Which of the following is a laboratory finding consistent with von Willebrand disease?

Which of the following is a laboratory finding consistent with von Willebrand disease?

• A. Normal ristocetin cofactor activity
• B. Decreased ristocetin cofactor activity
• C. Elevated platelet count
• D. Normal bleeding time

Answer: (B)

The key concept is how von Willebrand factor enables platelets to adhere and form a plug, which is tested by the ristocetin cofactor assay. In this test, ristocetin induces platelets to clump in a manner that depends on functional von Willebrand factor. When vWF is deficient or dysfunctional, this ristocetin-induced platelet agglutination is reduced, so the ristocetin cofactor activity is decreased. That’s why a decreased vWF ristocetin cofactor activity is the lab finding consistent with von Willebrand disease. A normal result would suggest normal vWF function, an elevated platelet count isn’t typical for vWD, and bleeding time is usually prolonged rather than normal in vWD.

The key concept is how von Willebrand factor enables platelets to adhere and form a plug, which is tested by the ristocetin cofactor assay. In this test, ristocetin induces platelets to clump in a manner that depends on functional von Willebrand factor. When vWF is deficient or dysfunctional, this ristocetin-induced platelet agglutination is reduced, so the ristocetin cofactor activity is decreased. That’s why a decreased vWF ristocetin cofactor activity is the lab finding consistent with von Willebrand disease. A normal result would suggest normal vWF function, an elevated platelet count isn’t typical for vWD, and bleeding time is usually prolonged rather than normal in vWD.