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
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 healthcare 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
Protein S deficiency. This is another rare disease that causes people to make abnormal
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 healthcare
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
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
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 healthcare
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.