PB1141 - Congenital Haemophilia A with very low titre inhibitor, undetectable by routine methods and presenting with discrepant factor VIII assays post treatment with recombinant FVIII

Background: An 8 week old male presented with a subdural haemorrhage and was diagnosed with severe Haemophilia A (SHA). He was managed conservatively with 250IU recombinant FVIII (Advate) bi-daily (BD). Factor VIII recovery was disappointing leading to suspicion of an inhibitor being developed.

Aims: Assess the use of the Anti-FVIII ELISA for low level inhibitor detection
Report of findings of discrepant FVIII assay results in post treatment dose samples

Methods: Chromogenic (CFVIII) (Hyphen Biomed, Neuville-Sur-Oise, France) and one-stage (OS) FVIII were performed on the ACL TOP 750 (Werfen, Bedford, USA). Anti-FVIII ELISA (Hyphen Biomed) and Bethesda assays were performed on heat treated plasma.

Results: From day 18 of admission, the patient displayed reduced half-life and increments, with post dose levels only reaching 0.42IU/mL. This prompted dose escalation to 250IU 6 hourly. OS FVIII and CFVIII results became discrepant, with results of the CFVIII being double that seen in the one stage assay. The Bethesda assay was positive on one occasion. However, by Anti-FVIII ELISA, one sample was positive and one borderline. By day 33, increments hadn’t improved and the patient was immune-tolerated with 500IU rFVIII BD. The inhibitor was eradicated as determined by both methods and factor assays were in agreement.

Conclusion(s): In severe Haemophilia A, if suspected, timely identification of an inhibitor is essential. In this case, the Anti-FVIII ELISA was more sensitive to the low-level inhibitor and better reflected the clinical picture of the patient. The factor assay discrepancy also appeared to correlate with clinical progression and resolved following immune-tolerance. Parallel testing of FVIII is not routine practice so it is unknown whether this is a common finding but further investigation is merited. Possible mechanisms may be the prolonged incubation in the chromogenic assay, different dilutions, as well as the supraphysiological factor concentrations present in chromogenic reagents (Marlar 2020).