• Donald F Brophy, Erika J Martin, Marisa Ninivaggi, Bassem M Mohammed, John C Barrett, Janice Kuhn, Melinda E Nolte, Emily K Waters, and Mirella Ezban.
• Coagulation Advancement Laboratory, Department of Pharmacotherapy & Outcomes Science, Virginia Commonwealth University (VCU), Richmond, Virginia.
• Haemophilia. 2018 Sep 1; 24 (5): 815-822.

IntroductionThe thrombin generation assay (TGA) can be used to monitor factor replacement therapy in patients with haemophilia. The TGA assay is typically performed using tissue factor as the reaction activator; however, activating with FIXa or FXIa can enhance assay sensitivity when FVIII < 1%.AimsTo evaluate the sensitivity of the TGA when FIXa (5 nmol/L) and FXIa (0.22 nmol/L) are used to activate the assay in platelet-poor plasma and to compare these data to the one-stage and chromogenic assays.MethodsPlasma from 10 severe FVIII-deficient subjects was supplemented with FVIII (0%, 0.1%, 0.4%, 1.2%, 4%, 11% and 33%), using either Novo Eight® , Advate® , Eloctate® , turoctocog alfa pegol or a control standard. The one-stage and chromogenic assays quantified the FVIII levels. The TGA assay was activated using either FIXa or FXIa.ResultsBoth FIXa- and FXIa-activated TGA were sensitive across FVIII concentrations, with intra-assay coefficient of variation (CV) < 10%. The FXIa-activated assay had 25% CV at the lowest level of FVIII compared to 10% CV with FIXa activation. There were strong correlations between the FIXa- and FXIa-activated TGA tests (R2  = 0.9912) and between the one-stage and chromogenic assays (R2  = 0.9469). However, there were poor relationships between the TGA tests and one-stage and chromogenic assays.ConclusionsBoth FIXa- and FXIa activation results in similar TGA profiles across a FVIII range of 0.1%-33%; however, FIXa activation was more robust at the lowest levels of FVIII compared with FXIa activation.© 2018 John Wiley & Sons Ltd.

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