Validation of an automated assay to determine activity of ristocetin cofactor of von Willebrand factor

Von Willebrand disease (VWD) is caused by a defective qualitative or quantitative von Willebrand factor (VWF) molecule. Low VWF:Ag suggests but does not distinguish VWD subtypes. Therefore, it is also necessary to determine the functionality of VWF to complete the diagnosis. The reference method to study VWF function is the ristocetin cofactor assay (VWF:RCo) based on the VWF ability to induce platelet aggregation in the presence of ristocetin. Recently, automated methods for determining the activity of ristocetin cofactor have been developed. The aim of this study is to evaluate the performance of VWF: RCo automated immunoturbidimetric assay: with VWF activity assay (VWF: Act) used as a screening of VWD and the traditional VWF: RCo test by aggregometry. Intra-assay precision was 3.1% and interassay precision was: 3.2% in normal control and 5.1% low control: When VWF: Act and immunoturbidimetric VWF: RCo results were compared in 60 patients, the correlation coefficient was 0.96 with a -3.5% bias. In a subset of 30 patients VWF:Rco was compared, determined by aggregation and in an automated manner, yielding a correlation of 0.90 with an 18.9% bias. VWO: Rco values obtained were -with a 15% allowable total error-statistically comparable with those determined by the VWF: Act method, and the VWF: RCo values determined by aggregometry in the patients studied.