Antithrombin

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Also known as: Functional Antithrombin III; AT III; AT 3
Formal name: Antithrombin (Activity and Antigen)

At a Glance

Why Get Tested?

To help investigate the cause of recurrent inappropriate blood clotting; to help diagnose an antithrombin deficiency

When to Get Tested?

A couple of months after getting a blood clot (thrombotic episode) or when you are not responding as expected to heparin anticoagulation therapy

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

Antithrombin is a protein produced by the liver that helps regulate blood clot formation (i.e., a naturally occurring mild blood thinner). Antithrombin testing measures the activity (function) and the amount (quantity) of antithrombin in an individual's blood and is used to evaluate the person for excessive blood clotting.

Normally, when a blood vessel is injured, the body initiates a complex process called hemostasis to form a blood clot and prevent further blood loss. Part of this complex process involves the activation of several proteins called coagulation factors in a series of steps referred to as the coagulation cascade. Antithrombin helps to regulate this process by inhibiting the action of several activated coagulation factors, including thrombin and factors Xa, IXa, and XIa, to slow down the process and prevent excessive or inappropriate clotting (thrombosis).

People with an excessive clotting disorder due to an inherited or acquired antithrombin deficiency are at increased risk of developing blood clots, especially in deep veins such as in the legs (venous thrombosis). Inherited deficiencies are rare, affecting about 1 in 5,000 people. For people who inherit one defective gene and one normal gene (heterozygous), episodes of inappropriate blood clot formation typically start at about 20 to 30 years of age. Very rarely, a person may inherit two defective antithrombin genes, resulting in severe clotting problems soon after birth. 

Acquired antithrombin deficiencies may occur at any age. They are associated with a variety of conditions, including liver disease, extensive thrombosis, disseminated intravascular coagulation (DIC), blood loss, cancer, and nephrotic syndrome – a form of kidney disease.

There are two types of antithrombin deficiency. With type 1, antithrombin functions normally, but the quantity is insufficient. With type 2, there is a sufficient quantity of antithrombin produced, but it is dysfunctional. These types can be differentiated and assessed by testing:

  • Antithrombin activity, which evaluates the function of antithrombin.
  • Antithrombin antigen, which measures the quantity of antithrombin present.

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?

Typically, no test preparation is needed. Note that testing for antithrombin deficiency is not recommended if a person's condition may affect the antithrombin level (e.g., DIC) and/or after the person has been treated with heparin for a blood clot.

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

Dugdale, D. (Updated 2012 June 5). Antithrombin III. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003661.htm. Accessed September 2012.

Dugdale, D. (Updated 2011 February 28). Congenital antithrombin III deficiency. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000558.htm. Accessed September 2012.

Harper, J. (Updated 2011 August 1). Antithrombin III Deficiency. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/954688-overview through http://emedicine.medscape.com. Accessed September 2012.

Hart, K. et. al. (Updated 2011 November). Hypercoagulable States – Thrombophilia. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/Thrombophilia.html?client_ID=LTD#tabs=0 through http://www.arupconsult.com. Accessed September 2011.

(© 1995–2012). Unit Code 9030: Antithrombin Activity, Plasma. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/9030 through http://www.mayomedicallaboratories.com. Accessed September 2012.

(© 1995–2012). Unit Code 9031: Antithrombin Antigen, Plasma. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/9031 through http://www.mayomedicallaboratories.com. Accessed September 2012.

Lusky, K. (2010 September). In coagulation, reflexive testing algorithms do the trick. CAP Today [On-line information]. Available online through http://www.cap.org. Accessed September 2012.

Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 105-107.

(January 10, 2012) Rajun A, et. al. Antithrombin Deficiency. Medscape Reference. Available online at http://emedicine.medscape.com/article/198573-overview through http://emedicine.medscape.com/. Accessed September 2012.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Antithrombin III pp. 100-101. Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

Brose, M. (2003 June 1, Updated). Antithrombin III. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003661.htm.

Cohen, E. (2003 October 28, Updated). Congenital antithrombin III deficiency. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000558.htm.

(2002 November 19, Updated). Antithrombin. Massachusetts General Hospital, Pathology Service Laboratory Medicine Coag Test Handbook [On-line information]. Available online at http://www.mgh.harvard.edu/labmed/lab/coag/handbook/CO000300.htm through http://www.mgh.harvard.edu.

(2003 September, Modified). Antithrombin III Functional Assay and Antigen. Duke University Medical Center, Clinical Coagulation Laboratory, Coagulation Test Descriptions [On-line information]. Available online at http://pathology.mc.duke.edu/coag/TestDes.htm through http://pathology.mc.duke.edu.

(2003 Spring). Current approaches to the work-up of hypercoagulability: Antithrombin deficiency. College of American Pathologists, Newspath V 17 (2). Available online at http://www.cap.org/apps/docs/newspath/spring_2003/clinical.html through http://www.cap.org.

(© 2004) Antithrombin, Antigen. ARUP's Guide to Clinical Laboratory Testing [On-line information]. Available online at http://www.arup-lab.com/guides/clt/tests/clt_al65.jsp#3952516 through http://www.arup-lab.com.

Rodgers, G. et. al. (© 2004) Hemostasis/Thrombosis, General Information. ARUP's Guide to Clinical Laboratory Testing [On-line information]. Available online at http://www.arup-lab.com/guides/clt/tests/clt_fro8.jsp#1238723 through http://www.arup-lab.com.

(© 2004). Antithrombin, Enzymatic (Activity). ARUP's Guide to Clinical Laboratory Testing [On-line information]. Available online at http://www.arup-lab.com/guides/clt/tests/clt_al66.jsp#3952541 through http://www.arup-lab.com.

(2001 June 26, Updated). Antithrombin Deficiency. University of Illinois - Urbana/Champaign, Carle Cancer Center, Hematology Resource Page, Patient Resources [On-line information]. Available online at http://www-admin.med.uiuc.edu/hematology/PtAntithrombin.htm through http://www-admin.med.uiuc.edu. 

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007).  Mosby's Diagnostic and Laboratory Test Reference 8th Edition:  Mosby, Inc., Saint Louis, MO. Pp 104-105.

Clarke, W. and Dufour, D. R., Editors (2006).  Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC.  Pp 231.

Wu, A. (2006).  Tietz Clinical Guide to Laboratory Tests, Fourth Edition.  Saunders Elsevier, St. Louis, Missouri. Pp 138-139.

Levin, M. (2007 March 9, Updated). Antithrombin III. MedlinePlus Medical Encyclopedia. Available online through http://www.nlm.nih.gov/medlineplus. Accessed on 3/9/08.

Nanda, R. (2007 April 27). Deep venous thrombosis. MedlinePlus Medical Encyclopedia. Available online through http://www.nlm.nih.gov/medlineplus. Accessed on 3/9/08.

Harper, J. (2007 August 16). Antithrombin III Deficiency. eMedicine. Available online through http://www.emedicine.com. Accessed on 3/9/08.