Differential sensitivity of von Willebrand factor (VWF) 'activity' assays to large and small VWF molecular weight forms: a cross-laboratory study comparing ristocetin cofactor, collagen-binding and mAb-based assays.

BACKGROUND: von Willebrand disease (VWD), the most common inherited bleeding disorder, is caused by deficiencies and/or defects in von Willebrand factor (VWF). An effective diagnostic and VWD typing strategy requires plasma testing for factor VIII, and VWF antigen plus one or more VWF 'activity' assays. VWF activity is classically assessed by using VWF ristocetin cofactor activity (VWF:RCo), although VWF collagen-binding (VWF:CB) and VWF mAb-based (VWF activity [VWF:Act]) assays are used by some laboratories.
OBJECTIVE: To perform a cross-laboratory study to specifically evaluate these three VWF activity assays for comparative sensitivity to loss of high molecular weight (HMW) VWF, representing the form of VWF that is most functionally active and that is absent in some types of VWD, namely 2A and 2B.
METHODS: A set of eight samples, including six selectively representing stepwise reduction in HMW VWF, were tested by 51 different laboratories using a variety of assays.
RESULTS: The combined data showed that the VWF:CB and VWF:RCo assays had higher sensitivity to the loss of HMW VWF than did the VWF:Act assay. Moreover, within-method analysis identified better HMW VWF sensitivity of some VWF:CB assays than of others, with all VWF:CB assays still showing better sensitivity than the VWF:Act assay. Differences were also identified between VWF:RCo methodologies on the basis of either platelet aggregometry or as performed on automated analyzers.
CONCLUSIONS: We believe that these results have significant clinical implications for the diagnosis of VWD and monitoring of its therapy, as well as for the future diagnosis and therapy monitoring of thrombotic thrombocytopenic purpura.