Validation of an automated latex particle-enhanced immunoturbidimetric von Willebrand factor activity assay

Background: Laboratory diagnosis of von Willebrand disease (VWD) requires accurate measurement of plasma von Willebrand factor (VWF) activity. Objectives: To evaluate laboratory characteristics, diagnostic accuracy and testing utilities of an automated latex particle-enhanced immunoturbidimetric VWF assay (VWF:Lx) based on a monoclonal antibody recognizing the VWF-platelet glycoprotein (GP) Ib binding domain. Methods: Laboratory characteristics including lower detection limit, linearity, precision, sample stability, and method comparison between VWF:Lx and VWF ristocetin cofactor activity by platelet aggregometry (VWF:RCo) were examined. To assess VWF:Lx diagnostic accuracy, 492 patient plasma samples, including 40 previously characterized VWD patient samples, were tested for VWF antigen (VWF:Ag) and VWF:RCo by either aggregometry or flow cytometry, and VWF:Lx with supplemental VWF multimer analysis when indicated. Based on results of VWF:Ag, VWF:RCo and VWF multimer analysis, and available clinical information, samples were categorized as: normal; VWD types 1, 2A/B, 2M, or severe 1 vs. 2M; or acquired VWF abnormalities (AVWA) due to subtle loss of highest molecular weight multimers. Results: VWF:Lx had excellent laboratory characteristics and linear correlation with VWF:RCo (R(2) = 0.93). VWF:Lx accurately classified virtually all normal and VWD patient samples. Compared with VWF:RCo, VWF:Lx had superior sensitivity and specificity for distinguishing severe type 1 vs. 2M VWD and identifying AVWA. A proposed screening panel comprising VWF:Ag and VWF:Lx had 100% and 83% sensitivity for detecting VWD and AVWA, respectively. Conclusions: VWF:Lx has excellent laboratory characteristics and diagnostic accuracy compared with VWF:RCo, and can be used as part of an initial VWD screening panel and as a supplementary test.