Alternative names 
Antithrombin III; AT III

This is a blood test that measures the amount of antithrombin III (AT III), a protein that helps prevent and regulate blood clotting.

How the test is performed

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test
The health care provider may limit certain medications shortly before the test to assure an accurate sample. Usually this will include monitoring drugs that may affect the amount of antithrombin in the bloodstream.

For infants and children:
The preparation you can provide for this procedure depends on your child’s age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

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
This test is indicated when there are repeated episodes of Blood clots or when individuals do not respond to anticoagulant medications. It can help to determine the cause of hypercoagulation (increased blood coagulation).

Normal Values

The normal range is 0.20-0.45 mg/ml (milligrams per milliliter).

What abnormal results mean

Lower-than-normal AT III may indicate an increased risk of clotting. Examples of disorders and/or conditions associated with increased blood clotting include:

Other conditions that can be related to lower-than-normal AT III include:

  • DIC (disseminated intravascular coagulation)  
  • other coagulation disorders  
  • liver disorders  
  • nephrotic syndrome

Higher-than-normal AT III may indicate:

  • use of anabolic steroids

What the risks are

  • 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

Special considerations

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.

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.