Pilot study of on-treatment platelet reactivity at low shear stress and platelet activation status on aspirin or clopidogrel monotherapy in patients with TIA or ischaemic stroke

BACKGROUND: Simultaneously-collected data regarding platelet reactivity and activation in ischaemic cerebrovascular disease (CVD) patients starting antiplatelet agents are limited. METHODS: This prospective observational pilot study assessed platelet reactivity and activation in TIA/ischaemic stroke patients before (baseline; N = 73), 14 ± 7 days (14d; N = 59) and ≥ 90 days (90d; N = 38) after commencing aspirin or clopidogrel monotherapy. Platelet reactivity at low shear-stress (Multiplate® Aspirin/ADP assays) and platelet activation status (% expression of CD62P, CD63, and leucocyte-platelet complexes by flow cytometry) were quantified in whole blood. Prevalence of high on-treatment platelet reactivity (HTPR) was determined on Multiplate with 'case-control definitions' (Aspirin-HTPR:>40 U; Clopidogrel-HTPR:>46 U), and innovative 'longitudinal-HTPR definitions' (failure to reduce aggregation compared with the patient's baseline by more than twice the co-efficient of variation of the relevant assay). RESULTS: Prevalence of case-control aspirin-HTPR was 23.8 % at 14d and 30.8 % at 90d, and of longitudinal aspirin-HTPR was 4.8 % at 14d and 0 % at 90d. Prevalence of clopidogrel-HTPR was significantly higher with case-control than longitudinal definitions at 14d (60.5 % vs. 21 %) and 90d (52 % vs. 24 %) (P ≤ 0.03). % Neutrophil-platelet complexes (P = 0.04) and lymphocyte-platelet complexes (P = 0.002) were higher in patients with vs. without case-control clopidogrel-HTPR at 14d. Median % lymphocyte-platelet complexes significantly decreased between baseline and 14d (P = 0.019), and monocyte-platelet complexes decreased between baseline and 90d (P = 0.017) only in the clopidogrel-patient subgroup whose platelets were adequately inhibited by clopidogrel. CONCLUSIONS: Antiplatelet-HTPR prevalence is higher in CVD patients with case-control than longitudinal definitions on the Multiplate analyser. Quantifying leucocyte-platelet complexes improves our understanding of cellular mechanisms contributing to case-control clopidogrel-HTPR.