[Show abstract][Hide abstract] ABSTRACT: Measuring von Willebrand factor (VWF) activity is essential to the diagnosis of von Willebrand disease (VWD). The VWF activity is usually assessed based on measurement of the ristocetin cofactor (VWF:RCo). However, that test is technically challenging and has high intra- and inter-assay variabilities. The HemosIL VWF activity (VWF:AC) is a fully automated assay, recently proposed as a good alternative to VWF:RCo for VWD diagnosis. This study was undertaken to assess this new method. First, the analytical performance of VWF:AC on an automated coagulo-meter (ACLTop) was determined, and then this new method was compared with VWF:RCo and the platelet function analyzer (PFA100) for 160 patients referred for VWD screening. The VWF:AC achieved acceptable precision with within-run and between-run coefficients of variation ranging from 2.3% to 14.1%, and linearity from 10% to 100%. Despite some marked differences between VWF:AC and VWF:RCo for 10 plasmas tested, their agreement for VWD diagnosis was good. The VWF:AC had sensitivity similar to that of PFA100 (close to 100%), but better specificity (97.7% vs. 66% or 60%, depending on the cartridge used). The good analytical performance, and the sensitivity and specificity of VWF:AC to detect VWF deficiency renders it a suitable method for VWD screening. Our findings support VWF:AC use for the diagnostic work-up of VWD, paying close attention to concomitant clinical signs and bleeding score, as recommended for VWD.