Also Known As
Functional Antithrombin III
Formal Name
Antithrombin; Activity and Antigen
This article was last reviewed on
This article waslast modified on 10 June 2021.
At a Glance
Why Get Tested?

To help investigate the cause of recurrent or inappropriate blood clotting; to help diagnose an antithrombin deficiency

When To Get Tested?

Preferably a couple of months after a thrombotic episode and not whilst taking, or for at least 6 weeks after taking anticoagulants. Occasionally testing may be requested if you are not responding as your doctor expected to heparin anticoagulation therapy

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

No test preparation is needed.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

Antithrombin testing measures the function and quantity of antithrombin in the blood.  Antithrombin is a protein produced by the liver to help control blood clotting. Normally, when a blood vessel is injured, the body activates a series of coagulation factors, in a process called the coagulation cascade, to form a blood clot and prevent further blood loss. Antithrombin helps to regulate this process by inhibiting the action of several activated coagulation factors, chiefly thrombin and factor Xa to slow down the process and prevent excessive or inappropriate clotting.

There are two major causes of antithrombin deficiency:


2. Acquired

Patients with inherited or acquired antithrombin deficiency are at increased risk of venous thrombosis, including deep vein thrombosis (DVT) and pulmonary embolism (PE).

1. Inherited deficiencies are rare (about 1 in 5000 patients). If a person has one defective gene and one normal gene (heterozygous), then inappropriate clotting episodes typically start at about 20 to 30 years of age. Very rarely, an individual has two defective antithrombin genes (homozygous), resulting in severe thrombotic problems soon after birth. There are two types of inherited antithrombin deficiency. With Type 1, normal antithrombin is produced, but the quantity made is insufficient. With Type 2, there is a sufficient quantity of antithrombin produced, but it is dysfunctional. These types can be detected, differentiated, and assessed using two antithrombin tests. They are:

  • Antithrombin activity, which evaluates the function of a patient’s antithrombin
  • Antithrombin antigen, which measures the quantity of antithrombin present

2. Acquired antithrombin deficiencies may occur at any age. These result in a quantitative deficiency of antithrombin (Type 1 deficiency). They are associated with a variety of conditions, including liver disease, extensive thrombosis, disseminated intravascular coagulation (DIC), blood loss, poor nutrition, surgery, cancer, nephrotic syndrome – a form of kidney disease, metastatic (widespread) tumours and chemotherapy with asparaginase.

Accordion Title
Common Questions
  • How is it used?

    Antithrombin testing is primarily requested to investigate the cause of recurrent venous thrombosis and is normally part of a wider ‘thrombophilia screen’. The antithrombin activity test is performed before the antigen test, to evaluate whether the total amount of functional antithrombin is normal. Activity will be decreased with both Type 1 and Type 2 antithrombin deficiencies, so this test can be used as an initial screen for both. If the antithrombin activity is low, then the antithrombin antigen test is performed to determine the quantity of antithrombin present. If a deficiency is detected, both antithrombin tests are repeated at a later date to confirm test findings. Antithrombin testing may be used to evaluate patients not responding as expected to heparin.

  • When is it requested?

    Antithrombin activity testing is requested, along with other proteins involved in clotting regulation (such as Protein S and Protein C), when a patient has been experiencing recurrent venous thrombosis. Antithrombin should be measured after a blood clot has been treated and resolved as both the presence of the clot, and the therapy used to treat it, will affect antithrombin results.

    Antithrombin testing may be requested in the close blood relatives of someone with low levels of antithrombin to see if they share the problem.

    Antithrombin testing may be used when a patient is not responding as expected to heparin anticoagulation - when unusually high doses of heparin are necessary to achieve the desired level of anticoagulation.

  • What does the test result mean?

    Decreased antithrombin activity and decreased quantities of antithrombin antigen suggest a Type 1 antithrombin deficiency. In this case, the activity is decreased because there is less antithrombin available to participate in clotting regulation. Reduced antithrombin activity and normal levels of antithrombin antigen suggest a Type 2 antithrombin deficiency. This means that there is sufficient antithrombin, but it is not functioning as it should.

    If the antithrombin activity is normal, then the antithrombin antigen test is usually not performed. In this case, the antithrombin is functioning adequately and the recurrent thrombotic episodes being investigated are likely due to another cause.

    Increased levels of antithrombin are not usually considered a problem.  They may be seen with:

    • Acute cholestasis
    • Kidney transplant
    • Vitamin K deficiency
    • Warfarin (coumadin) anticoagulation therapy (sometimes)

    Testing for antithrombin deficiency is not recommended unless the patient’s condition is stable. Temporarily or chronically decreased antithrombin levels may be seen with conditions that affect its consumption or production, such as:

    • an acute or chronic condition characterised by the consumption of clotting factors. An affected patient may bleed and/or thrombose. 
    • DVT (deep vein thrombosis – a blood clot usually in a deep leg vein)
    • Pulmonary embolism (a clot which travels in the circulation to the lung from another part of the body)
    • Pregnancy
    • Liver disease
    • Nephrotic syndrome
    • Protein-losing condition
    • Neonates during the first few days of life
    • Oestrogen therapy
    • Metastatic tumours
    • Chemotherapy with asparaginase
  • Is there anything else I should know?

    If a patient with an antithrombin deficiency also has other coagulation risks, such as a Protein C or S deficiency, a Factor V Leiden mutation, or oral contraceptive use, then they may be at a significantly elevated risk of developing a blood clot.

    Antithrombin deficiency can increase the risk of recurrent miscarriage.

    Patients with antithrombin deficiency may need to take preventative anticoagulation prior to medical or surgical procedures.

    Antithrombin concentrates may be used to temporarily correct acute or chronic antithrombin deficiencies.

    Genetic testing of the antithrombin gene (SERPINC1) may be performed in select cases to identify the mutation causing the faulty gene. This can determine whether the patient has one defective gene (heterozygous) or two defective genes (homozygous) and can confirm the type of antithrombin deficiency (Type 1 or 2). Some mutation types may be associated with a higher or lower risk of thrombosis.


  • How can I determine my risk of developing a blood clot?

    If you have a personal or strong family history of recurrent blood clots, your doctor may do a physical to determine your general state of health and request a series of tests to determine your risk of having a hypercoagulable disorder. The more inherited or acquired risk factors you have (such as a Factor V Leiden or PT 20210 mutation, or a Protein C or S deficiency), the higher your relative risk of clotting. This risk can be further increased if you are overweight, sedentary, smoke, and/or take oral contraceptives. It is important to remember, however, that any overall relative risk that is determined is still a statistical risk. No one can predict whether a particular patient will actually experience recurrent clotting.

  • Should I tell my dentist or other doctors about my antithrombin deficiency?

    Yes.  This is important information that your health care providers should know about as it affects your body’s ability to clot appropriately during and after a medical procedure.