VWF and hemorrhagic disease: However, up to 60% of patients with an MBD may be diagnosed with a bleeding disorder of unknown cause (BDUC),3 where there is no identifiable cause following haematological evaluation (all current known tests, such as full blood count, prothrombin time, activated partial thromboplastin time, factor VIII, or von Willebrand factor [vWF] antigen, are negative).