American Board of Family Medicine (ABFM) ITE Practice Test

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.