Antithrombin III
Also known as: AT-III, Antithrombin
What Does Antithrombin III Measure?
Antithrombin III (AT-III) is a protein produced by the liver that acts as a natural anticoagulant — meaning it helps prevent blood from clotting too much. This test measures the activity or concentration of AT-III in your blood. AT-III works by inhibiting thrombin and other clotting factors (particularly Factor Xa and Factor IXa), essentially acting as a brake on the clotting cascade to keep blood flowing smoothly through your vessels.
Why Does Antithrombin III Matter?
Antithrombin III is clinically significant because low levels can lead to an increased risk of abnormal blood clots (thrombosis), including deep vein thrombosis (DVT), pulmonary embolism, and clots in unusual locations such as the mesenteric or cerebral veins. AT-III deficiency can be inherited (congenital) or acquired due to liver disease, nephrotic syndrome, or other conditions. Monitoring AT-III is especially important when evaluating unexplained clotting events, assessing clotting risk during pregnancy, or monitoring patients on heparin therapy, since heparin requires AT-III to function effectively.
Normal Ranges
Males
80–120% of normal activity (or approximately 21–30 mg/dL)
Females
80–120% of normal activity (or approximately 21–30 mg/dL)
Children
Varies by age; newborns typically 40–80%, reaching adult levels by 6 months
Causes of High Levels
- Use of anticoagulant medications such as warfarin or oral anticoagulants
- Acute hepatitis or early-stage liver inflammation with increased synthesis
- Kidney transplant or recovery from acute illness
- Use of anabolic steroids or certain androgenic hormones
- Post-blood transfusion or fresh frozen plasma administration
- Vitamin K deficiency in some settings (relative increase in AT-III ratio)
Causes of Low Levels
- Inherited (congenital) antithrombin III deficiency — autosomal dominant genetic mutation
- Liver disease (cirrhosis, hepatitis) reducing protein synthesis
- Nephrotic syndrome — excess loss of AT-III through urine
How to Improve Your Antithrombin III
Diet
- Consume adequate protein from lean meats, eggs, legumes, and dairy to support liver synthesis of AT-III
- Eat foods rich in omega-3 fatty acids (salmon, sardines, walnuts, flaxseed) to support vascular health and reduce inflammation
- Increase intake of leafy greens and antioxidant-rich vegetables to support liver function
- Stay well-hydrated with at least 8 glasses of water daily to maintain healthy blood viscosity
- Limit alcohol consumption to reduce liver stress and support adequate AT-III production
Supplements
- AT-III concentrate (antithrombin III concentrate) — prescribed by physicians for severe deficiency, typically dosed to bring levels to 80–120%
- Fresh frozen plasma (FFP) — used medically to temporarily restore AT-III in acute deficiency settings
- Vitamin C (500–1000 mg/day) to support liver health and collagen synthesis
- Milk thistle (silymarin, 140 mg three times daily) to support liver function and protein production
- Omega-3 fish oil (2–4 g/day EPA+DHA) to support cardiovascular and endothelial health
Related Biomarkers
Frequently Asked Questions
What does it mean if my Antithrombin III level is low?
A low AT-III level means your blood may have a reduced ability to control clot formation, which increases your risk of developing blood clots (thrombosis) in veins or arteries. This can lead to conditions like deep vein thrombosis (DVT) or pulmonary embolism. Low AT-III can be inherited or caused by liver disease, kidney disorders, or conditions that consume clotting proteins rapidly. Your doctor will typically investigate further with additional tests and may recommend anticoagulant therapy or preventive measures.
Is Antithrombin III deficiency hereditary?
Yes, congenital AT-III deficiency is an inherited condition passed down in an autosomal dominant pattern, meaning you only need one defective gene copy from one parent to be affected. It affects approximately 1 in 2,000 to 1 in 5,000 people and is one of the recognized causes of hereditary thrombophilia. People with inherited AT-III deficiency have a significantly elevated lifetime risk of blood clots and often need long-term anticoagulation management, especially during high-risk periods like surgery or pregnancy.
Why is Antithrombin III important for patients on heparin therapy?
Heparin works by binding to and dramatically amplifying the activity of AT-III. Without adequate AT-III levels, heparin cannot effectively prevent clotting. This means that patients with AT-III deficiency may appear 'heparin resistant' — requiring much higher doses of heparin to achieve therapeutic anticoagulation. In such cases, doctors may administer AT-III concentrate or fresh frozen plasma alongside heparin, or consider switching to an alternative anticoagulant that does not depend on AT-III.