Clot formation assays performed with CTI-treated plasmas differentiate FXI-deficient bleeders from nonbleeders. Clotting was triggered in CTI-treated plasmas from healthy individuals and FXI-deficient patients by recalcification and addition of tissue factor and phospholipids. Clot formation was monitored by turbidity. (A) Onset time, (B) rate (Vmax), (C) time to plateau, and (D) peak turbidity change for controls, all FXI-deficient patients, and only FXI-deficient patients with partial deficiency (16-60 IU/dL). Symbols represent plasmas from individual subjects; lines show median and interquartile range. *P < .05, **P < .005. B, bleeders; C, controls; mOD, milli optical density; NB, nonbleeders.