AT III Test: Normal Range, Purpose & Results

When Blood Clots Form Without Warning: Investigating Antithrombin III

In my clinical practice, I often see patients puzzled by sudden deep vein thrombosis or pulmonary embolism without obvious triggers. One of the first laboratory assessments in such cases is the Antithrombin III (AT III) test. This natural anticoagulant is a cornerstone of our body's delicate balance between bleeding and clotting.

What is Antithrombin III (AT III)?

Antithrombin III is a protein synthesised primarily in the liver. It functions as a serine protease inhibitor, directly neutralising key clotting factors like thrombin and Factor Xa. Essentially, it acts as a vital brake within the coagulation cascade, preventing excessive and potentially dangerous clot formation.

Why is the Antithrombin III Test Performed?

We typically order this test to investigate unexplained thrombotic events, especially in younger individuals or those with a family history of clotting disorders. It's also valuable in evaluating recurrent pregnancy loss, assessing patients before major surgery, and monitoring those with known liver disease or nephrotic syndrome.

Normal Antithrombin III Reference Ranges

Antithrombin III levels are most commonly reported as a percentage of normal functional activity. The following table outlines standard reference intervals.

Understanding Your Antithrombin III Test Results

What Causes Low Antithrombin III Levels?

Low levels, or antithrombin deficiency, significantly increase thrombosis risk. This can be inherited (congenital) or acquired. Acquired causes include liver cirrhosis, disseminated intravascular coagulation (DIC), nephrotic syndrome, major surgery, and certain medications like heparin, which can transiently lower levels.

Is High Antithrombin III Dangerous?

Elevated AT III is uncommon and generally not a clinical concern. It may even confer a protective effect against clot formation. Persistently high levels are occasionally seen in vitamin K deficiency or with some oral anticoagulants, but they rarely require specific intervention.

Antithrombin III During Pregnancy

Pregnancy induces a hypercoagulable state. AT III levels often decrease modestly, particularly in the third trimester. A significant drop below 70% can elevate the risk of venous thromboembolism, necessitating closer monitoring in at-risk women.

How is Antithrombin III Deficiency Diagnosed?

Diagnosis involves measuring both AT III activity (functional test) and antigen level (quantity). In cases suggesting inherited deficiency, genetic testing may be offered. My patients typically undergo this evaluation after a first unexplained clot or with a compelling family history.

Managing Antithrombin III Disorders

For individuals with congenital deficiency and a history of thrombosis, long-term anticoagulation with warfarin or direct oral anticoagulants is standard. In high-risk periods like surgery, childbirth, or acute thrombosis, AT III concentrate infusions can be used to temporarily boost levels and prevent clots.

When to Discuss Testing with Your Doctor

Consider speaking with your physician about AT III testing if you or a close relative has experienced unexplained blood clots, especially at a young age, or if there are multiple affected family members. Proactive assessment can guide effective preventive strategies.