Comparison of automated von Willebrand factor activity assays.

INTRODUCTION: Von Willebrand Disease (VWD) is the most common inherited bleeding disorder. Measurement of von Willebrand factor (VWF) activity in plasma is often based on platelet agglutination stimulated by the ristocetin cofactor activity. Novel assays, based on latex beads with recombinant glycoprotein Ib instead of platelets, have recently been developed but it is unclear whether these can improve the diagnostic capability for VWD. AIM: To compare four automated VWF activity methods in a mixed population of patients referred for evaluation of bleeding tendency. METHODS: The analytical performances of three ristocetin and one non-ristocetin cofactor activity assays were compared in 170 consecutive plasma samples from patients referred for VWD evaluation. RESULTS: All methods correlated well with concordance correlation coefficients ranging from 0.90-0.95. However, when comparing the VWF activity/antigen ratios in samples classified as having VWD (activity <0.4 IU/mL) the number of samples below a ratio of 0.7 differed between 16 and 8%. CONCLUSION: Despite overall correlation between assays we found that differences in classification power might interfere with the interpretation of individual samples.