Antithrombin Ill
Accurate Antithrombin III test to evaluate clotting function and thrombosis risk.
Turnaround time
3 days
Biomarkers count
1
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Details about Antithrombin Ill
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
- Book your appointment at your chosen London Blood Tests location.
- A trained clinician performs a venous blood draw.
- Your sample is processed in a UKAS-accredited laboratory.
- Do tOptional GP consultation available to discuss results.
At-Home Service – Phlebotomy Blood Draw
- Book your visit and choose a preferred time slot.
- A professional phlebotomist attends your home or hotel for an additional £60.
- Your blood sample is collected safely and sent to the lab.
- 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.
How It Works
Simple steps to get your results
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Choose your test online and book in seconds. Select your preferred clinic location or home visit option.
Visit or Test at Home
Attend one of our UK or London clinics, arrange a home nurse visit, or use a finger-prick kit where available.
Get Your Results
Your sample is analysed by accredited UK laboratories, with secure results delivered directly to you.
Take control of your health with London Blood Tests
WhatsApp UsHow to Prepare for Your Test
Follow these guidelines for accurate results
Sample Timing
Where possible, attend your blood test between 7am and 12pm. Please wait until any short-term illness or infection has fully resolved before testing. Avoid intense exercise for 24–48 hours beforehand, as this can affect certain markers.
Fasting & Hydration
Fasting is not always required, but some tests may recommend it. If fasting is advised, avoid food for 8–12 hours before your appointment and drink water only. Stay well hydrated, as this helps with sample collection and accuracy.
Medications & Supplements
Continue prescribed medications unless advised otherwise by your clinician. Avoid vitamin, mineral, or biotin supplements for at least 24–48 hours before your test, as these can interfere with results. If you are unsure about any medication or supplement, please let us know before your appointment.
Hormones & Menstrual Cycle
For hormone-related tests, timing within your menstrual cycle may be important. If relevant, follow any specific guidance provided on your test page.
If you have any questions or special circumstances, our team is happy to advise before your appointment.