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 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 thrombin 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 breaks off and travels
through the bloodstream and gets stuck in the blood vessels of a 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 3 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 measures how well the protein inhibits thrombin. The
antithrombin antigen test measures how much antithrombin protein your body has made,
regardless of how well it functions. As explained below, the 2 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 may need 1 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 belly 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 healthcare provider 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?
Test results may vary depending on your age, gender, health history, the method used
for the test, and other things. Your test results may not mean you have a problem.
Ask your healthcare provider what your test results mean for you.
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. The normal range for newborns is usually about 44% to 76%. 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. This means that 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
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 is done with a blood sample. A needle is used to draw blood from a vein in
your arm or hand.
Does this test pose any risks?
Having a blood test with a needle carries some risks. These include bleeding, infection,
bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may
feel a slight sting or pain. Afterward, the site may be sore.
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 anticlotting medicine 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 blood-thinner (anticoagulant)
medicines after a severe blood-clotting episode, your antithrombin should be tested
again 2 weeks or more after you have stopped taking the medicines to get more accurate
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 healthcare provider also knows about all medicines,
herbs, vitamins, and supplements you are taking. This includes medicines that don't
need a prescription and any illegal drugs you may use.