An external quality assessment program for von Willebrand factor laboratory analysis: an overview from the European concerted action on thrombosis and disabilities foundation.

The laboratory diagnosis of von Willebrand disease (vWD) is complex and requires a panel of different laboratory tests. Because of this complexity, a proper quality control process is necessary. Since 2003, the European Concerted Action on Thrombosis and Disabilities Foundation has provided an external quality control program for several laboratory tests included in the diagnosis of vWD. Currently, ~180 different laboratories participate in this program, of which the vast majority perform both von Willebrand factor (vWF):antigen (Ag) and activity tests. The lowest between-laboratory variation was observed for the vWF antigen assay (10 to 24%), with a better performance for the latex immunoassay (8 to 24%) than the enzyme immunoassay (13 to 25%). Both the ristocetin cofactor activity assay (RCo) and the collagen-binding assay showed a higher between-laboratory variation (20 to 40% and 17 to 29%, respectively). We have observed that the within-laboratory repeatability for normal samples ranged from 0 to 40% for the antigen assay and from 0 to 86% for the ristocetin cofactor activity assay. Normal samples were interpreted correctly by the majority of the participants. However, type 1 vWD samples were wrongly interpreted by 20 to 40% of the participants, which was mainly caused by a discordance in the vWF:RCo/vWF:Ag ratio. It can be concluded that further improvement in the laboratory diagnosis of vWD is necessary.