Characterization with a simple clotting assay of the tissue factor inhibiting activity in serum

The serum inhibiting activity on tissue factor (TFI) was evaluated as follows: normal human serum was incubated at room temperature with highly diluted rabbit brain thromboplastin (TF) and calcium ions; after 30 min. rabbit brain cephalin, calcium and normal human plasma were added to a small aliquot of TF/serum mixture; the clotting time at 37 degrees C was then recorded. With this assay TFI activity is proportional to the serum level; it is low for incubation periods shorter than 30 min., is missing at TF concentrations higher than 8%, is strictly connected to the presence of F VII and F Xa in the serum sample, depends upon the presence of calcium ions and finally, is independent of antithrombin III. This inhibiting activity also seems to be saturable and reversible.