Given the following laboratory results: normal platelets, slightly prolonged aPTT, normal platelet aggregation studies, and abnormal aggregation with collagen, ADP, and ristocetin, what is the appropriate reflex test?

• A. Ristocetin cofactor assay
• B. D-dimer
• C. PK screen
• D. TT

Answer: (A)

Ristocetin-induced platelet aggregation depends on von Willebrand factor binding to platelet GPIb; when vWF is deficient or dysfunctional, aggregation with ristocetin is impaired. The reflex test to confirm this pattern is the ristocetin cofactor assay, which specifically measures vWF activity. A reduced ristocetin cofactor activity supports von Willebrand disease and explains the slight prolongation of the aPTT due to less stabilization of factor VIII. The other tests listed do not assess vWF function and therefore aren’t the appropriate reflex test in this scenario.