-dose aspirin therapy show that the inability of aspirin to inhibit platelet COX for both symptomatic and asymptomatic patients with high-grade internal carotid artery stenosis is a very rare event. So-called aspirin resistance detected quite frequently by platelet function analyzer measurement is most likely from COX-independent mechanisms.

Published 4 June 2007 in J Vasc Surg, 45(6): 1142-7; discussion 1147.
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