Use of the collagen‐binding assay for von Willebrand factor in the analysis of type 2M von Willebrand disease: a comparison with the ristocetin cofactor assay

Open Access
Publisher Website
Google Scholar
Abstract
Summary. This study compares the utility of two functional assays for von Willebrand factor (VWF), the ristocetin cofactor assay (VWF:RCo) and the collagen‐binding assay (VWF:CBA). We analysed a group of 32 patients with type 2 von Willebrand disease (VWD) (25 patients with type 2M, six with type 2A and one with type 2B) and 22 normal control subjects. VWF:RCo/VWF antigen (VWF:Ag) ratios and VWF:CBA/VWF:Ag ratios were compared between the patient and control groups. In the six patients with type 2A VWD, both VWF:RCo/VWF:Ag ratios and VWF:CBA/VWF:Ag ratios were discordant (≤ 0·7). In the 25 type 2M VWD patients, the VWF:CBA/VWF:Ag ratios were concordant (> 0·7), but the VWF:RCo/VWF:CBA ratios were discordant (≤ 0·7) (P = 0·001) compared with control subjects. Thus, VWF:RCo/VWF:Ag ratios were discordant in both type 2M and 2A VWD patient groups indicating a functional abnormality. However, VWF:CBA/VWF:Ag ratios were discordant in the type 2A VWD group but not in the type 2M VWD group. Our study showed that VWF:CBA is sensitive to functional variants associated with the loss of high‐molecular‐weight multimers, i.e. type 2A and 2B in VWD, but the assay was unable to discriminate defective platelet‐binding VWD variants with normal multimeric patterns such as type 2M VWD. It was concluded that the VWF:CBA assay should be used in association with rather than as a replacement for the VWF:RCo assay.