Ristocetin is an antibiotic that causes vWF to bind to GP1b, causing agglutination in normal blood.

In patients with either defective vWF or GP1b (BSS), platelets do not aggregate in the presence of ristocetin. Addition of normal plasma corrects this defect in von Willebrand's disease but not in BSS (because the platelet receptor remains defective).

Reference: First Aid for the USMLE Step 1 (2016): pp. 385

Case Scenario:

An alcoholic cirrhosis case with bleeding diathesis and deranged coagulation profile, particularly PTT; Vit. K is given and observed in 2 days but no improvement. Why?

Ans: likely due to Factor VII deficiency;

not factor VIII, not dietary vit. K deficiency, not platelet, vWF problems

Factor VII, part of the extrinsic pathway, has the shortest half-life of all coagulation factors.