Thrombin−Antithrombin Complex

Thrombin-antithrombin complexes (TAT) form covalently following thrombin generation and have a plasma half-life of 10 to 15 minutes. The presence of TAT indicates ongoing thrombin formation and the consumption of antithrombin. Upon activation of coagulation, antithrombin complexes with thrombin as well as other serine proteases. Complex formation is greatly enhanced by the presence of heparin or other glycosaminoglycans. The reaction initially is reversible, but becomes irreversible following the formation of a covalent bond between antithrombin and thrombin. This binding results in complete inhibition of thrombin's activity. Elevated levels of TAT may be associated with advancing age, pregnancy, septicemia, disseminated intravascular coagulation, multiple trauma, acute pancreatitis, acute and chronic leukemia, preëclampsia, acute and chronic liver disease, and other predisposing causes of thrombosis. Increased levels are also reported during heparin and fibrinolytic therapy. TAT levels are markedly reduced in the first 24 hours after receiving oral anticoagulants. The TAT assay can detect the intravascular generation of thrombin and provides valuable information in the diagnosis of thrombotic events. Decreasing TAT levels can also indicate the resolution of a thrombotic event. A normal TAT level in the presence of an elevated D-dimer may indicate an old thrombus. Elevated TAT measurements may be accompanied by increased levels of prothrombin fragment 1+2, fibrinopeptide A, fibrin(ogen) degradation products, and D-dimer. D-dimer has greater sensitivity for detection of deep venous thrombosis.