What is the Antithrombin III Blood Test

The Antithrombin III test measures the level and activity of antithrombin, a natural anticoagulant produced by the liver. Antithrombin regulates blood clot formation by inhibiting thrombin and Factor Xa. If your antithrombin level is too low, the blood may clot too easily, increasing the risk of deep vein thrombosis, pulmonary embolism or pregnancy-related complications. This test helps identify hereditary antithrombin deficiency, acquired deficiency or other clotting abnormalities.

Benefits of the Test

• Helps diagnose inherited or acquired antithrombin deficiency
• Assesses risk of abnormal blood clots including DVT and pulmonary embolism
• Supports investigation of recurrent miscarriages and pregnancy complications
• Useful before major surgery or when considering hormonal treatments
• Helps monitor liver disease or nephrotic syndrome, where antithrombin levels may drop
• Guides clinicians when evaluating unexplained clotting or anticoagulation resistance
• Offers early detection of clotting imbalances that may otherwise go unnoticed

Why You Might Need an Antithrombin III Test

You may benefit from this test if you experience any of the following:

Symptoms or history

• Unexplained blood clots
• Recurrent deep-vein thrombosis
• Pulmonary embolism
• Clots occurring at a young age
• Recurrent pregnancy loss
• Family history of thrombophilia
• Swelling, redness or pain in the legs
• Shortness of breath without clear cause

Who may need this test

• Individuals with unexplained or recurrent clotting events
• Patients with suspected inherited thrombophilia
• Pregnant women with past complications or clotting disorders
• People with liver disease, nephrotic syndrome or active malignancy
• Individuals starting estrogen-containing contraception or HRT
• Patients who show poor response to heparin therapy

Step by Step: How It Works

In-Clinic Service – Available Across Multiple UK Locations

1. Book your appointment at your chosen London Blood Tests location.
2. A trained clinician performs a venous blood draw.
3. Your sample is processed in a UKAS-accredited laboratory.
4. Do tOptional GP consultation available to discuss results.

At-Home Service – Phlebotomy Blood Draw

1. Book your visit and choose a preferred time slot.
2. A professional phlebotomist attends your home or hotel for an additional £60.
3. Your blood sample is collected safely and sent to the lab.
4. Results arrive digitally, with the option of GP review if required.

How to Interpret Your Antithrombin III Levels

Antithrombin III (ATIII) results are typically reported as a percentage (%) of normal activity.
Use the guide below to understand what your levels may indicate:

Low Antithrombin III (< 80%)

Low ATIII activity may suggest:

• Increased risk of abnormal blood clotting
• Possible hereditary antithrombin deficiency
• Recent thrombosis (DVT, PE)
• Liver disease
• Nephrotic syndrome (protein loss)
• Heparin therapy affecting results
  
  

Normal Antithrombin III (80–120%)

This range indicates:

• Normal clot-regulation function
• No biochemical evidence of ATIII deficiency

High Antithrombin III (> 120%)

Higher-than-normal levels can be associated with:

• Acute inflammation
• Severe infections
• Certain cancers
• Estrogen therapy
• Heparin therapy rebound

Important Considerations

• Pregnancy, oral contraceptives, acute clotting events, and current anticoagulant medication may influence results.
• Interpretation should always consider clinical symptoms and medical history.