| Definition: |
A blood test that measures the amount of Antithrombin III (AT III; a protein that helps prevent and/or regulate blood clotting).
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| Alternative Names: |
A-Th3; AT III
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| How the test is performed: |
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (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 tourniquet 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.
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.
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| 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.
Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
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| 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.
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| Why the test is performed: |
This test is indicated when there are repeated episodes of blood clots and/or when individuals do not respond to anticoagulant medications. It can help to determine the cause of hypercoagulation (increased blood coagulation).
A number of so-called protease inhibitors are normally present in the blood. These will inactivate the coagulation factors after a period of time. The most important of these is antithrombin III. Heparin works as an anticoagulant by activating antithrombin III.
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| Normal Values: |
The normal range is 0.20 to 0.45 mg/ml (or more than 50% of the laboratory control value).
Note: mg/ml = milligrams per milliliter
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| What abnormal results mean: |
Reduced levels (a deficiency) 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 deficient AT III include:
Elevated levels of Antithrombin III may indicate:
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| What the risks are: |
- excessive bleeding
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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
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| Special considerations: |
The use of oral contraceptives (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.
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