• Also Known As:
  • TT
  • Thrombin Clotting Time
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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?


You may be able to find your test results on your laboratory’s website or patient portal. However, you are currently at Testing.com. You may have been directed here by your lab’s website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab’s website or portal, or contact your healthcare practitioner in order to obtain your test results.

Testing.com is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called “normal” values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are “within normal limits.”

For more information, please read the article Reference Ranges and What They Mean.

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 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).

Common Questions

How is the test used?

The thrombin time may sometimes be used as part of an investigation of a possible bleeding disorder or inappropriate blood clot formation (thrombotic episode), particularly to evaluate the level and function of fibrinogen.

The clotting-based functional fibrinogen assay is now routinely available in clinical laboratories and has largely replaced the need for thrombin time for evaluation of fibrinogen.

This test is very sensitive to the anticoagulant heparin. Because of this, it was once used to monitor unfractionated heparin therapy and to detect heparin contamination in a blood sample. While it is still sometimes used for these purposes, other tests and heparin neutralization procedures have largely replaced it.

When is it ordered?

A thrombin time may be ordered by itself or along with a combination of other tests when you have bleeding or clotting episodes, experience recurrent miscarriages, or have unexplained prolonged results on primary coagulation tests such as prothrombin time (PT) or partial thromboplastin time (PTT).

This test may be ordered when your healthcare practitioner suspects that you may have a disorder associated with decreased or dysfunctional fibrinogen. However, as mentioned previously, the functional fibrinogen assay (with or without a fibrinogen antigen assay) has largely replaced thrombin time for evaluating fibrinogen.

A thrombin time may sometimes be ordered when heparin contamination of a sample is suspected or when you are receiving heparin therapy, although these uses have declined.

What does the test result mean?

Significantly prolonged thrombin times are most commonly found when there is contamination of the blood sample by the anticoagulant heparin, direct thrombin inhibitor (such as dabigatran), and may be seen with heparin-like substances and inhibitors (e.g., fibrinogen/fibrin degradation products). Contamination can occur when you are on heparin therapy and/or when heparin is used as a periodic flushing agent to keep intravenous catheters from clotting.

A prolonged thrombin time may indicate decreased fibrinogen level and/or abnormal fibrinogen function. Conditions such as disseminated intravascular coagulation (DIC), a serious condition that can cause clotting and bleeding, or abnormal fibrinolysis impair fibrin formation and can also lead to a prolonged thrombin time, as can conditions such as end stage liver disease or malnutrition.

If thrombin time is abnormal, additional tests may be needed, including fibrinogen activity assay (now routinely available in clinical laboratories), fibrinogen antigen assay, and liver function tests.

Is there anything else I should know?

A significant percentage of people with decreased or dysfunctional fibrinogen will have no symptoms or mild symptoms and may only be diagnosed if the abnormality is discovered during testing for another reason, or because they have unexpected or prolonged bleeding following a surgical procedure or trauma.

Fibrinogen is one of several blood factors that are called acute phase reactants. Blood levels of fibrinogen along with other acute phase reactants rise sharply with conditions causing acute tissue inflammation or damage.

Genetic molecular testing is occasionally performed on those with inherited fibrinogen-related disorders to identify the genetic mutation responsible. Testing for this mutation may also be performed on other family members.

Some recurrent miscarriages are thought to be related to inappropriate clotting in placental blood vessels.

Can the thrombin time be performed in my doctor's office?

With the exception of a PT test, bleeding disorder tests, including the thrombin time, require specialized equipment and reagents and are typically performed in a laboratory.

Can I have decreased or abnormal fibrinogen and not know it?

Yes. For many people, the clotting process functions relatively normally even when fibrinogen concentrations and/or activity are decreased. Your condition may not be identified unless pre-surgery screening identifies a potential problem or you bleed longer than expected after a surgical procedure or trauma.

Are there treatments available for decreased or dysfunctional fibrinogen?

Treatment is not needed in most cases, but people with significant bleeding may be given fibrinogen replacements like cryoprecipitate or fresh frozen plasma on a short-term basis to replace fibrinogen. Those who have recurrent inappropriate clotting may require anticoagulation therapy with anticoagulants such as warfarin (Coumadin) or heparin. See the article on Transfusion Medicine for more about these treatments.

What is a reptilase time (RT) and how is it used?

If heparin is suspected as the cause of a prolonged thrombin time, heparin assays or reptilase time (RT) can be used to confirm heparin presence. Reptilase time measures the time it takes for a clot to form after reptilase has been added to plasma. Thrombin time may be ordered with a reptilase time to investigate a prolonged clotting time. Since thrombin time, but not reptilase time, is affected by the anticoagulant heparin, prolongation of both thrombin time and reptilase time indicates decreased fibrinogen level and/or abnormal function of fibrinogen. If thrombin time is prolonged but reptilase time is normal, heparin contamination or presence of an interfering substance is likely the cause. The clotting-based functional fibrinogen assay is now routinely available in clinical laboratories and has largely replaced the need for thrombin time and reptilase time for evaluation of fibrinogen.

Can thrombin time be used for monitoring dabigatran therapy?

Dabigatran, a direct thrombin inhibitor, is one of the newly approved oral anticoagulants. Routine therapeutic monitoring is not required; however, monitoring may be needed in certain clinical conditions. Thrombin time or a modified plasma-diluted thrombin time (dTT) has been advocated for monitoring dabigatran therapy, but it has not been widely accepted as the standard practice. Instead, direct measurement of dabigatran level should be used if monitoring is indicated.

View Sources

Sources Used in Current Review

(Jan 30, 2014) Teruya, J. Thrombin Time. Medscape. Available online at http://emedicine.medscape.com/article/2086278-overview#showall. Accessed July 2020.

(© 2020). Thrombin Time (Bovine), Plasma. Mayo Clinic, Mayo Medical Laboratories. Available online at https://www.mayocliniclabs.com/test-catalog/Overview/602184. Accessed July 2020.

(Oct 28, 2019) Mir, M. Nonplatelet Hemostatic Disorders. Medscape Reference. Available online at https://emedicine.medscape.com/article/210467-overview#a2. Accessed July 2020.

(© 2020). Thrombin Time with Reflex to Thrombin Time 1:1 Mix. ARUP Laboratories. Available online at https://ltd.aruplab.com/Tests/Pub/0030260. Accessed July 2020.

©2020 University of Rochester Medical Center. Health Encyclopedia. Thrombin Time. Available online at https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=thrombin_time. Accessed July 2020.

Sources Used in Previous Reviews

Brick, W. et. al. (Updated 2009 November 17). Dysfibrinogenemia. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/199723-overview. Accessed July 2010.

(© 1995–2010). Unit Code 9059: Thrombin Time (Bovine), Plasma. Mayo Clinic, MayoMedical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/9059. Accessed July 2010.

Israels, S. (Updated 2009 February 12. Inherited Abnormalities of Fibrinogen. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/960677-overview. Accessed July 2010.

(Updated 2009 September 18). Thrombin Time [CO004600]. Massachusetts General Hospital, Pathology Service [On-line information]. Available online at http://www2.massgeneral.org/pathology/coagbook/CO004600.htm. Accessed July 2010.

Dugdale, D. (Updated 2009 March 2). Congenital afibrinogenemia. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/001313.htm. 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.

Teruya, J., et al (Updated 2014 January 30). Thrombin Time. Medscape. Available online at http://emedicine.medscape.com/article/2086278-overview#showall. Accessed April 2014.

Burgess, R. et. al. (Updated 2012 January 10). Dysfibrinogenemia. Medscape. Available online at http://emedicine.medscape.com/article/199723-overview. Accessed April 2014.

(© 1995–2014). Unit Code 9059: Thrombin Time (Bovine), Plasma. Mayo Clinic, MayoMedical Laboratories. Available online at http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/9059. Accessed April 2014.

Basala, V. (Updated 2012 May 6). Inherited Abnormalities of Fibrinogen. eMedicine. Available online at http://emedicine.medscape.com/article/960677-overview. Accessed April 2014.

Wilczynski, C. et al. (Updated 2014 February 12). Fibrinogen. Medscape. http://emedicine.medscape.com/article/2085501-overview. 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 http://ltd.aruplab.com/Tests/Pub/0030268. Accessed April 2014.

(© 1995–2014). Reptilase Time, Plasma. Mayo Clinic, MayoMedical Laboratories. Available online at http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/9078. Accessed April 2014.


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