Influence of blood group, von Willebrand factor levels, and age on factor VIII levels in non‐severe haemophilia A

BACKGROUND: Data on the effect of ABO blood group (ABO), von Willebrand factor (VWF) levels, and age on factor VIII (FVIII) in non‐severe haemophilia A (HA) is scarce. OBJECTIVE: To investigate if ABO, VWF levels, and age have an influence on the variability of FVIII levels and consequently on the assessment of severity in non‐severe HA. PATIENTS/METHODS: Eighty‐nine patients with non‐severe HA and 82 healthy controls were included. Data on ABO was collected and FVIII clotting activity (FVIII:C) with one‐stage clotting assay (FVIII:C OSA) and chromogenic substrate assay (FVIII:C CSA), FVIII antigen (FVIII:Ag) and VWF antigen (VWF:Ag) and activity (VWF:Act) were determined. RESULTS: In HA, FVIII:C OSA and CSA and FVIII:Ag were not different between non‐O (n = 42, median 15.5, interquartile range 10.4‐24.0; 10.0, 6.8‐26.0 and 15.2, 10.7‐24.9) and O (n = 47, 14.1, 9.0‐23.0; 10.0, 5.0‐23.0 and 15.2, 9.3‐35.5), whereas in healthy controls, non‐O individuals had significantly higher FVIII levels. FVIII:C showed no relevant correlation with VWF levels in HA, but we observed strong correlations in healthy controls. Age had only a minor influence in HA, but had a considerable impact on FVIII:C in healthy controls. In multivariable regression analysis ABO, VWF:Ag and age were not associated with FVIII:C in HA, whereas this model explained 61.3% of the FVIII:C variance in healthy controls. CONCLUSIONS: We conclude that in non‐severe HA ABO and VWF levels do not substantially influence the variability of FVIII levels and age has only minor effects on it, which is important information for diagnostic procedures.