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

Summary Introduction The development of an automated, von Willebrand factor ( VWF ) activity assay, Innovance ® VWF Ac ( VWF :Ac), which measures VWF binding to the platelet receptor glycoprotein Ibα 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 : RC o) 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 : RC o method. Although there was good correlation between VWF :Ac and VWF : RC o results among healthy controls and patient subjects, VWF :Ac results were undetectable and/or significantly lower than VWF : RC o 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 : RC o method. While most samples drawn on VWD therapy had similar VWF levels by VWF :Ac and VWF : RC o, a type 2B VWD subject on replacement had much lower activity estimated by VWF :Ac. Conclusion We conclude that Innovance ® VWF Ac is suitable for the diagnosis, classification, and monitoring of VWD , and that it has a number of advantages over VWF : RC o method.