Sensitivity of activated factor X (FXa)-initiated thrombin generation to factor XI (FXI). A–B, FXa (0.1 or 0.5 nM) was used to initiate thrombin generation in FXI-depleted plasma (FXI-dep) supplemented with 30 nM FXI or vehicle in the presence or absence of the anti-FXI mAb, 1A6 (30 μg/ml). C, Endogenous thrombin generation potential (ETP), (D) peak thrombin concentration, (E) lag time, and (F) peak time were estimated from the thrombin generation curves recorded for each condition. Data are means ± standard error of the mean (n = 3). *P < .05 with respect to FXI-dep+FXI