IntroductionResponse to ADP P2Y12receptor inhibition by clopidogrel can be evaluated by various techniques. Here, we compared a functional rapid point-of-care technique (PFA-P2Y) with the degree of biochemical inhibition assessed by the VASP/P2Y12assay.MethodsPlatelet response to clopidogrel was investigated in 173 patients undergoing elective intracerebral stenting (derivation cohortn= 117; validation cohortn= 56). High platelet reactivity (HPR) was defined as PFA-P2Y occlusion time <106 seconds or VASP/P2Y12platelet reactivity index (PRI) >50%.ResultsIn the derivation cohort, receiver operator characteristics analysis for the ability of PFA-P2Y to detect biochemical HPR showed high specificity (98.4%) but poor sensitivity (20.0%) and a very low area under the curve (0.59). The VASP/P2Y12assay revealed two coexisting platelet populations with different levels of vasodilator-stimulated phosphoprotein (VASP) phosphorylation: a fraction of highly phosphorylated, inhibited platelets and another of poorly phosphorylated, reactive platelets. Analysis of the PFA-P2Y curve shape revealed different types, categorized by time of occlusion (<106 seconds, 106 to 300 seconds, >300 seconds), and pattern (regular, irregular, and atypical). Noteworthy, curves with late occlusion and permeable curves with an irregular or atypical pattern correlated with VASP-PRI >50% and smaller sizes of the inhibited platelet subpopulation. Considering the PFA-P2Y shape of the curve for the detection of HPR improved sensitivity (72.7%) and preserved specificity (91.9%), with a rather high AUC (0.823). The validation cohort confirmed the VASP/P2Y12assay data and the usefulness of considering the PFA-P2Y curve shape.ConclusionIn patients treated with acetylsalicylic acid and clopidogrel for 7-10 days, the VASP/P2Y12assay reveals two coexisting subpopulations of differentially inhibited platelets, whose relative sizes predict global PRI and distinct PFA-P2Y curve patterns, indicating incomplete clopidogrel efficacy. The detailed analysis of both VASP/P2Y12and PFA-P2Y is necessary for optimal detection of HPR.