Functional assay that measures the ability of ristocetin to promote platelet aggregation in the presence of von Willebrand factor (vWF). Ristocetin is an antibiotic found to agglutinate platelets due to binding of vWF and platelet glycoprotein Ib (GPIb). Ristocetin binds to vWF, inducing a conformational change in vWF which increases its binding to platelets.