Antithrombin III (AT III) is a protein that helps control blood clotting. A blood test can determine the amount of AT III present in your body.
How the Test is Performed
A blood sample is needed.
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. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
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 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 blood 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 does not function properly and is 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
- Disseminated intravascular coagulation (DIC)
- AT III deficiency, an inherited condition
- 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)
Antithrombin; AT III; AT 3; Functional antithrombin III
Schmaier AH, Miller JL. Coagulation and fibrinolysis. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 39.
Schafer AI. Thrombotic disorders: hypercoagulable states. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 179.
Update Date 5/29/2014
Updated by: Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital, Boston, MA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.