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Antithrombin (Activity and Antigen)

Antithrombin (Activity and Antigen)

Does this test have other names?

Functional antithrombin III, functional AT, AT activity

What are these tests?

The antithrombin activity and antigen tests are used to help find out what may be causing abnormal blood clots in your body. A blood clot (thrombus) can be good or bad, depending on the case. Your body needs to be able to form blood clots in order to stop too much bleeding in case of injury. But it's important to prevent abnormal clots that cut off blood flow.

Normally you have a healthy balance between chemicals in your body that help clotting and chemicals that stop clotting. One important protein that helps clotting is thrombin. The protein that blocks clotting is called antithrombin. Antithrombin works to thin the blood slightly so that it doesn't clot too much. A lack of antithrombin (AT) can make it more likely for you to form blood clots.

An example of a dangerous clot is deep vein thrombosis, or DVT. This clot can form in a leg or arm, in your abdomen (belly), or near the brain. Another dangerous clot is a pulmonary embolism, or PE. This can happen when a clot travels through the bloodstream and gets stuck in the blood vessels of the lung.

AT deficiencies may be either type 1 or type 2.

In type 1 AT deficiency, your body does not make enough antithrombin. Type 1 AT deficiency may be either inherited or acquired. Inherited means it was passed on from a parent. The three main causes of acquired AT deficiency are:

  • Liver failure. This is because antithrombin is made in the liver.

  • Kidney disease. This may cause too much antithrombin to be sent out of your body in your urine.

  • Spreading cancer (metastatic disease)

Type 2 AT deficiency is always passed down from a parent. In type 2 AT deficiency, your body may make a normal amount of the antithrombin protein. But much of that protein doesn't work the way it should.

The antithrombin activity test and antigen test measure how much good-quality antithrombin you have. As explained below, the two tests can be used together to find out if you have type 1 or type 2 AT deficiency. 

Why do I need this test?

You might have one or both of these tests if you have had a problem with blood clots. Your health care provider might especially recommend an antithrombin test if you have had any of the following:

  • A venous thromboembolism before you are 50 years old. This is a clot that forms in a vein and then travels (embolizes) to another part of the body.

  • A blood clot that becomes stuck in the lung (pulmonary embolism)

  • A clot that forms in the abdomen or near the brain

  • A family history of blood-clotting problems

Your provider usually will not order an antithrombin test for blood clots that form in arteries instead of veins. This is because clots in the arteries usually are not caused by antithrombin deficiency. 

What other tests might I have along with this test?

Antithrombin deficiency is one possible cause of a tendency to form unwanted blood clots. Your doctor may want to test for other causes as well. These include:

  • Protein C deficiency. This is a rare genetic disease that causes people to make abnormal blood clots.

  • Protein S deficiency. This is another rare disease that causes people to make abnormal blood clots.

  • Factor V Leiden. This is a more common disorder passed down in families that raises the risk of making abnormal blood clots.

  • Factor II G20210A. This is an abnormal gene that makes higher levels of the clotting factor prothrombin. Having more clotting factor means clots are more likely to form when they should not. 

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

The results for both activity and antigen tests are given as percentages. Different labs use slightly different normal ranges, but in general, 80% to 120% is considered normal for adults. Newborns usually have about half as much antithrombin as adults. Thrombin levels in infants rise to adult levels by about 6 months of age.

People with genetically inherited antithrombin deficiency typically have test results between 40% and 60%.

In both type 1 and type 2 AT deficiency, the antithrombin activity test shows a low result because you don't have as much working antithrombin as you should have. When the AT activity test shows that levels are low, the antithrombin antigen test can then be used to find out whether the deficiency is type 1 or type 2.

If the follow-up antithrombin antigen test shows a lower-than-normal result, then you probably have a type 1 deficiency. You do not have enough antithrombin protein. But if the antigen test shows a normal result, then the AT deficiency shown by the activity test is likely to be type 2. This means you have enough antithrombin protein, but its quality is poor. This problem is caused by a defect in the antithrombin protein. 

No evidence exists that higher-than-normal antithrombin levels cause bleeding problems or have any health significance. Therefore, the main concern is with AT deficiency.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

As with any blood draw, you may have minor, temporary pain when the needle pricks your arm. There are also small risks such as bleeding, infection, bruising, and dizziness.

What might affect my test results?

In some cases, your antithrombin levels may be low for a short while. These include:

  • Acute blood clots

  • Use of the anti-clotting drug heparin

  • Severe trauma

  • Severe burns

  • Chemotherapy with the medicine asparaginase

  • Disseminated intravascular coagulation, or DIC. This is a blood disorder that often happens with an infection of the bloodstream (sepsis) or blood poisoning.

If you have been taking warfarin or heparin, or taking anticoagulant drugs after a severe blood-clotting episode, your antithrombin should be tested again 2 weeks or more after you have stopped taking the drugs to get more accurate results.

How do I get ready for this test?

You don't need to do anything special to prepare for this test. Make sure your health care provider knows if you are taking warfarin or heparin or getting chemotherapy with asparaginase. And be sure your doctor also knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use. 

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