Health Encyclopedia

Antithrombin III

Definition

Antithrombin III is a blood test that measures the amount of antithrombin III (AT III), a protein that helps control blood clotting.

Alternative Names

A-Th3; AT III

How the test is performed

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to prepare for the test

Certain medicines may affect the results of the test. Your health care provider may tell you to stop taking certain medicines or reduce their dose before the test. Do not stop taking any medicine before speaking with your doctor.

How the test will feel

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

Your doctor may order this test if you have repeated blood clots or if blood thinning medicine does not work.

Normal Values

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

Lower-than-normal AT III may mean you have an increased risk of clotting. Abnormal results may not show up until you are an adult.

Examples of disorders and conditions associated with increased blood clotting include:

Deep venous thrombosis
Phlebitis (vein inflammation)
Pulmonary embolus (blood clot traveling to lung)

Lower than normal AT III may be due to:

Bone marrow transplant
DIC (disseminated intravascular coagulation)
Other coagulation disorders
An inherited condition causing lower blood clotting protein levels
Liver disorders
Nephrotic syndrome

Higher than normal AT III may be due to:

Use of anabolic steroids

What the risks are

Excessive bleeding
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
Multiple punctures to locate veins

Special considerations

Birth control pills can cause a slight decrease in AT III levels.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Review Date:6/10/2008
Reviewed By:Sean O. Stitham, MD, private practice in Internal Medicine, Seattle, Washington; and James R. Mason, MD, Oncologist, Director, Blood and Marrow Transplantation Program and Stem Cell Processing Lab, Scripps Clinic, Torrey Pines, California. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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