VWF and von Willebrand disease (hereditary or acquired): Normal platelet aggregation responses to physiological platelet agonists—with the exception of ristocetin, which induces platelet agglutination by the VWF-mediated crosslinking of the GPIb/V/IX receptor complex—are indicative of biallelic BSS when observed with macrothrombocytopenia and when VWD could be excluded.