Background: Stroke occurrence despite chronic antiplatelet drug (APD) treatment is frequent. We aimed at evaluating the relevance of platelet aggregation testing in the identification of stroke etiology in this context. Methods: Patients admitted for a suspected acute ischemic stroke, while under APD (aspirin and/or clopidogrel), were prospectively included. The efficacy of the APD was evaluated using a Multiplate™ assay. Resistance was confirmed using light transmission aggregometry. A standardized diagnostic work-up was performed to identify stroke mechanism according to the TOAST and the ASCO classifications. We evaluated the influence of APD functional status on stroke severity and identified potential determinants of resistance. Results: APD resistance was observed in 53 of the 287 patients (18.5%). No difference in stroke mechanism depending on APD efficacy was observed. Patients sensitive to APD had less severe initial stroke severity (mean National Institutes of Health Stroke Scale 3.9 ± 5.6 vs. 7.2 ± 6.8; p < 0.01). Main determinants for APD resistance were a worse control of the diabetes and higher baseline levels of inflammation (mean CRP 26.4 ± 56.0 vs. 9.3 ± 21.0; p < 0.01). Conclusions: Platelet function testing does not provide orientation concerning stroke mechanism in patients who were previously on APDs. However, the high frequency of APD resistance and its association with inflammation and stroke severity are confirmed.

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