Antithrombin
- Also Known As:
- Functional Antithrombin III
- AT III
- AT 3
- Formal Name:
- Antithrombin (Activity and Antigen)

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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?
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 and/or after the person has been treated with heparin for a blood clot.
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?
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 (known as deep venous thrombosis or DVT). 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 that cause decreased production, excessive consumption, or loss of antithrombin. These conditions include 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
Common Questions
View Sources
Sources Used in Current Review
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