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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 (or thrombus) can be good or bad, depending on the situation. Your body needs to be able to form blood clots in order to stop excessive bleeding in case of injury. But it's important to prevent abnormal clots that cut off necessary blood flow.

Normally, there is a healthy balance between chemicals in the body that promote clotting and chemicals that inhibit clotting. One important clot-promoting protein is called thrombin, and the protein that blocks it is called antithrombin. Antithrombin works to thin the blood slightly so that it doesn't clot too much. An antithrombin (AT) deficiency can lead to a dangerous tendency to form blood clots.

An example of a dangerous clot is deep vein thrombosis, or DVT, which can form in the leg, arm, abdomen (belly), or near the brain. Another hazardous clot is a pulmonary embolism, or PE, which may occur when a clot travels through the bloodstream and becomes 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 produce enough antithrombin. Type 1 AT deficiency may be either genetically inherited, which means passed on from a parent, or acquired. The three main causes of acquired AT deficiency are:

  • Liver failure, because antithrombin is produced in the liver

  • Kidney disease that may cause too much antithrombin to be expelled in the urine

  • Spreading cancer, also called metastatic disease

Type 2 AT deficiency is always inherited. In type 2 AT deficiency, your body may produce a normal quantity of the antithrombin protein, but much of that protein is poor quality and doesn't work to stop clotting. In other words, the antithrombin protein does not work the way it should.

The antithrombin activity test and antigen test measure how much effective antithrombin a patient has. As explained below, the two tests can be used together to determine whether a patient has 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 doctor might especially recommend an antithrombin test if you have had any of the following:

  • A venous thromboembolism, or a clot that forms in a vein and then travels – or "embolizes" – to another part of the body) before you turn 50

  • A pulmonary embolism, or a blood clot that becomes lodged in the lung

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

  • A family history of blood clotting problems

Blood clots that form in arteries, as opposed to veins, do not generally prompt a doctor to do an antithrombin test. This is because antithrombin deficiency is not associated with arterial clotting. 

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, a rare genetic disease that causes people to make abnormal blood clots

  • Protein S deficiency, another rare disease that causes people to make abnormal blood clots

  • Factor V Leiden, a more common inherited disorder that raises the risk of making abnormal blood clots

  • Factor II G20210A, a gene mutation associated with increased levels of the clotting factor prothrombin. This 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 expressed as percentages. Different labs use slightly different normal ranges, but in general, 80 to 120 percent 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 percent.

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 determine 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: simply not 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 an adequate quantity of antithrombin protein but its quality is poor. This problem results from a defect in the antithrombin protein. 

No evidence exists that higher-than-normal antithrombin levels cause bleeding problems or have any medical 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 situations, antithrombin levels may appear temporarily low. These include:

  • Acute blood clots

  • Use of the anti-clotting drug heparin

  • Severe trauma

  • Severe burns

  • Chemotherapy with the drug called asparaginase

  • Disseminated intravascular coagulation, or DIC; this is a blood disorder that often occurs with sepsis, an infection of the bloodstream 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 two 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.