Clotting in factor XI-deficient plasma as a function of TF and factor XI. Clot time in factor XI-deficient plasma (<1%) was measured as a function of TF and factor XI (as described in the Materials and Methods). The ordinate is shown in seconds (left-hand axis) and minutes (right-hand axis). Two curves are presented: one for factor XI-deficient plasma without factor XI replacement (<1%, ▪) and a second curve for the same plasma with 1 U/mL factor XI (25 nmol/L, •). Smooth curves following each set of data were drawn, meeting near 24 pmol/L TF. The curves become increasingly divergent as TF concentration is reduced to 6 pmol/L TF, where the difference is most pronounced.