From the same study also emerged that the influence of ABO blood groups on plasma VWF levels can make difficult the diagnosis of type 1 VWD since the normal range for VWF:Ag in group O individuals extends below 50 IU/dL (mean ± 2SD: 36–157 IU/dL), which is commonly accepted as the lower normal limit. The gene discussed is VWF; the disease is von Willebrand disease 1.