Which test assesses the functional activity of von Willebrand factor?

• A. Ristocetin cofactor activity
• B. Von Willebrand factor antigen
• C. Factor VIII activity
• D. Platelet aggregation with ADP

Answer: (D)

Ristocetin cofactor activity is the test that directly measures von Willebrand factor’s functional ability to support platelet adhesion. In this assay, patient plasma is evaluated for its capacity to cause ristocetin-induced binding of vWF to the platelet GPIb receptor, leading to platelet agglutination. If vWF is functioning properly, this agglutination occurs; if vWF is deficient or dysfunctional, the reaction is diminished or absent.

Von Willebrand factor antigen, by contrast, tells you how much vWF protein is present but not how well it works. Factor VIII activity reflects the activity of factor VIII, which is carried by vWF but is not a direct measure of vWF function. Platelet aggregation with ADP tests general platelet aggregation ability and is not specific to vWF function.