Drug‐Specific Calibration: A Solution to Reagent Variability in Extended Half‐Life Factor IX Activity Measurements

ABSTRACT Introduction Discrepancies in factor IX activity (FIX:C) measurements between one‐stage clotting assays (OSAs) have been observed following infusion with recombinant factor IX extended half‐life concentrates (EHL‐rFIX) in the treatment of haemophilia B. These variations, primarily due to differences in activated partial thromboplastin time (APTT) reagents, complicate clinical decision‐making. Objectives The aim of this study was to evaluate whether drug‐specific calibrations for albumin‐fused recombinant FIX (rFIX‐FP) and Fc‐fused recombinant FIX (rFIX‐Fc) could reduce inter‐reagent discrepancies. Methods In a multicentre field study involving 20 laboratories, plasma samples spiked with rFIX‐FP, rFIX‐Fc or standard rFIX were tested using different APTT reagents. FIX:C was measured with usual and drug‐specific calibration. Data were analysed for compliance (target range: 80%–120% of expected values), interlaboratory variability and intralaboratory reproducibility. Results Usual calibration resulted in significant discrepancies among reagents, with compliance rates varying widely. Drug‐specific calibration significantly improved compliance for all reagents tested, except for one concentration with rFIX‐Fc. Interlaboratory variability decreased markedly, with coefficients of variation dropping from 19.4%–36.0% (usual calibration) to 6.0%–12.4% (specific calibration). Intralaboratory reproducibility was consistent whatever the type of calibration. Conclusion Drug‐specific calibration for EHL‐rFIX reduces reagent‐related variability in OSA FIX assays, ensuring reliable and standardised results. This approach facilitates monitoring with a single reagent system, improving laboratory efficiency. Wider availability of validated calibrators remains crucial for broader adoption and standardisation.