Comparison of von Willebrand factor platelet-binding activity assays: ELISA overreads type 2B with loss of HMW multimers

Background: A number of new assays with different measuring principles are available to measure VWF GPIb-binding activity, but little is known about how these assays might behave differently for subtypes of VWD. Objectives: The COMPASS-VWF study was designed to compare all available VWF GPIb-binding activity assays for von Willebrand factor. We specifically searched for particular assay behavior differences. Patients/Methods To sort out random differences from systematic assay behavior deviations, all assays were performed in different laboratories on the same samples in a blinded fashion. Samples from 53 normal controls and 42 well-characterized VWD patients were re-analysed in this study to dissect assay-specific discrepancies. Results: No assay behavior differences were found for 53 normal controls. For VWD patients, we found the following systematic assay behavior patterns: (i) All ELISA assays for VWF:GPIbR as well as VWF:GPIbM are insensitive to detect the low VWF activity of VWD type 2B patients with loss of HMW multimers. (ii) VWF:Ab assay reports higher activity for the p.V1665E mutation than all other assays, and (iii) all ristocetin-based assays (including VWF:RCo using fixed platelets) but the AcuStar assay report discrepantly low VWF activity for the p.P1467S polymorphism. No systematic assay-specific difference was observed for either the particle agglutination VWF:GPIbM assay, or the AcuStar assay using magnetic beads. Conclusions: Different assay principles may lead to discrepant results for certain VWD types or mutations. Therefore, a more extensive study for a large number of patients is needed to better characterise the incidence and relevance of such assay specific differences.