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Also known as: Functional antithrombin III; AT III
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 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?


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.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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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.