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

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.