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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 excessive bleeding or inappropriate blood clot formation (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?

Thrombin is an enzyme in blood that acts on the clotting factor fibrinogen to form fibrin, helping blood to clot. The thrombin time (TT) assesses the activity of fibrinogen.

When an injury occurs and bleeding begins, the body begins to form a clot at the injury site to help stop the bleeding. Small cell fragments called platelets adhere to, aggregate, and are activated at the injury site. At the same time, the coagulation cascade begins and proteins called coagulation factors, including fibrinogen, are activated. Fibrinogen is then converted by thrombin into insoluble threads called fibrin that crosslink together to form a fibrin net that adheres to the injury site. Along with the platelets adhering, this forms a stable blood clot and prevents additional blood loss, remaining in place until the injury has healed.

For a stable clot to form, there must be enough normally functioning platelets and coagulation factors. If there are dysfunctional factors or platelets, or too few of them, it can lead to bleeding episodes and/or to inappropriate blood clotting (thrombosis).

The thrombin time evaluates that part of the hemostatic process where soluble fibrinogen is changed into fibrin threads. It measures the time required for a fibrin clot to form following the addition of a standard amount of thrombin to plasma. It is affected by the level and/or function of fibrinogen and the presence of inhibitors (e.g., heparin, fibrinogen/fibrin degradation products, direct thrombin inhibitor). With the addition of thrombin to the test sample, the thrombin time bypasses the rest of the coagulation factors and focuses on the function of fibrinogen.

It is now understood that blood 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, there are several laboratory tests used to evaluate specific components of the hemostasis system. (For more on this, see the explanation of the Coagulation Cascade).

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.

Sources Used in Current Review

Teruya, J., et al (Updated 2014 January 30). Thrombin Time. Medscape. Available online at through Accessed April 2014.

Burgess, R. et. al. (Updated 2012 January 10). Dysfibrinogenemia. Medscape. Available online at through Accessed April 2014.

(© 1995–2014). Unit Code 9059: Thrombin Time (Bovine), Plasma. Mayo Clinic, MayoMedical Laboratories. Available online at through Accessed April 2014.

Basala, V. (Updated 2012 May 6). Inherited Abnormalities of Fibrinogen. eMedicine. Available online at through Accessed April 2014.

Wilczynski, C. et al. (Updated 2014 February 12). Fibrinogen. Medscape. through Accessed April 2014.

Pagana, K. D. & Pagana, T. J. (© 2013). Mosby's Diagnostic and Laboratory Test Reference 11th Edition: Mosby, Inc., Saint Louis, MO. Pp 445-446.

(© 2014). Thrombic Risk Reflexive Panel. ARUP Laboratories. Available online at through Accessed April 2014.

(© 1995–2014). Reptilase Time, Plasma. Mayo Clinic, MayoMedical Laboratories. Available online at through Accessed April 2014.

Sources Used in Previous Reviews

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.