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

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

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.

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.

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

The normal range is 0.20 - 0.45 mg/ml (milligrams per milliliter).

Lower-than-normal AT III may mean you have an increased risk of clotting. Examples of disorders and conditions associated with increased blood clotting include:

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

Lower-than-normal AT III may be due to:

• DIC (disseminated intravascular coagulation)
• Other coagulation disorders
• Liver disorders
• Nephrotic syndrome

Higher-than-normal AT III may be due to:

• Use of anabolic steroids

• 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

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.