VWF and von Willebrand disease type 2A: In comparison, for the diagnosis of hereditary von Willebrand disease type 2A, the ratio of VWF Ristocetin co-factor activity (VWF:RCo) to the VWF antigen level (VWF:Ag) of less than 0.7 [5,[14], [15], [16]] or 0.6 [17,18] is used for the diagnosis.