title: Does the presence of von Willebrand factor in FVIII-deficient plasma influences the measurement of FVIII inhibitor titres in haemophilia A patients?
abstract: BACKGROUND
  Reliable measurement of FVIII inhibitor is critical in the follow-up of haemophilia A patients. We performed a multicentre study to evaluate whether the presence of von Willebrand factor (VWF) in FVIII-deficient plasma (FVIII-DP) influences FVIII inhibitor titres.
  
  
  METHODS
  Six French haematology laboratories participated in this study. Three samples with varying FVIII inhibitor titres (1, 5 and 15 BU/mL) and one sample without any detectable FVIII inhibitor were tested using four different procedures for FVIII inhibitor assay. The Nijmegen method and a modified assay with imidazole were performed using FVIII-DP with and without VWF in the control mixture and as substrate plasma in the FVIII one stage assay (OSA). Each mixture (reference and test) was incubated for two hours at 37 °C with buffered normal pool plasma.
  
  
  RESULTS
  Higher inhibitor titres were measured in 5 and 15 BU/mL samples when assays were performed with the Nijmegen method and FVIII-DP without VWF. When samples were diluted in imidazole buffer, similar inhibitor titres, close to expected values, were measured whether VWF was present in the FVIII-DP or not. The data obtained were also more accurate when residual FVIII activity levels between 40% and 60% were used to calculate inhibitor titres, despite a linear type I reaction kinetics.
  
  
  CONCLUSIONS
  These results support the hypothesis that reliable FVIII inhibitor titres can be measured without the use of FVIII-DP containing VWF when an imidazole-modified assay is used.
authors.affiliation: Haemostasis Laboratory, UMR CNRS 7292, Hôpital Trousseau, University François Rabelais Tours, Tours, France.
authors.forename: C
authors.initials: C
authors.lastname: Pouplard
authors.forename: C
authors.initials: C
authors.lastname: Desconclois
authors.forename: F
authors.initials: F
authors.lastname: Sobas
authors.forename: M F
authors.initials: MF
authors.lastname: Aillaud
authors.forename: C
authors.initials: C
authors.lastname: Ternisien
authors.forename: C
authors.initials: C
authors.lastname: Caron
mesh_terms: D019774:Blood Coagulation Factor Inhibitors / Q000097:blood*; D001780:Blood Coagulation Tests / Q000379:methods / Q000592:standards; D005169:Factor VIII / Q000037:antagonists & inhibitors*; D005602:France; D006467:Hemophilia A / Q000097:blood* / Q000175:diagnosis; D006801:Humans; D015203:Reproducibility of Results; D014841:von Willebrand Factor / Q000378:metabolism*
chemical_list: D019774:Blood Coagulation Factor Inhibitors; D014841:von Willebrand Factor; D005169:Factor VIII
keywords: FVIII inhibitor; FVIII-deficient plasma; Nijmegen-Bethesda assay; von Willebrand Factor