Antithrombin
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 a thrombotic episode or if you are not responding as your doctor 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 testing measures the function and quantity of antithrombin. Antithrombin is a protein produced by the liver to help control blood clotting. Normally, when a blood vessel is injured, the body activates a series of coagulation factors, in a process called the coagulation cascade, to form a blood clot and prevent further blood loss. 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.
Patients with inherited or acquired antithrombin deficiency are at increased risk of venous thrombosis. Inherited deficiencies are rare (about 1 in 5000 patients). If a person has one defective gene and one normal gene (heterozygous), then inappropriate clotting episodes typically start at about 20 to 30 years of age. Very rarely, an individual has two defective antithrombin genes, resulting in severe thrombotic 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, normal antithrombin is produced, but the quantity made is insufficient. With type 2, there is a sufficient quantity of antithrombin produced, but it is dysfunctional. These types can be detected, differentiated, and assessed using two antithrombin tests. They are:
- Antithrombin activity, which evaluates the function of a patient’s 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?
No test preparation is needed.
The Test
How is it used?
Antithrombin testing is primarily ordered, along with other tests for hypercoagulable disorders, to investigate to investigate the cause of recurrent venous thrombosis. The antithrombin activity test is performed before the antigen test, to evaluate whether the total amount of functional antithrombin is normal. Activity will be decreased with both type 1 and type 2 antithrombin deficiencies, so this test can be used as an initial screen for both. If the antithrombin activity is low, then the antithrombin antigen test is performed to determine the quantity of antithrombin present. If a deficiency is detected, both antithrombin tests are repeated at a later date to confirm test findings. Antithrombin testing may be used to evaluate patients not responding as expected to heparin.
When is it ordered?
Antithrombin activity testing is ordered, along with other tests for hypercoagulable disorders (such as Protein C and Protein S, and Lupus anticoagulant), when a patient has been experiencing recurrent venous thrombosis. Antithrombin should be measured after a blood clot has been treated and resolved as both the presence of the clot, and the therapy used to treat it, will affect antithrombin results.
Antithrombin testing may be ordered when a patient is not responding as expected to heparin anticoagulation - when unusually high doses of heparin are necessary to achieve the desired level of anticoagulation.
What does the test result mean?
Decreased antithrombin activity and decreased quantities of antithrombin antigen suggest a type 1 antithrombin deficiency. In this case, the activity is decreased because there is less antithrombin available to participate in clotting regulation. Reduced antithrombin activity and normal levels of antithrombin antigen suggest a type 2 antithrombin deficiency. This means that there is sufficient antithrombin, but it is not functioning as it should.
If the antithrombin activity is normal, then the antithrombin antigen test is usually not performed. In this case, the antithrombin is functioning adequately and the recurrent thrombotic episodes being investigated are likely due to another cause.
Increased levels of antithrombin are not usually considered a problem. They may be seen with:
- Acute hepatitis/cholestasis
- Kidney transplant
- Vitamin K deficiency
- Warfarin (COUMADIN®) anticoagulation therapy (sometimes)
Testing for antithrombin deficiency is not recommended unless the patient’s condition is stable. Temporarily or chronically decreased antithrombin levels may be seen with conditions that affect its consumption or production, such as:
- DIC (Disseminated intravascular coagulation), an acute or chronic condition characterized by the consumption of clotting factors. An affected patient may bleed and/or thrombose.
- DVT (deep vein thrombosis – a blood clot usually in a deep leg vein)
- Liver disease
- Nephrotic syndrome
- Protein-losing condition
- Pulmonary embolism
- Neonates during the first few days of life
- Estrogen therapy
Is there anything else I should know?
If a patient with an antithrombin deficiency also has other coagulation risks, such as a Protein C or S deficiency, a Factor V Leiden mutation, or oral contraceptive use, then she may be at a significantly elevated risk of developing a blood clot.
Antithrombin deficiency can increase the risk of recurrent miscarriage.
Patients with antithrombin deficiency may need to take preventative anticoagulation prior to medical or surgical procedures.
Antithrombin concentrates may be used to temporarily correct acute or chronic antithrombin deficiencies.
Common Questions
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Related Pages
On This Site
Conditions: Excessive Clotting Disorders, Bleeding Disorders, Liver Disease
Elsewhere On The Web
Article Sources
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
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 at http://www.nlm.nih.gov/medlineplus. Accessed on 3/9/08.
Nanda, R. (2007 April 27). Deep venous thrombosis. MedlinePlus Medical Encyclopedia. Available online at http://www.nlm.nih.gov/medlineplus. Accessed on 3/9/08.
Harper, J. (2007 August 16). Antithrombin III Deficiency. eMedicine. Available online at http://www.emedicine.com. Accessed on 3/9/08.
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.
