INTRODUCTION: Patients with Haemophilia A treated with Mim8 may require concomitant Factor VIII (FVIII) replacement
hence, accurate assessment of FVIII activity (FVIII:C) and FVIII inhibitors in the presence of Mim8 is important. AIM: Assess which chromogenic substrate assays (CSAs) accurately monitor FVIII:C and FVIII inhibitor levels in the presence of Mim8. METHODS: FVIII and Mim8 were spiked into congenital FVIII-deficient plasma to obtain mixtures containing various FVIII:C levels (0-100 IU/dL) and Mim8 concentrations (0, 3, 6 and 12 µg/mL). Five CSAs using different activated Factor IX and X sources (bovine, bovine-human or human) were used to measure FVIII:C. Bovine and bovine-human CSAs were then used to measure FVIII:C of standard half-life (SHL) and extended half-life (EHL) FVIII products. FVIII inhibitor levels were assessed using two different bovine CSAs in congenital FVIII-deficient plasma spiked with various Mim8 concentrations (5, 10, 20 and 40 µg/mL) and FVIII inhibitor levels (0.2, 1.0 and 4.8 BU). RESULTS: High levels of interference were observed using human CSAs. Bovine CSAs accurately measured FVIII:C of SHL and EHL FVIII products in the presence of Mim8 without interference. In bovine-human CSAs, interference was observed at 5 IU/dL FVIII, increasing up to four-fold with increasing Mim8 levels. FVIII inhibitor levels were accurately measured using bovine CSAs without Mim8 interference. CONCLUSION: FVIII:C of SHL and EHL products and FVIII inhibitor levels can be accurately monitored in the presence of Mim8 using bovine CSAs at all FVIII levels, and bovine-human CSAs at FVIII concentrations >
20 IU/dL.