Low FXIa concentrations do not trigger TG but promote TG in the presence of added procoagulant stimuli. (A) Effect of increasing FXIa concentrations on thrombin peak height in recalcified, but not activated, plasma (black) and TF-activated plasma (red). The red shaded area indicates the range of silent FXIa concentrations that do not initiate TG but enhance TG when TF is introduced. Shown representative of 2 repeats. (B) TG curves in TF-triggered (0.65 pM TF, 20 mM CaCl₂) NPP (from panel A). Shown representative of 2 repeats. (C) TG curves of the recalcified NPP without TF (from panel A). Shown representative of 2 repeats. (D) Effect of increasing FXIa concentrations on thrombin peak height in recalcified plasma supplemented with the indicated β-thrombin concentrations. Red shaded area indicates the range of silent FXIa from the panel A. Shown representative of 2 repeats.