Measurement of von Willebrand factor binding to a recombinant fragment of glycoprotein Ib alpha in an enzyme-linked immunosorbent assay-based method: performances in patients with type 2B von Willebrand disease

Type 2B von Willebrand disease (VWD) is characterised by an increased affinity of von Willebrand factor (VWF) for its platelet receptor glycoprotein Ib (GPIb). This feature is usually studied in vitro by a ristocetin-dependent VWF platelet-binding assay, which has some limitations as it requires [e.g. (radio)-labelled anti-VWF antibodies and normal formaldehyde-fixed platelets]. We, here, extended the applicability of an enzyme-linked immunosorbent assay-based method previously described for the measurement of ristocetin co-factor activity that used a recombinant fragment of GPIb (rfGPIb alpha) and horseradish peroxidase-labelled rabbit anti-human VWF antibodies for measuring the captured ristocetin-VWF complexes on the rfGPIb alpha. Thirty-one type 2B VWD patients from 15 families with eight different known mutations were studied. VWF in plasma from 28 of these patients bound better than normal VWF at 0.2 mg/ml ristocetin, with the ratio, optical density (OD) patient/OD normal pool plasma, higher than 1.8. For two of the three other patients with no enhanced response of plasma VWF, the platelet lysate VWF showed an enhanced binding capacity; for the last patient, the results in other members of the family are unequivocal. We conclude that, this new method for measurement of plasma or platelet VWF-binding capacity offers great advantages for correct type 2B VWD diagnosis.