VWF and Von Willebrand disease: Although there are differences in the reference ranges cited by coagulation laboratories, the normal plasma range for both factor VIII and factor IX is 50–200 IU/dL.6 In practical terms, normal equates to levels of factor VIII, factor IX and von Willebrand factor antigen > 50 IU/dL for haemophilia A, haemophilia B and type 1 von Willebrand disease.