What is Anti Thrombin III?

Also Known As: Functional Antithrombin III AT III AT 3

Formal Name: Antithrombin (Activity and Antigen)

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 test used?

Antithrombin testing is primarily ordered, along with other tests for excessive clotting disorders, to investigate the cause of recurrent blood clot formation (thrombosis). Testing evaluates the activity (function) and the amount (quantity) of antithrombin and helps identify antithrombin deficiency.

The activity test is performed first, to evaluate whether the total amount of functional antithrombin is normal. If the antithrombin activity is low, then the antithrombin antigen test is performed to determine the quantity of antithrombin present. These two tests can be used to differentiate between type 1 and type 2 antithrombin deficiencies. If a deficiency is detected, both antithrombin tests are typically repeated at a later date to confirm test findings.

Antithrombin testing may sometimes be used to evaluate people who are not responding as expected to heparin. Heparin is an anticoagulant drug that is given to people who have a blood clot or are at an increased risk of forming inappropriate blood clots. The effects of heparin are mediated by antithrombin. Heparin can greatly increase antithrombin activity, thus inhibiting the formation of blood clots, but those who are antithrombin-deficient are resistant to heparin treatment.