Bridging the Gap in Laboratory Monitoring of Extended Half-Life Factor VIII and IX: Can Thrombin Generation Assays Overcome Assay Discrepancies?

Abstract: The management of hemophilia A has been transformed by the introduction of extended half-life factor VIII (EHL-FVIII) products. Similarly, the development of extended half-life factor IX (EHL-FIX) products has also impacted the treatment of hemophilia B, improving treatment adherence and reducing bleeding episodes. These bioengineered molecules, created through Fc-fusion, PEGylation, single-chain technologies, or albumin conjugation (particularly for FIX), offer patients reduced infusion frequency while maintaining effective bleed protection. However, the structural modifications of both EHL-FVIII and EHL-FIX pose significant challenges for accurate laboratory monitoring. Traditional one-stage and chromogenic substrate assays often yield discrepant results, varying by reagent, analytical platforms, and product type, complicating clinical decision-making and pharmacokinetic assessments. The thrombin generation assay (TGA) has emerged as a powerful global hemostasis tool that provides a functional readout of a patient's overall coagulation potential. By measuring the enzymatic capacity to generate thrombin over time, TGA offers a more integrated view of hemostatic efficacy than conventional factor activity tests. Although promising, TGA remains limited by lack of standardization, interplatform variability, and absence of validated clinical thresholds. Current data support its use as a complementary tool rather than a replacement for conventional assays. Future prospective studies integrating laboratory and clinical endpoints are required to define its role in precision hemophilia care and to determine whether TGA can better reflect bleeding phenotype and guide individualized therapy. Ultimately, the integration of functional assays into clinical practice will depend on harmonization efforts and demonstration of added clinical value in real-world settings and trials.