Antithrombin III

Written by Darla Burke
Medically Reviewed by George Krucik, MD

What Is an Antithrombin III Test?

When you bleed, your body has natural defenses that keep you from losing too much blood. These defenses are collectively known as clotting. Chemicals that aid in the clotting process are known as clotting factors. Clotting factors also help maintain the consistency of blood flow in your vessels.

Although clotting factors are important for protecting you from blood loss, there are other proteins that are responsible for ensuring that excessive blood clotting does not occur. When too much clotting occurs, you can develop life-threatening clots that block blood flow to your vital organs. One protein that regulates this clotting process is antithrombin.

Antithrombin acts as a natural blood thinner. If you develop clotting disorders, such as venous thrombosis, your doctor may order a test known as antithrombin III. This test is used to measure the amount of antithrombin protein in your body. It will help to determine if you have an antithrombin deficiency.

The antithrombin III test is also known as:

  • functional antithrombin III
  • AT III
  • antithrombin

Why Is an Antithrombin III Test Performed?

Your doctor may order the antithrombin III test if you develop venous thrombosis more than once. Deep vein thrombosis (DVT) occurs when a clot, or thrombus, develops in one of the veins deep in your body. This type of clot can develop anywhere, but is most common in the legs. DVT can be a life-threatening condition. The clot in your vein may break free and travel to other parts of the body. If the clot travels to your lung, it can cause a pulmonary embolus (clot in the lungs).

Recurring venous thrombosis indicates that you may not have enough antithrombin in your body to prevent clots from forming. Antithrombin deficiencies can develop due to other health problems, such as atherosclerosis (hardening of the arteries). Antithrombin deficiencies can also be caused by genetic disorders, such as Factor V Leiden thrombophilia.

Preparing for the Test

There are certain medications that may interfere with the results of the test, especially blood thinners, such as ibuprofen and warfarin. You may need to stop taking these medications before the test. Talk with your doctor about all medications you are taking, including prescription and non-prescription drugs.

How Is the Antithrombin III Test Administered?

The antithrombin III test is typically administered by a nurse or lab technician in a clinical setting. You will simply be required to provide a blood sample. The blood sample is usually drawn from the arm using a small needle. The blood is collected in a tube and sent to a lab for analysis. Once the lab reports the results, your doctor will be able to provide you with more information about what they mean.

What Are the Risks of the Antithrombin III Test?

If you have an antithrombin III test, you may experience some discomfort when the blood sample is drawn. Needle sticks may result in pain at the insertion site. After the test, you may experience lightheadedness or throbbing at the puncture site.

The risks of the antithrombin III test are minimal. These risks are common to all routine blood tests, and include:

  • difficulty obtaining a sample, resulting in multiple needle sticks
  • excessive bleeding at the puncture site
  • fainting or lightheadedness
  • the accumulation of blood under the skin, known as a hematoma
  • development of an infection where the skin is broken by the needle

Understanding the Test Results

You will need to talk with your doctor about your results and what they mean for you. Typically, if your antithrombin level is lower than normal, you have an antithrombin deficiency. If you have this deficiency, you are at an increased risk for developing venous thrombosis and other conditions, including:

  • phlebitis (inflammation of the veins)
  • pulmonary embolus (a blood clot in the lung)
  • stroke
  • heart attack
  • thrombophlebitis (inflammation of the veins that causes a clot to form)

Additional testing will be required to determine whether your antithrombin deficiency may be caused by:

  • liver failure caused by cirrhosis (scarring)
  • kidney disease (nephrotic syndrome)
  • certain types of cancer, such as pancreatic cancer
  • trauma

Test results may indicate higher-than-normal levels of antithrombin. However, these values do not indicate any significant health problems.

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