PB1226 Acquired Haemophilia A in Finland

Methods: Inhibitor concentration of ≥0.6 was defined as positive for NBA.Residual activity was measured with a one-stage FVIII assay using SynthasIL and HemosIL calibration plasma (Werfen, Barcelona).For patients on emicizumab residual FVIII activity was measured with a chromogenic assay using bovine reagents (Coamatic, Chromogenix, Werfen).ELISA was performed with Lifecodes factor VIII antibody screen (Immucor, Paris, France).Results: A total of 44 samples from 10 patients with AHA were analyzed with both NBA and ELISA.Results were concordant for 31 samples with both techniques: 26 were positive and 5 negative.For 10 samples we had a discrepancy with positive NBA (0.6-18.3 BU) and negative ELISA.These samples were issued from 5 different patients.One patient was never positive with ELISA during the course of his disease (lupus anticoagulant was ruled out).For the other 4 patients discrepant results were only seen during follow-up and not at diagnosis.In three samples (from 2 patients) a borderline NBA (0.4-0.5 BU) with a weak positive ELISA was seen. Table:Distribution of negative and positive inhibitor samples for the Nijmegen Bethesda assay and the anti-FVIII ELISA. Conclusion(s):In our small patient group with AHA we saw several discrepant results between NBA and anti-FVIII ELISA.NBA was often positive with a negative ELISA.In the majority of patients this was seen in follow-up samples.These results should be taken into consideration when deciding which assay to perform for the detection of FVIII inhibitors.