A sensitive ristocetin co-factor activity assay with recombinant glycoprotein Iba for the diagnosis of patients with low von Willebrand factor levels

The assay of ristocetin co-factor activity of von Willebrand factor (VWF:RCo) is used in the screening of patients with suspected von Willebrand's disease (VWD), the most frequent inherited bleeding disorder. A correct diagnosis of VWD relies on platelet agglutination tests that have a low accuracy within and between assays. A more accurate VWF:RCo assay would improve VWD diagnosis and classification. DESIGN AND METHODS: We describe here an ELISA method in which a recombinant fragment of the alpha-subunit of platelet glycoprotein Ib-IX-V complex (rGPIbalpha) is bound to an anti-GPIbalpha monoclonal antibody immobilized onto microtiter plate wells and which captures plasma VWF in the presence of ristocetin. The results obtained with this ELISA assay were compared blindly with values calculated from the agglutination test in normal subjects (n=60) and in type 1 (n=8), type 2A (n=16), type 2B (n=13), type 2M (n=17) or type 2M Vicenza (n=8) VWD patients which were characterized by low VWF levels. RESULTS: The two assays gave similar results in both normal subjects and VWD patients (r=0.93), but the ELISA test showed higher sensitivity (0.1 versus 6.25 U/dL). The repeatability and reproducibility gave coefficients of variation of 9% and 10%, respectively, for the ELISA, as compared to 14% and 15% for the agglutination test. INTERPRETATION AND CONCLUSIONS: This ELISA assay can be useful in the identification and classification of VWD patients in that it may provide a more accurate distinction between type 2 with abnormal VWF function and type 1 with a low plasma VWF concentration.