Summary
A multilaboratory collaborative study was undertaken to assess the feasibility of
using a plasma standard for expressing the results of assays for the von Willebrand
factor content of von Willebrand factor concentrates and of factor VIII concentrates.
Thirteen laboratories tested six concentrates for von Willebrand factor antigen, ristocetin
cofactor activity, and multimer content using the World Health Organization plasma
standard for factor VIII/von Willebrand factor, 87/718, as a standard. Only a few
assays were invalid because of nonparallelism or nonlinearity. Significant interlaboratory
and interassay differences were found for both von Willebrand factor antigen and ristocetin
cofactor activity. There was generally good agreement between the laboratories with
respect to the multimer content in the preparations. With respect to assay validity,
a plasma standard could be suitable for assaying concentrated preparations of von
Willebrand factor.
