Ristocetin

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Ristocetin is an antibiotic, obtained from Amycolatopsis lurida, previously used to treat staphylococcal infections. It is no longer used clinically because of its toxicity. It causes platelet agglutination and blood coagulation and is used to assay those functions in vitro, e.g. to diagnose von Willebrand disease (vWD) or the Bernard-Soulier syndrome. Platelet agglutination caused by ristocetin can occur only in the presence of large multimers of von Willebrand factor, so if ristocetin is added to blood lacking the factor (or its receptor -- see below), it will not coagulate.

In some types of vWD (types 2B and platelet-type), lower than normal amounts of ristocetin cause platelet aggregation when the patient's platelet-rich plasma is used.¹ This paradox is explained by these types having gain-of-function mutations which cause the vWD high molecular-weight multimers to bind more tightly to their receptors on platelets (the alpha chains of glycoprotein Ib (GPIb) receptors). In the case of type 2B vWD, the gain-of-function mutation involves von Willebrand's factor (VWF gene), and in platelet-type vWD, the receptor is the object of the mutation (GPIb). This increased binding causes vWD because the high-molecular weight multimers are removed from circulation in plasma since they remain attached to the patient's platelets. Thus, if the patient's platelet-poor plasma is used, the ristocetin cofactor assay will not agglutinate "standardized (ie., pooled platelets from normal donors which are fixed in formalin)" platelets, similar to the other types of vWD.

In all forms of the ristocetin assay, the platelets are fixed in formalin prior to the assay to prevent von Willebrand's factor stored in platelet granules from being released and participating in platelet aggregation. Thus, the ristocetin cofactor activity depends only upon high-molecular multimers of the factor present in circulating plasma.

¹McPherson & Pincus: Henry's Clinical Diagnosis and Management by Laboratory Methods, 21st ed., Copyright © 2006 W. B. Saunders Company, pp. 760-2.