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

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.

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.

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

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

• 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.