Also known as: Antithrombin III, AT III
Name: Antithrombin
Related tests: aPTT, Clotting factors, D-dimer, Factor V Leiden, Fibrinogen, Homocysteine, Lupus anticoagulant, Protein C, Protein S, PT, PT 20210
Why is the test done?
To investigate the cause of inappropriate blood clotting and to help diagnose antithrombin deficiency
When is the test done?
About two months after a thrombotic episode; when heparin treatment does not have the expected effect
Sample required?
A blood sample taken from a vein in the arm
What is being tested?
It measures the function and amount of antithrombin, a protein involved in controlling the clotting process. When a blood vessel is injured, the body activates a series of clotting factors (in a process called the coagulation cascade) to form a clot and prevent further blood loss. Antithrombin helps regulate this process by inhibiting the activation of several factors (thrombin and factors X, IX, XI and XII), slowing the process down and preventing excessive or inappropriate clotting.
An acquired or inherited antithrombin deficiency increases the risk of thrombotic episodes – the formation of an inappropriate clot in a vein or artery. There are two types of antithrombin deficiency. In type 1, antithrombin works normally but not enough is produced. In type 2, enough antithrombin is produced but it does not work properly. These types of deficiency are detected, distinguished and assessed using two tests:
– Antithrombin activity, which assesses function
– Antithrombin antigen, which measures the amount
How is the sample collected?
A blood sample taken from a vein in the arm
How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?
How is it used?
Antithrombin testing is mainly requested, together with other tests for hypercoagulability disorders, to investigate the cause of recurrent venous thrombosis. The antithrombin activity test is done before the antigen test to assess whether the total amount of functional antithrombin is normal. Activity will be reduced in both type 1 and type 2 antithrombin deficiency, so this test can be used as an initial screen for both. If antithrombin activity is low, the antithrombin antigen test is done to determine how much antithrombin is present. If a deficiency is found, both antithrombin tests are repeated at a later date to confirm the findings. Antithrombin tests can also be used to assess patients who do not respond to heparin as expected.
When is it requested?
Antithrombin activity tests are requested, together with other tests for hypercoagulability disorders (such as protein C, protein S and lupus anticoagulant), when a patient has had recurrent venous thrombosis. Antithrombin should be measured after a blood clot has been treated and has resolved, since both the presence of the clot and the treatment used for it will affect antithrombin results.
Antithrombin tests may be requested when a patient does not respond as expected to anticoagulation with heparin, when unusually high doses of heparin are needed to reach the desired level of anticoagulation
What does the test result mean?
Reduced antithrombin activity and reduced amounts of antithrombin antigen suggest type 1 antithrombin deficiency. In this case activity is reduced because less antithrombin is available to take part in the coagulation cascade. Reduced antithrombin activity with normal antithrombin antigen levels suggests type 2 antithrombin deficiency. This means there is enough antithrombin, but it is not working as it should.
If antithrombin activity is normal, the antithrombin antigen test is not usually done. In this case antithrombin is working properly and the recurrent thrombotic episodes being investigated are probably due to another cause.
Raised antithrombin levels are not usually considered a problem. They can be found with:
• acute hepatitis (cholestasis)
• Kidney transplant • Vitamin K deficiency • Anticoagulant treatment with warfarin (Coumadin) (sometimes)
Testing for antithrombin deficiency is not recommended unless the patient’s condition is stable. Temporarily or chronically reduced antithrombin levels can be found in conditions that affect its consumption or production, such as:
• DIC (disseminated intravascular coagulation), an acute or chronic condition characterised by consumption of clotting factors. An affected patient may bleed and/or develop clots.
• DVT (deep vein thrombosis, a blood clot usually in a deep vein of the leg)
• liver disease
• Nephrotic syndrome
• Protein-losing conditions
• Pulmonary embolism
• Newborn babies in the first days of life
• Oestrogen therapy
Is there anything else I should know?
If a patient with antithrombin deficiency also has other clotting defects, such as protein C or S deficiency or a factor V Leiden mutation, or uses oral contraceptives, they may be at a considerably higher risk of developing a blood clot.
Antithrombin deficiency can increase the risk of repeated miscarriages.
Patients with antithrombin deficiency may need to take preventive anticoagulants before medical or surgical procedures.
Antithrombin concentrates can be used temporarily to correct acute or chronic antithrombin deficiency.
1. How can I find out my risk of developing a blood clot?
2. Should I tell my dentist or other doctors about my antithrombin deficiency?
1. How can I find out my risk of developing a blood clot?
If you have a personal or family history of recurrent blood clots, your doctor may examine you to assess your general state of health and request a series of tests to determine your risk of a hypercoagulability disorder. The more inherited or acquired risk factors you have (such as factor V Leiden or the PT 20210 mutation, or protein C or S deficiency), the higher your relative risk of a clot. This risk increases further if you are overweight, inactive or smoke, and/or take oral contraceptives. It is important to remember, however, that any relative risk found is still a statistical risk. No one can predict whether a specific patient will actually have recurrent clots.
2. Should I tell my dentist or other doctors about my antithrombin deficiency?
Yes. This is important information that the people caring for your health should know, as it concerns your body’s ability to clot blood properly during and after a medical procedure.