Recombinant FIX Fc fusion protein activity assessment with the one‐stage clotting assay: A multicenter, assessor‐blinded, prospective study in Japan (J‐Field Study)

INTRODUCTION: The one‐stage clotting assay is used to measure factor IX (FIX) activity in patients’ plasma samples and in FIX products for hemophilia treatment. However, the diversity of reagents and instruments has resulted in significant FIX assay variability. METHODS: The accuracy of the one‐stage clotting assay to measure recombinant FIX Fc fusion protein (rFIXFc) activity was evaluated by major Japanese hemophilia treatment centers and commercial laboratories that measure factor IX activity for a majority of hemophilia B patients in Japan. Plasma‐derived FIX (pdFIX) and recombinant FIX (rFIX) products were used as comparators. FIX‐deficient plasma was spiked with four levels of FIX products based on label potency and measured under blinded conditions by routine one‐stage clotting assay procedures in 19 participating laboratories. Interlaboratory coefficient of variation and spike recovery were calculated. RESULTS: Interlaboratory coefficient of variation of rFIXFc was not significantly different from that of rFIX, but appeared larger than that of pdFIX. Mean spike recovery for rFIXFc was generally comparable to rFIX and pdFIX. However, larger discrepancies between pdFIX and rFIX were observed in three of nine laboratories using ellagic acid‐based activated partial thromboplastin time reagents. CONCLUSION: Recombinant FIX Fc fusion protein activity was found to be similar to that of rFIX or pdFIX by the one‐stage clotting assay. However, minimizing interlaboratory variability is vital for optimizing future patient care.