Does the presence of von Willebrand factor in FVIII-deficient plasma influences the measurement of FVIII inhibitor titres in haemophilia A patients?

IntroductionReliable 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. MethodsSix French haematology laboratories participated in this study. Three samples with varying FVIII inhibitor titres (1, 5 and 15BU/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 degrees C with buffered normal pool plasma. ResultsHigher inhibitor titres were measured in 5 and 15BU/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. ConclusionThese 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.