Which tests best screen for von Willebrand disease?

• A. PT and aPTT
• B. vWF antigen with ristocetin cofactor activity
• C. Platelet aggregation with ADP
• D. Factor IX assay

Answer: (B)

Screening for von Willebrand disease relies on assessing both how much von Willebrand factor is present and how well it functions. Measuring vWF antigen tells you the amount of vWF in the blood, while ristocetin cofactor activity (vWF:RCo) tests vWF’s functional ability to mediate platelet agglutination in the presence of ristocetin. In von Willebrand disease, both the quantity and/or function of vWF can be reduced, so using both tests together gives the most sensitive screen for detecting most forms of the disorder.

Other tests listed are not as informative for screening: PT and aPTT are nonspecific and can be normal in many vWD cases; platelet aggregation with ADP assesses platelet function broadly but not specifically vWF-related pathways; Factor IX assay measures a different coagulation factor entirely.