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

AT III

A blood sample is needed. For information on how this is done, see: Venipuncture

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. This can occur when there is not enough AT III in your blood, or when there is enough AT III in your blood, but the AT III doesn't work right and less active.

Abnormal results may not show up until you are an adult.

Examples of complications associated with increased blood clotting are:

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

Lower than normal AT III may be due to:

• Bone marrow transplant
• DIC (disseminated intravascular coagulation)
• AT III deficiency, an inherited condition causing lower blood clotting protein levels
• Liver cirrhosis
• Nephrotic syndrome

Higher than normal AT III may be due to:

• Use of anabolic steroids

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.

Other risks may include:

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

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

Schafer AI. Thrombotic disorders: hypercoagulable states. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 179.