Thrombin Time

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Also known as: TT; Thrombin Clotting Time
Formal name: Thrombin Time

At a Glance

Why Get Tested?

As part of an investigation of a bleeding or thrombotic episode, particularly to evaluate the level and function of fibrinogen; to detect heparin contamination

When to Get Tested?

When you have bleeding or thrombotic episodes, or recurrent miscarriages; when a PT and/or PTT test is prolonged, particularly if abnormal fibrinogen level or function is considered; when heparin contamination of a blood sample is suspected

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


The Test Sample

What is being tested?

The thrombin time (TT) is the time required for a fibrin clot to form following the addition of a standard amount of thrombin to plasma. The TT measures the conversion of fibrinogen to fibrin after thrombin is added, and therefore is affected by the level and/or function of fibrinogen and the presence of inhibitors (e.g., heparin, fibrinogen/fibrin degradation products).

Thrombin and fibrinogen are two coagulation factors that are essential for proper blood clot formation in the body. When a body tissue or blood vessel wall is injured, a process called hemostasis begins to form a plug at the injury site to help stop the bleeding. At the beginning of the process, small cellular fragments called platelets adhere to and then aggregate at the injury site and are activated. At the same time, a process called the coagulation cascade is initiated and coagulation factors are activated. As the process progresses, fibrinogen is converted by thrombin into insoluble fibrin threads. These threads then crosslink together to form a fibrin net that stabilizes at the injury site. The fibrin net adheres to the site of injury along with the platelets to form a stable blood clot. This barrier prevents additional blood loss and remains in place until the injured area has healed.

There must be an adequate amount of platelets and of each of the coagulation factors and each must function normally in order for a stable blood clot to form. Too little or dysfunctional factors can lead to bleeding episodes and/or to inappropriate blood clotting (thrombosis).

There are several laboratory tests that can be used to evaluate hemostasis. It is now understood that coagulation tests are based on what happens artificially in the test setting (in vitro), and thus do not necessarily reflect what actually happens in the body (in vivo). Nevertheless, the tests can be used to evaluate specific components of the hemostasis system. (For more on this, see the explanation of the coagulation cascade). The TT evaluates that part of the hemostatic process in which soluble fibrinogen is changed into fibrin threads. With the addition of thrombin to the test sample, the TT test bypasses the rest of the coagulation factors and focuses on the function of fibrinogen.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Brick, W. et. al. (Updated 2009 November 17). Dysfibrinogenemia. eMedicine [On-line information]. Available online at through Accessed July 2010.

(© 1995–2010). Unit Code 9059: Thrombin Time (Bovine), Plasma. Mayo Clinic, MayoMedical Laboratories [On-line information]. Available online at through Accessed July 2010.

Israels, S. (Updated 2009 February 12. Inherited Abnormalities of Fibrinogen. eMedicine [On-line information]. Available online at through Accessed July 2010.

(Updated 2009 September 18). Thrombin Time [CO004600]. Massachusetts General Hospital, Pathology Service [On-line information]. Available online at through Accessed July 2010.

Dugdale, D. (Updated 2009 March 2). Congenital afibrinogenemia. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed July 2010.

Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 1028-1029.

Clarke, W. and Dufour, D. R., Editors (© 2006). Contemporary Practice in Clinical Chemistry: AACC Press, Washington, DC. Pp 227-238.

Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007 Pp 729-731, 736.