When distinguishing qualitative vs quantitative vWF defects, which tests are most informative?

• A. vWF antigen (quantity) and ristocetin cofactor activity (function)
• B. Factor VIII level alone
• C. Platelet count alone
• D. PT and aPTT simultaneously

Answer: (A)

Distinguishing qualitative from quantitative von Willebrand disease relies on knowing both how much von Willebrand factor is present and how well it works. The amount is measured with the vWF antigen test, while the function is assessed with the ristocetin cofactor activity test (vWF:RCo), which reflects vWF’s ability to mediate platelet clumping in the presence of ristocetin. If the antigen level is normal or near-normal but the ristocetin cofactor activity is reduced, this points to a qualitative defect—the protein is present but dysfunctional (type 2 vWD). If both the antigen and the ristocetin cofactor activity are reduced, this indicates a quantitative deficiency (types 1 or 3). Other tests, like Factor VIII level, can be affected by vWF levels but don’t distinguish between qualitative and quantitative defects. Platelet count and PT/aPTT lack the specificity to differentiate vWD defect types. So measuring both vWF antigen and ristocetin cofactor activity provides the most informative distinction.