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SummaryEssentials    Performance of the one‐stage clotting (OSC) assay varies with the clotting activator used.  Recombinant FIX‐albumin fusion protein (rIX‐FP) was reliably monitored with most OSC reagents.  rIX‐FP shows comparable reagent‐dependent variability to other rFIX products in the OSC assay.  Actin ®  FS and kaolin‐based reagents underestimated rIX‐FP activity by around 50% in the OSC assay.Summary   Background  Measuring factor  IX  activity ( FIX :C) with one‐stage clotting ( OSC ) assays, based on the activated partial thromboplastin time ( APTT ), is the current mainstay of diagnostic techniques for hemophilia B. Assessing the performance of new recombinant  FIX  (r FIX ) products in  OSC  assays is essential, as  APTT  reagents from different manufacturers yield different potency estimates for r FIX .    Objectives  To evaluate the extent to which choice of reagent composition influences r FIX  potency measurements of recombinant  FIX –albumin fusion protein (r IX ‐ FP ,  IDELVION ) activity in  OSC  assays.    Methods  r IX ‐ FP  was added to  FIX ‐deficient plasma, and  FIX :C was assessed centrally and locally in a multicenter international field study with a variety of commercial  OSC APTT  reagents. Paired sample analysis of clinical samples was performed to compare values of  FIX :C from local and central laboratories. In‐house bioanalytical investigations with spiked samples were conducted to compare the  APTT ‐reagent dependent variability of r IX ‐ FP  with unmodified r FIX  and r FIX  Fc fusion protein (r FIXF c).    Results  Central and local assessments of  FIX :C from 10 countries and 21 participating centers showed comparable results to those from the central laboratory across the majority of 18 different  APTT  reagents from both clinical and spiked samples. There was a consistent underestimation of r IX ‐ FP  activity of ≈ 50% with  OSC  assays using Actin  FS  or kaolin‐based  APTT  reagents. In the bioanalytical study, r IX ‐ FP  showed comparable variability in  OSC  assays to unmodified r FIX  and r FIXF c.    Conclusions  r IX ‐ FP  activity can be accurately measured by the use of  OSC  assays with the majority of commercial reagents. Actin  FS  or kaolin‐based reagents will probably lead to a 50% underestimation of activity.

Performance of a recombinant fusion protein linking coagulation factor IX with recombinant albumin in one‐stage clotting assays