It is now 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 (or its receptor—see below), the platelets will not clump.
The assay is now used exclusively for evaluating platelet function in vitro for diagnosing conditions like von Willebrand disease and Bernard–Soulier syndrome. Platelet aggregation induced by ristocetin only occurs when von Willebrand factor multimers are present; therefore, if ristocetin is added to blood lacking the factor or its receptor (see below), platelets will not clump together.