The thrombin time (TT), also known as the thrombin clotting time (TCT), is a

blood test A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a hypodermic needle, or via fingerprick. Multiple tests for specific blood components, such as a glucose test or a cholester ...

that measures the time it takes for a clot to form in the plasma of a blood sample containing

anticoagulant Anticoagulants, commonly known as blood thinners, are chemical substances that prevent or reduce coagulation of blood, prolonging the clotting time. Some of them occur naturally in blood-eating animals such as leeches and mosquitoes, where the ...

, after an excess of thrombin has been added. It is used to diagnose blood

coagulation Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism o ...

disorders and to assess the effectiveness of fibrinolytic therapy. This test is repeated with pooled plasma from normal patients. The difference in time between the test and the 'normal' indicates an abnormality in the conversion of fibrinogen (a soluble protein) to fibrin, an insoluble protein. The thrombin time compares the rate of clot formation to that of a sample of normal pooled plasma. Thrombin is added to the samples of plasma. If the time it takes for the plasma to clot is prolonged, a quantitative (fibrinogen deficiency) or qualitative (dysfunctional fibrinogen) defect is present. In blood samples containing

heparin Heparin, also known as unfractionated heparin (UFH), is a medication and naturally occurring glycosaminoglycan. Since heparins depend on the activity of antithrombin, they are considered anticoagulants. Specifically it is also used in the treatm ...

, a substance derived from snake venom called

batroxobin Batroxobin, also known as reptilase, is a snake venom enzyme with Venombin A activity produced by ''Bothrops atrox'' and ''Bothrops moojeni'', venomous species of pit viper found east of the Andes in South America. It is a hemotoxin which acts as ...

(formerly reptilase) is used instead of thrombin. Batroxobin has a similar action to thrombin but unlike thrombin it is not inhibited by heparin. Normal values for thrombin time are 12 to 14 seconds. If batroxobin is used, the time should be between 15 and 20 seconds. Thrombin time can be prolonged by heparin, fibrin degradation products, and fibrinogen deficiency or abnormality.

Test procedure

After separating the plasma from the whole blood by

centrifugation Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate ...

, bovine thrombin is added to the sample of plasma. Clot formation is detected optically or mechanically by a coagulation instrument. The time between the addition of the thrombin and the clot formation is recorded as the thrombin clotting time.

Specimen requirements

Whole blood is taken with either citrate or oxalate additive (if using the vacutainer system, this is a light blue top tube). As with other coagulation assays, the tube must not be over- or under-filled in order to ensure the correct anticoagulant-to-blood ratio: one part anticoagulant per nine parts blood.

Reference ranges

The

reference ranges In medicine and health-related fields, a reference range or reference interval is the range or the interval of values that is deemed normal for a physiological measurement in healthy persons (for example, the amount of creatinine in the blood, or ...

of the thrombin clotting time is generally <22 seconds, and from 14 to 16 seconds. Laboratories usually calculate their own ranges, based on the method used and the results obtained from healthy individuals from the local population. Separate ranges are used for infants.

Limitations

Blood samples that are more than eight hours old can give inaccurate results when tested.

References

External links