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.
Antithrombin; AT III; AT 3; Functional antithrombin III
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:
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.