Ristocetin-induced platelet aggregation

The ristocetin-induced platelet aggregation (RIPA) is an ex vivo assay for live platelet function. It measures platelet aggregation with the help of von Willebrand factor (vWF) and exogenous antibiotic ristocetin added in a graded fashion^[1] used to diagnose von Willebrand disease (vWD). It is similar to the ristocetin cofactor activity but has the added benefit in that it can diagnose type 2B vWD and Bernard-Soulier syndrome because it uses patient's live endogenous platelets, whereas Ristocetin Cofactor Assay tests the function of only the vWF and not the platelets. Riscocetin cofactor assay uses platelet-poor plasma (with vWF but no platelets) and adds ristocetin and exogenous formalin-fixed platelets which can passively agglutinate (but not actively "aggregate", as they are dead).

In an unknown fashion, 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 of fixed platelets or initiates the initial agglutination phase of aggregation of live platelets. In von Willebrand disease, where von Willebrand factor is absent or defective, abnormal agglutination occurs:

• In type 1 vWD: hypoactive agglutination occurs
• In type 2A vWD: hypoactive agglutination occurs
• In type 2B vWD: hyperactive agglutination occurs
• In type 2M vWD: hypoactive agglutination occurs activity^[2]
• In type 2N vWD: normal agglutination occurs
• In type 3 vWD: no agglutination occurs

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