Antithrombin III is a blood test that measures the amount of antithrombin III (AT III), a protein that helps control blood clotting.
How the test is performed:
A blood sample is needed. For information on how this is done, see: Venipuncture
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, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
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 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:
Lower than normal AT III may be due to:
Higher than normal AT III may be due to:
What the risks are:
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.