Functional Assay of VWF Measured by Its Interaction with Platelet GPIb without the Use of Ristocetin.

The clinical laboratory assessment of von Willebrand factor (VWF) includes measurement of the amount of VWF protein (VWF:Ag) and the functional interaction of VWF with platelets. The universally accepted measure of VWF function utilizes the ability of VWF to bind to platelets (or platelet GPIb) in the presence of ristocetin resulting in VWF-dependent agglutination of platelets measured quantitatively as ristocetin cofactor activity (VWF:RCo) by platelet aggregometry or turbidometry. An international reference standard has been assigned a biologic activity in international units by the WHO and the SSC of the ISTH. VWF:RCo has a recognized wide variability and reproducibility using different assays or when performed in different clinical laboratories. Alternative assays with improved reproducibility have recently been suggested that use recombinant GPIb in an ELISA assay of VWF binding, but this binding still requires ristocetin. Patients have been reported that have reduced interaction with ristocetin such that the VWF:RCo is markedly reduced (