Ristocetin induced platelet agglutination (RIPA) is similar to the Ristocetin Cofactor Assay but RIPA measures platelet agglutination induced by Ristocetin-mediated VWF binding to the platelet Gp1b receptor. WITH RIPA, the ristocetin is added directly to the patient's platelet rich plasma and there are no serial dilutions of the plasma sample.