Limitations of Current Plasma VonWillebrand Factor (VWF) Activity Tests: A Comprehensive Comparison of Five VWF Activity Assays.

Accurate measurement of plasma von Willebrand factor (VWF) activity is essential for the laboratory diagnosis and treatment monitoring of von Willebrand disease (VWD).

Currently available VWF activity assays include VWF ristocetin cofactor activity by manual light transmission platelet aggregometry (VWF:RCo–Agg) or flow cytometry (VWF:RCo–FL), collagen I and III binding activity (VWF:Co–I and –III) (Technozym), and platelet activity by latex agglutination (VWF:Lx) (Instrumental Laboratory). In this study we evaluated and compared the accuracy and precision of these 5 assay methods. Plasma samples from 11 normal donors and 41 patients categorized as type 1 (n=20) or type 2 (n=21) VWD based on clinical evaluation, fVIII:C activity, VWF:RCo–Agg, VWF antigen (VWF:Ag) level and plasma VWF multimer analysis by agarose gel electrophoresis were assayed for VWF activity by VWF:RCo–FL, VWF:Co–I, VWF:Co–III and VWF:Lx methods. The VWF:Ag/VWF activity ratio by VWF activity assay method was calculated for each sample. For normal donors and type I VWD patients, VWF:RCo–FL and VWF:Lx correlated well with VWF:RCo–Agg (R²=0.87, and 0.97, respectively), while VWF:Co–I and –III were lower compared to VWF:RCo-Agg. For type 2 VWD patients, different VWF:Ag/VWF activity ratio cutoffs (range 0.3–0.7) were used (Figure). Both VWF:RCo–Agg and –FL were sensitive (95%) and specific (97%) for type 2 VWD while the VWF:Lx was slightly less sensitive (81%) but was very specific (100%). VWF:Co–I and –III were the least sensitive (<90%) and specific (<90%); both methods had high false positive and negative rates for type 2 VWD. In Summary, for normals and type 1 VWD patients, VWF:RCo–FL and VWF:Lx correlate well with VWF:RCo–Agg and have similar sensitivities and specificities for type 2 VWD. VWF:Co–I and –III are unreliable for assessing plasma VWF activity.