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

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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 (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

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.