Laboratory Testing for von Willebrand Factor: Factor VIII Binding for the Diagnosis or Exclusion of Type 2N von Willebrand Disease: An Update.

von Willebrand factor (VWF) is a large adhesive plasma protein that expresses several functional activities. One of these activities is to bind coagulation factor VIII (FVIII) and to protect it from degradation. Deficiency of, and/or defects in, VWF can give rise to a bleeding disorder called von Willebrand disease (VWD). The defect in VWF that affects its ability to bind to and protect FVIII is captured within type 2N VWD. In these patients, FVIII is produced normally; however, plasma FVIII quickly degrades as it is not bound to and protected by VWF. These patients phenotypically resemble those with hemophilia A, where instead, FVIII is produced in lower amount. Both hemophilia A and 2N VWD patients therefore present with reduced levels of plasma FVIII relative to VWF level. However, therapy differs, since patients with hemophilia A are given FVIII replacement products, or FVIII mimicking products; instead, patients with 2N VWD require VWF replacement therapy, since FVIII replacement will only be effective for a short term, given this replacement product will quickly degrade in the absence of functional VWF. Thus, 2N VWD needs to be differentiated from hemophilia A. This can be achieved by genetic testing or by use of a VWF:FVIII binding assay. The current chapter provides a protocol for the performance of a commercial VWF:FVIII binding assay.