Tissue Factor Activity and Antigen Levels during Cardiopulmonary Bypass and Disseminated Intravascular Coagulation.

Background: in healthy subjects small amounts of tissue factor antigen but little if any active tissue factor can be detected circulating in blood. An increased level of circulating active tissue factor has been proposed as a possible mechanism of systemic coagulation activation.

Methods: tissue factor antigen and activity levels were measured in healthy controls (n=20) and in two patient groups associated with systemic coagulation activation: open heart surgery utilizing cardiopulmonary bypass (CPB, n=4) and disseminated intravascular coagulation (DIC, n=15). In heart surgery patients, samples were obtained at 9 time points before, during and after CPB. Samples were obtained from patients with presumed DIC based on clinical history (9 trauma, 4 acute hemorrhage for other reasons, and 1 lymphoma) and an ISTH DIC score of 5 or greater, mean 6, range 5 to 8 (score based on low fibrinogen, low platelet count, high D-dimer and prolonged prothrombin time). DIC samples were obtained during episodes of bleeding and acute resuscitation. Tissue factor antigen was measured using an enzyme immunoassay. Tissue factor activity was measured in citrated plasma using a clot-based assay with corn trypsin inhibitor added to block contact system activation and human activated factor VII added to improve sensitivity to low levels of tissue factor. Both antigen and activity assays were calibrated with lipidated recombinant human tissue factor (rhTF, Recombiplastin) diluted into tissue factor deficient (immunodepleted) plasma. One arbitrary unit (AU) of tissue factor activity was defined as equal to 1 pg of lipidated rhTF. The activity assay showed a linear response between the log of clotting time versus the log of tissue factor concentration.

Results: the reference range for 20 healthy subjects (10 men, 39±15 years and 10 women 38±12 years) was 190±65 pg/mL of tissue factor antigen and 1.4±0.5 AU/mL of tissue factor activity. In the heart surgery patients, baseline levels prior to CPB were fibrinogen 4.37±1.46 mg/mL, tissue factor antigen 125±17 pg/mL and tissue factor activity 1.3±0.3 AU/mL. Within 5 minutes after going on CPB, fibrinogen levels fell 44% then remained stable consistent with hemodilution of the patient’s blood by the priming solution in the CPB circuit. Tissue factor antigen levels followed a similar pattern to fibrinogen falling 49% after starting CPB then remaining constant for the remainder of CPB. In contrast tissue factor activity levels increased 3-fold to 3.6±0.3 AU/mL after starting CPB and remained elevated throughout the procedure. Tissue factor activity was 15-fold higher in patients with DIC during resuscitation (18.7±20.4 AU/mL, p = 0.0006, unpaired t-test) than healthy controls. In contrast, tissue factor antigen was slightly lower in patients with DIC (126±99 pg/mL, p = 0.03, unpaired t-test). In 88 samples including normal, CPB, DIC and other subjects, a comparison of tissue factor antigen and activity results showed little correlation (r² = 0.01)

Conclusions: Elevated tissue factor activity in plasma during cardiopulmonary bypass (3-fold above normal) and during DIC (15-fold above normal) may contribute to the coagulation activation that occurs during these disorders. Tissue factor antigen was not elevated during CPB or DIC during resuscitation. In CPB and DIC patients, tissue factor antigen and activity levels did not change in parallel and represented different aspects of tissue factor in blood.