Procoagulant effect of factor IX concentrate is caused by contamination with factor IXa. (A) Factor XI–depleted plasma was supplemented with factor IXa at the indicated concentrations. Thrombin generation was initiated with 1 pM tissue factor. (B) Normal pooled plasma was supplemented with recombinant factor IX (FIX B, Benefix) at the indicated concentrations in the presence of inhibitory anti-factor XI antibodies (Moab aFXI). Thrombin generation was initiated with 1 pM tissue factor. (C) Factor XI–depleted plasma was supplemented with recombinant factor IX (FIX B, Benefix) at the indicated concentrations in the presence of inhibitory anti-factor XI antibodies (Moab aFXI). Thrombin generation was initiated with 1 pM tissue factor. (D) Whole blood was spiked with inhibitory anti-factor XI antibodies (Moab aFXI) and recombinant factor IX (FIX B, Benefix) at the indicated concentrations. Thromboelastography was performed with 0.11 pM tissue factor, and the clot times are depicted.