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

blood test A blood test is a medical laboratory, 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 ...

that measures the time it takes for a

clot A thrombus ( thrombi) is a solid or semisolid aggregate from constituents of the blood (platelets, fibrin, red blood cells, white blood cells) within the circulatory system during life. A blood clot is the final product of the blood coagulatio ...

to form in the plasma of a blood sample containing

anticoagulant An anticoagulant, commonly known as a blood thinner, is a chemical substance that prevents or reduces the coagulation of blood, prolonging the clotting time. Some occur naturally in blood-eating animals, such as leeches and mosquitoes, which ...

, after an excess of

thrombin Prothrombin (coagulation factor II) is encoded in the human by the F2-gene. It is proteolytically cleaved during the clotting process by the prothrombinase enzyme complex to form thrombin. Thrombin (Factor IIa) (, fibrose, thrombase, throm ...

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 thrombus, blood clot. It results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The process of co ...

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 Fibrinogen (coagulation factor I) is a glycoprotein protein complex, complex, produced in the liver, that circulates in the blood of all vertebrates. During tissue and vascular injury, it is converted Enzyme, enzymatically by thrombin to fibrin ...

(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 suspected to contain

heparin Heparin, also known as unfractionated heparin (UFH), is a medication and naturally occurring glycosaminoglycan. Heparin is a blood anticoagulant that increases the activity of antithrombin. It is used in the treatment of myocardial infarction, ...

, a substance derived from snake venom called

batroxobin Batroxobin, also known as reptilase, is a snake venom Snake venom is a highly toxic saliva containing zootoxins that facilitates in the immobilization and digestion of prey. This also provides defense against threats. Snake venom is usually i ...

(formerly

reptilase 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 a ...

) is used for comparison to thrombin time. Batroxobin has a similar action to thrombin but unlike thrombin it is not inhibited by heparin, so

reptilase time Reptilase time (RT) is a blood test used to detect deficiency or abnormalities in fibrinogen, especially in cases of heparin contamination. Reptilase, an enzyme found in the venom of Bothrops snakes, has activity similar to thrombin. Unlike th ...

and thrombin time can be used concurrently to distinguish anticoagulant effect from

hypofibrinogenemia Fibrinogen deficiency, also known as factor I deficiency, is a rare inherited bleeding disorder related to fibrinogen function in the coagulation cascade. It is typically subclassified into four distinct fibrinogen disorders: afibrinogenemia, hy ...

or dysfibrinogenemia.Mackie I, Casini A, Pieters M, Pruthi R, Reilly-Stitt C, Suzuki A. International council for standardisation in haematology recommendations on fibrinogen assays, thrombin clotting time and related tests in the investigation of bleeding disorders. Int J Lab Hematol. 2024 Feb;46(1):20-32. doi: 10.1111/ijlh.14201. Epub 2023 Nov 20. PMID: 37984807. Normal values for thrombin time may be 12 to 14 seconds, but the test has significant reagent variability. 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. Thrombin time is not affected by anti-Xa anticoagulants such as

rivaroxaban Rivaroxaban, sold under the brand name Xarelto among others, is an anticoagulant medication (blood thinner) used to treat and reduce the risk of blood clots. Specifically it is used to treat deep vein thrombosis and pulmonary emboli and prev ...

apixaban Apixaban, sold under the brand name Eliquis, is an anticoagulant medication used to treat and prevent blood clots and to prevent stroke in people with nonvalvular atrial fibrillation through directly inhibiting factor Xa. It is used as an al ...

, but is very sensitive to

direct thrombin inhibitors Direct thrombin inhibitors (DTIs) are a class of medication that act as anticoagulants (delaying coagulation, blood clotting) by directly inhibiting the enzyme thrombin (factor IIa). Some are in clinical use, while others are undergoing clinical dev ...

including

dabigatran Dabigatran, sold under the brand name Pradaxa among others, is an anticoagulant used to treat and prevent blood clots and to prevent stroke in people with atrial fibrillation. It is commonly used to prevent blood clots following hip or kn ...

argatroban Argatroban is an anticoagulant that is a small molecule direct thrombin inhibitor. In 2000, argatroban was licensed by the US Food and Drug Administration (FDA) for prophylaxis or treatment of thrombosis in people with heparin-induced thrombocyt ...

, and

bivalirudin Bivalirudin, sold under the brand names Angiomax and Angiox, among others, is a specific and reversible direct thrombin inhibitor (DTI). Chemically, it is a synthetic congener of the naturally occurring drug hirudin, found in the saliva of t ...

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

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 Citric acid is an organic compound with the formula . It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry Biochemistry, or biological chemistry, is the study of chemical processes within and relati ...

oxalate Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula . This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (), and several esters such as ...

additive (if using the

vacutainer A vacutainer blood collection tube is a sterile glass or plastic test tube with a colored rubber stopper creating a vacuum seal inside of the tube, facilitating the drawing of a predetermined volume of liquid. Vacutainer tubes may contain addit ...

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 of the thrombin clotting time is generally <22 seconds, and often 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. Variability arises from differences in thrombin concentration, dilution of plasma, presence and/or concentration of calcium ions, as well as the influence of analyser type. TT may also be sensitive to citrate pH. 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