Thrombin time (TT) test

Thrombin is an enzyme in blood that acts on fibrinogen (a clotting factor) to form fibrin, causing blood to clot. The thrombin time (TT), also known as the thrombin clotting time (TCT), is a blood test that measures the time it takes for a fibrin clot to form in the plasma of a blood sample. This assesses the activity of fibrinogen and is used to investigate excessive bleeding or inappropriate blood clot formation. The TT test is also sensitive to the presence of inhibitors in the plasma, such as heparin. The reference range for the thrombin time is usually less than 20 seconds. Prolonged time is suggestive of fibrinogen deficiency, abnormal fibrinogen structure or inhibition of thrombin by fibrinogen degradation products or heparin.