von Willebrand factor: measuring its antigen or function? Correlation between the level of antigen, activity, and multimer size using various detection systems.

von Willebrand factor (vWF) from normal human plasma was purified and separated into three fractions containing high, medium, and low molecular weight vWF multimers. vWF fractions were tested for (1) vWF-antigen (vWF:Ag); (2) vWF-ristocetin cofactor activity (vWF:RiCof); (3) vWF-collagen binding activity (vWF:CBA); and (4) a monoclonal antibody-binding ELISA (mAB-binding ELISA), based on the vWF binding to immobilized monoclonal antibody directed to the glycoprotein Ib-binding region within the A1 domain of vWF. The three different fractions of vWF showed a correlation between multimer size and vWF:RiCof/vWG:Ag and vWF:CBA/vWF:Ag, respectively. In contrast, results obtained with the mAB-binding ELISA showed identical levels of mAB-binding/vWF:Ag, without regard for the multimer size present in the tested fraction. Our results therefore suggest that in the case of structurally normal vWF the mAB-binding ELISA reflects the concentration of vWF:Ag rather than vWF function. It is feasible that while the mAB-binding ELISA may show reduced levels for abnormal vWF protein, structurally altered within the A1 domain of vWF as in some patients with vWD type 2, this assay does not appear to be suitable for functional analysis of structurally intact vWF.