Performance Evaluation of Different FIX Activity Assays for Determining Nonacog Beta Pegol (N9-GP, Refixia®) Activity Using Reagent-/Platform-Specific Conversion Factors

Nonacog beta pegol (N9-GP) is a glycoPEGylated FIX replacement product with extended half-life for treatment of haemophilia B patients. Monitoring of N9-GP with clotting-based one-stage FIX assays is complicated by high variations, mainly due to reagent-specific interference with polyethylene glycol. In 11 distinct specialized coagulation laboratories in Austria, N9-GP spiked samples were measured in replicates in two distinct surveys, 3 years apart, using five different one-stage assay reagents and one chromogenic FIX assay. Regression analysis was used to investigate if back-calculation of N9-GP levels is feasible. We could demonstrate a linear relationship between the spiked N9-GP concentration and measured FIX activity levels for all examined assays, suggesting that N9-GP activity may be back-calculated using reagent-/platform-specific conversion factors. Within-laboratory variation after 3 years was acceptable in most, but not all, laboratories. We demonstrate that back-calculation of N9-GP activity levels may be possible when using one-stage FIX assays. However, we recommend that every laboratory ascertain its own conversion factor. When measuring real patient samples, we encourage simultaneous measurement of N9-GP spiked control material with known concentrations to ensure the validity of the current back-calculation.