Antithrombin III Activity

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Also known as: AT 3, AT III, Functional Antithrombin III

Antithrombin Iii Activity

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The Antithrombin III Activity test contains 1 test with 1 biomarker.

Brief Description: The Antithrombin III Activity test measures the activity of antithrombin, a protein in the blood that naturally inhibits clot formation. It's one of the primary inhibitors of clotting factor enzymes, particularly thrombin and Factor Xa. Adequate antithrombin levels are vital for controlling and balancing coagulation. The activity test provides insights into how well the antithrombin is functioning, rather than just its concentration in the blood.

Collection Method: Blood Draw

Specimen Type: Plasma

Test Preparation: No preparation required

When and Why an Antithrombin III Activity Test May Be Ordered

An Antithrombin III Activity test may be ordered:

  1. When an individual has an unexplained thrombotic episode, such as deep vein thrombosis (DVT) or pulmonary embolism.
  2. If someone has a family history of clotting disorders and is being evaluated for their risk of developing blood clots.
  3. In cases where recurrent miscarriages have occurred without a clear cause, to rule out clotting disorders as a factor.
  4. Before starting on certain anticoagulant therapies, particularly heparin, as antithrombin is crucial for heparin's effectiveness.

What an Antithrombin III Activity Test Checks For

The Antithrombin III Activity test checks for the functionality of antithrombin in the blood. It assesses how effectively antithrombin can inhibit clotting enzymes, indicating whether an individual has a normal ability to prevent excessive clotting.

Other Lab Tests Ordered Alongside an Antithrombin III Activity Test

When an Antithrombin III Activity test is ordered, it's often part of a broader assessment of clotting disorders. Here are some tests commonly ordered alongside it:

  1. Protein C and Protein S Activity:

    • Purpose: To measure the activity of proteins C and S, which, like antithrombin, help control blood clotting.
    • Why Is It Ordered: Deficiencies or abnormalities in these proteins can also increase the risk of thrombosis. Testing them together can help diagnose or rule out various causes of clotting disorders.
  2. Activated Partial Thromboplastin Time (aPTT):

    • Purpose: To assess the efficiency of the blood's clotting process.
    • Why Is It Ordered: Prolonged aPTT can be seen in antithrombin deficiency and other conditions affecting the intrinsic clotting pathway.
  3. Prothrombin Time (PT) and International Normalized Ratio (INR):

    • Purpose: To measure how long it takes for blood to clot.
    • Why Is It Ordered: To evaluate the extrinsic clotting pathway. These tests are also important in monitoring anticoagulant therapy.
  4. D-Dimer Test:

    • Purpose: To detect fragments of protein involved in blood clot breakdown.
    • Why Is It Ordered: Elevated levels can indicate recent or ongoing clot formation, although this test is not specific for diagnosing antithrombin deficiency.
  5. Factor V Leiden and Prothrombin G20210A Mutation Tests:

    • Purpose: To check for specific genetic mutations associated with an increased risk of thrombosis.
    • Why Is It Ordered: To identify a hereditary predisposition to clotting disorders.
  6. Lupus Anticoagulant and Anticardiolipin Antibodies:

    • Purpose: To test for the presence of these antibodies, which are associated with an increased risk of clotting and are markers for antiphospholipid syndrome.
    • Why Is It Ordered: To assess for other causes of thrombophilia, especially in individuals with a history of recurrent clots or miscarriages.
  7. Fibrinogen Level:

    • Purpose: To measure the level of fibrinogen, a protein that is essential for blood clot formation.
    • Why Is It Ordered: Abnormal levels can be associated with bleeding disorders or thrombotic conditions.
  8. Complete Blood Count (CBC):

    • Purpose: To evaluate overall blood health.
    • Why Is It Ordered: To identify any underlying conditions that might contribute to clotting disorders, such as polycythemia (increased red blood cell count).

These tests, when ordered alongside an Antithrombin III Activity test, provide a comprehensive evaluation of a person's risk for thrombophilia. They are crucial for identifying the cause of clotting disorders, assessing the risk of future clotting events, and guiding treatment and management decisions. The specific combination of tests will depend on the individual’s clinical history, family history, and any previous episodes of abnormal clotting.

Conditions or Diseases Requiring an Antithrombin III Activity Test

Several conditions or situations might prompt the use of this test:

  1. Antithrombin III Deficiency: A hereditary or acquired condition that reduces the level or functionality of antithrombin, increasing the risk of clots.
  2. Liver Disease: Since antithrombin is produced in the liver, liver diseases can affect its levels.
  3. Nephrotic Syndrome: A kidney disorder that might result in loss of antithrombin in the urine.
  4. Disseminated Intravascular Coagulation (DIC): A severe condition where small blood clots develop throughout the bloodstream.

How Health Care Providers Use the Results of an Antithrombin III Activity Test

The results of the Antithrombin III Activity test can provide crucial information about an individual's risk of developing blood clots:

  1. Normal levels: This typically means that the person's antithrombin activity is sufficient to help regulate coagulation.
  2. Decreased levels: This suggests a potential deficiency and an increased risk of clotting. The doctor may consider treatment with anticoagulant medication or further investigation to find the underlying cause.

Healthcare providers may also use the test results to monitor the effectiveness of treatment, particularly if antithrombin concentrates are being administered or if the patient is on anticoagulant therapy.

Most Common Questions About the Antithrombin III Activity test:

Purpose and Clinical Indications

Why is the Antithrombin III Activity test performed?

The Antithrombin III Activity test is performed to measure the activity of antithrombin, a protein that helps regulate blood clotting. By inhibiting certain clotting factors, antithrombin prevents excessive clotting, which can lead to thrombotic events such as deep vein thrombosis or pulmonary embolism.

How does Antithrombin III function in the body?

Antithrombin III functions as a primary inhibitor of thrombin and certain other clotting factors. When it binds to these clotting factors, it inactivates them, preventing the conversion of fibrinogen to fibrin, which is a key step in blood clot formation. Therefore, Antithrombin III plays a critical role in maintaining a balance between bleeding and clotting.

Interpretation of Results

What do decreased levels of Antithrombin III activity indicate in the Antithrombin III Activity test results?

Decreased levels of Antithrombin III activity can suggest a higher risk of excessive clotting. This could be due to a hereditary deficiency, where a person is born with low levels of antithrombin, or an acquired deficiency, which could result from conditions like liver disease, disseminated intravascular coagulation (DIC), or vitamin K deficiency.

What might elevated levels of Antithrombin III activity mean in the Antithrombin III Activity test results?

Elevated levels of Antithrombin III activity are less common and can be seen in certain conditions, such as after antithrombin supplementation or treatment. They may also be seen during certain stages of pregnancy, but it's less often associated with an increased clinical risk compared to decreased levels.

Clinical Implications

How is the Antithrombin III Activity test used in the management of patients on anticoagulant therapy?

For patients on anticoagulant therapy, especially heparin, the Antithrombin III Activity test is crucial. Heparin enhances the activity of antithrombin, making it more effective in inhibiting clotting factors. If a patient has a deficiency of antithrombin and is administered heparin, the drug may not be as effective. Thus, monitoring antithrombin levels in such patients ensures the effectiveness of heparin therapy.

How often should the Antithrombin III Activity test be done for those with known deficiencies?

For individuals with known antithrombin deficiencies, the frequency of the Antithrombin III Activity test varies based on clinical circumstances. Those who have had thrombotic events or are at a high risk might need more frequent monitoring, especially if they are on anticoagulant therapy. However, specific testing frequency should be individualized based on clinical judgment and the patient's history.

Relationships with Other Health Conditions

Are there specific conditions or diseases that affect Antithrombin III levels in the Antithrombin III Activity test?

Yes, various conditions can affect Antithrombin III levels. Liver disease can lead to decreased production of antithrombin, resulting in lowered levels. Nephrotic syndrome, a kidney condition, can lead to increased loss of antithrombin in urine. Disseminated intravascular coagulation (DIC), a complex systemic thrombohemorrhagic disorder, can consume antithrombin, also leading to decreased levels. Other conditions, such as inflammatory disorders, certain infections, or post-surgical states, may also affect antithrombin levels.

We advise having your results reviewed by a licensed medical healthcare professional for proper interpretation of your results.

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