FXIII-A₂ increases circulating FXIII-B in humans. (A) Time course of FXIII(a) activity measured by chromogenic assay after infusion of 35 IU/kg recombinant FXIII-A₂ in 23 patients with FXIII deficiency (mentor2 trial, adapted from Kerlin et al²⁶). (B-D) Plasma concentrations of FXIII-A₂ (B), FXIII-A₂B₂ heterotetramer (C), and free FXIII-B (lower detection limit, 0.845 μg/mL) (D) after FXIII-A₂ infusion in these patients, determined by ELISA. (E) Estimated total FXIII-B was determined by calculating the sum of FXIII-B in heterotetrameric FXIII-A₂B₂ and free FXIII-B at each timepoint. Curves from individual subjects are indicated in gray, and the geometric mean is shown with a colored line in each graph. The x-axis represents nominal time. Predose, baseline measurements (“Pre”) are indicated in each graph. ELISA, enzyme-linked immunosorbent assay; IU, international units.