What is an antithrombin III blood test?

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

Clotting protects you from blood loss. But too much clotting can cause life-threatening blood clots to form. These clots can block the flow of blood to your vital organs. Your body produces certain types of proteins that regulate the clotting process to stop that from happening. Antithrombin is one of those proteins.

Antithrombin acts as a natural blood thinner. If your blood clots when it isn’t supposed to, your doctor may order an antithrombin III blood test. This measures the amount of antithrombin protein in your body to learn if you have an antithrombin deficiency that’s causing your blood to clot more easily than normal.

The antithrombin III blood test is also known as the:

  • antithrombin test
  • functional antithrombin III test
  • AT III test

Your doctor may order an antithrombin III blood test if blood clots develop in your blood vessels, especially if it happens 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 it’s most likely to form in your legs. If the clot breaks free, it can travel to other parts of your body. If it travels to one of your lungs, it can cause a pulmonary embolus, or a clot in your lung. A DVT can be life-threatening.

If you develop recurring blood clots, it may mean that you don’t have enough antithrombin III or other clotting factors in your body to prevent clots from forming. Antithrombin deficiencies can result from other health problems, such as liver disease or certain types of kidney disease that decrease your body’s ability to produce functional forms of the antithrombin III protein. Deficiency may also occur if too much of the protein is being used up. You can also inherit antithrombin deficiencies through genetic mutations in the antithrombin gene.

Like all blood tests, the antithrombin III test involves some risks. These risks are common to all routine blood tests. They include:

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

This test is safe. The risks are minimal.

Certain medications can interfere with the results of an antithrombin III test, including blood thinners such as ibuprofen and warfarin. You may need to stop taking those medications before your test. Tell your doctor about all of the medications that you’re taking, including prescription and non-prescription drugs. Ask them if you should stop taking any of them in the days or hours leading up to your test.

For the antithrombin III test, you’ll need to give a blood sample in a clinical setting, such as your doctor’s office. A healthcare provider will likely draw a blood sample from one of your arms, using a small needle. They’ll collect your blood in a tube or vial. Then, they’ll send it to a lab for analysis.

Once the lab reports the results of your test, your doctor can help you understand what they mean. If your antithrombin level is lower than normal, you likely have an antithrombin deficiency. This puts you at an increased risk of developing DVT and other conditions, including:

  • phlebitis, or inflammation of the veins
  • thrombophlebitis, or inflammation of the veins that causes a clot to form
  • a pulmonary embolus, or a blood clot in the lung
  • a heart attack if the blood clot travels to arteries of the heart
  • a stroke if the blood clot travels to arteries of the brain

Your doctor may recommend additional testing to determine the cause of your antithrombin deficiency. Possible causes include:

  • liver failure due to cirrhosis, or scarring of the liver
  • nephrotic syndrome, or kidney disease
  • certain types of cancer, such as pancreatic cancer
  • trauma

Alternately, your test results may indicate normal or higher-than-normal levels of antithrombin. Higher-than-normal levels aren’t a sign of significant health problems.

Ask your doctor for more information about your results and follow-up steps.