Evaluation of an automated method for measuring von Willebrand factor activity in clinical samples without ristocetin

Introduction The development of an automated, von Willebrand factor (VWF) activity assay, Innovance((R)) VWF Ac (VWF:Ac), which measures VWF binding to the platelet receptor glycoprotein Ib alpha without ristocetin, led us to evaluate the assay for diagnosing von Willebrand disease (VWD) and monitoring therapy. Methods After validating that the assay could be performed on an instrument from a different manufacturer, we compared VWF:Ac to VWF ristocetin cofactor activity (VWF:RCo) findings, including ratios of activity/antigen, for 100 healthy controls and 262 consecutive clinical samples from 217 patients (197 adults, 64 children, n=1 age unknown) referred for VWF testing. Results There was excellent correlation (R-2=0.96) between VWF:Ac results run at two different sites on two different instruments. VWF:Ac had greater precision and sensitivity to low levels of VWF than the VWF:RCo method. Although there was good correlation between VWF:Ac and VWF:RCo results among healthy controls and patient subjects, VWF:Ac results were undetectable and/or significantly lower than VWF:RCo among patients who had types 2A, 2B, or 2M VWD. Additionally, a higher proportion of patient samples were classified as showing qualitative defects using the VWF:Ac compared with VWF:RCo method. While most samples drawn on VWD therapy had similar VWF levels by VWF:Ac and VWF:RCo, a type 2B VWD subject on replacement had much lower activity estimated by VWF:Ac. Conclusion We conclude that Innovance((R)) VWF Ac is suitable for the diagnosis, classification, and monitoring of VWD, and that it has a number of advantages over VWF:RCo method.