Antithrombin

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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 or inappropriate blood clotting; to help diagnose an antithrombin deficiency

When to Get Tested?

Preferably a couple of months after a thrombotic episode and not whilst taking, or for at least 6 weeks after taking anticoagulants. Occasionally testing may be requested if you are not responding as your doctor expected to heparin anticoagulation therapy

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

No test preparation is 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, chiefly thrombin and factor Xa to slow down the process and prevent excessive or inappropriate clotting.

There are two major causes antithrombin deficiency:

1.Inherited

2. Acquired

Patients with inherited or acquired antithrombin deficiency are at increased risk of venous thrombosis

1. 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 inherited 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

2. Acquired antithrombin deficiencies may occur at any age. These result in quantitative deficiency of antithrombin (type 1 deficiency). They are associated with a variety of conditions, including liver disease, extensive thrombosis, disseminated intravascular coagulation (DIC), blood loss, cancer, nephrotic syndrome – a form of kidney disease, metastatic (widespread) tumours and chemotherapy with asparaginase.

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

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.