Measures the ability of a patient’s plasma to agglutinate formalin fixed platelets in the presence of ristocetin.
The rate of ristocetin induced agglutination is related to the concentration of Von Willebrand factor, and the percent normal activity is obtained from a standard curve.
Values are determined by comparison to a standard curve, which allows quantization of percent ristocetin cofactor activity.
Ristocetin is a glycopeptide antibiotic, obtained from Amycolatopsis lurida, previously used to treat staphylococcal infections.
It is no longer used clinically because it caused thrombocytopenia and platelet agglutination.
It is used solely to assay those functions in vitro in the diagnosis of conditions such as von Willebrand disease (vWD) and Bernard–Soulier syndrome.
Platelet agglutination caused by ristocetin can occur only in the presence of von Willebrand factor multimers, so if ristocetin is added to blood lacking the factor the platelets will not clump.
The antibiotic ristocetin causes von Willebrand factor to bind the platelet receptor glycoprotein Ib (GpIb), so when ristocetin is added to normal blood, it causes agglutination.
In some types of vWD (types 2B and platelet-type), even very small amounts of ristocetin cause platelet aggregation when the patient’s platelet-rich plasma is used.
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 the platelet alpha chains of glycoprotein Ib (GPIb) receptors.
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.
The ristocetin cofactor activity depends only upon high-molecular multimers of the factor present in circulating plasma.