The Correction Factor for Monitoring Efanesoctocog Alfa Is Not Always 2.5

INTRODUCTION: Efanesoctocog alfa (Altuvoct) is a recombinant super-extended half-life factor (F)VIII concentrate licensed in the United Kingdom for patients with severe and moderate haemophilia A. Concentrate potency is assigned by one-stage assays (OSA) using Actin FSL. Chromogenic FVIII:C assays (CA) and some OSA are inaccurate for Altuvoct; the product information sheet suggests results of these be divided by 2.5 to approximate FVIII:C. UK experts recommend against using correction factors (CF), with guidance that only validated OSA should be used for Altuvoct, or, in the presence of FVIII-mimetics, that CA results are reported with a caveat about assay inaccuracy. METHODS: Over four months, we assessed the impact of a fixed CF on FVIII:C, testing 70 samples collected during pharmacokinetic assessment when patients switched to Altuvoct. All were tested using OSA (Actin FSL and Actin FS) and CA (BIOPHEN and Siemens FVIII:C) on a Sysmex CN-6500 analyser, calibrated against a plasma standard. RESULTS: For samples with Altuvoct between 10 and 269 IU/dL, CF increased as FVIII:C increased when using CA and decreased when using OSA (Actin FS), suggesting that a single CF across that range is inappropriate. CONCLUSIONS: A universal CF for Altuvoct assays may lead to clinically significant FVIII:C underestimation across the measurement range, and is inappropriate in most cases. Should product-specific calibrators (and controls) become available, laboratories should verify their assays for Altuvoct using them so that CF are not required.