Antithrombin III
One of the body's natural blood thinners — a protein that keeps clotting under control. A low level (antithrombin deficiency), inherited or acquired, tips the balance toward clotting and raises the risk of abnormal clots in veins, so it is tested when someone has an unexplained or unusual clot.
What it measures
Blood clotting is a careful balance between proteins that promote clots and proteins that keep clotting in check. Antithrombin III is one of the most important of these natural anticoagulants: it inactivates thrombin and other clotting factors, damping the cascade so that clots form only where and when they are needed. The test usually measures antithrombin activity as a percentage of normal. Its clinical role is in investigating a tendency to form clots (thrombophilia). A low antithrombin level shifts the balance toward clotting and raises the risk of venous thrombosis — clots in the deep veins of the legs or in the lung. This can be inherited (a genetic antithrombin deficiency, one of the stronger inherited clotting risks) or acquired — through liver disease (less is made), kidney disease that loses protein in the urine, or being consumed during a large clot or DIC. Antithrombin deficiency also explains why some people respond poorly to the blood thinner heparin, which works through antithrombin. It is tested, alongside protein C and protein S, as part of a thrombophilia work-up, ideally away from an acute clot and off anticoagulants, since both affect the level.
What a high value can mean
- A normal or high antithrombin level is reassuring — it does not indicate a clotting problem.
- A high level has no adverse clinical significance.
What a low value can mean
- Inherited antithrombin deficiency — a genetic thrombophilia, one of the stronger inherited clotting risks.
- Liver disease — less antithrombin is made.
- Kidney disease (nephrotic syndrome) — protein is lost in the urine.
- Active clotting or DIC — antithrombin is consumed.
- Heparin resistance — low antithrombin blunts heparin's effect.
When to discuss with a doctor
Antithrombin is a specialist test, part of a thrombophilia work-up rather than a routine check. It is measured mainly when someone has had a blood clot that is unexplained, unusually severe, recurrent, in an unusual site, or at a young age, or when there is a strong family history — to look for an inherited tendency to clot. A confirmed antithrombin deficiency influences how a clot is treated and prevented, decisions about longer-term anticoagulation, and guidance for family members. Because an acute clot and blood thinners themselves lower the level, timing and interpretation are important and best handled by a haematologist. Mediora.AI shows antithrombin alongside protein C, protein S and D-dimer so the thrombophilia pattern is read together; confirming and acting on a deficiency belongs with your doctor or haematologist.