von Willebrand Factor Assay Proficiency Testing: The North American Specialized Coagulation Laboratory Association Experience

We evaluated the accuracy and precision of von Willebrand disease (vWD) testing performed by up to 50 North American Specialty Coagulation Laboratories from 2004 through 2009, using proficiency samples from healthy subjects (n = 7) and patients with type 1 vWD (n = 7) or type 2 vWD (n = 3). We analyzed 2,212 submitted results. Precision was highest for von Willebrand factor (vWF) antigen assays (coefficient of variation, 14%), which were performed predominantly by latex immunoassays, and lowest for ristocetin cofactor assays (coefficient of variation, 28%), which were increasingly replaced by collagen binding and immunofunctional methods during the 6-year evaluation period. Overall interpretation error rates ranged from 3% for normal samples, 28% for type 1 vWD, and 60% for type 2 vWD. Type 2 vWD samples were correctly identified by all laboratories using collagen binding/antigen ratios but by only one third of laboratories using ristocetin cofactor/antigen or immunofunctional/antigen ratios. In 2009, only 27% (12/45) of laboratories performed vWF multimer analysis, with error rates ranging from 7% to 22%.