Acquired von Willebrand syndrome (AVWS) is frequent in patients with myeloproliferative neoplasms (MPNs). For VWF functional evaluation, ristocetin cofactor activity by aggregometry (VWF: RCo) is considered the gold standard but have limitations, and automated activities measurement has been developed such as the HemosIl VWF:RCo Werfen® with particle agglutination (VWF:GPIbR). To evaluate the performance of VWF:GPIbR with HemosIL VWF:RCo Werfen® (VWF:GPIbR) versus VWF:RCo in patients with thrombocytosis in the context of MPNs (T-MPNs) and in patients with secondary thrombocytosis (ST). MPNs patients with thrombocytosis >450 G/L (T-MPNs) were compared to patients with secondary thrombocytosis (ST) due to inflammation or iron deficiency. VWF activity (VWF:Act) was analyzed using VWF:RCo or VWF:GPIbR. VWF analysis was completed by analysis of VWF multimers and VWF collagen binding (CB) assay (VWF:CB). 33 T-MPNs and 18 ST patients were included. Compared to aggregometry, evaluation of VWF:Act by VWF:GPIbR leaded to lower values in T-MPNs patients, but also in ST patients. Interestingly, whereas VWF:RCo/VWF:Ag ratio did not reveal differences between T-MPNs and ST patients, VWF:GPIbR/VWF:Ag ratio analysis allowed to suspect AVWS only in T-MPNs patients. Using the distribution of VWF multimers analysis and VWF:CB, we here demonstrated that VWF:GPIbR allows AVWS diagnosis in 9 T-MPNs as opposed to aggregometry. Evaluation of VWF:Act using VWF:GPIbR has a greater sensitivity compared to aggregometry to detect AVWS in T-MPNs patients.< Réduire