What the antithrombin III test shows
Antithrombin III — now usually shortened to antithrombin (AT) — is one of the body’s natural anticoagulants: a liver protein that switches off the clotting enzymes thrombin (factor IIa) and factor Xa, supplying most of the plasma’s brake on clotting. Heparin sharply accelerates its action, which is how heparin thins blood (see below). Per StatPearls, antithrombin is a serpin and, unlike the factors behind PT/INR, is not vitamin-K-dependent.
That makes it the mirror image of the other coagulation tests. Prothrombin time (PT/INR), aPTT and thrombin time time how fast blood clots and flag a tendency to bleed. Antithrombin measures a natural anticoagulant, so a low result flags the opposite: a tendency to clot (thrombophilia). It is ordered mainly to investigate unexplained venous clots — deep-vein thrombosis (DVT) or pulmonary embolism (PE) — usually within a thrombophilia panel.
Both the activity (functional) and antigen assays report as a percentage of normal. Activity is measured first, showing whether the antithrombin present works; if low, the antigen test shows how much protein is there — telling too little normal protein from a normal amount of faulty protein. MedlinePlus notes a low result means blood clots more easily than it should.
Antithrombin III normal range
Antithrombin is reported as a percentage of normal pooled plasma, identical in the conventional and SI systems, so US and European reports read alike. Typical orientation:
| Group | Antithrombin activity, % of normal |
|---|---|
| Adults (men and women) | ~80–120 |
| Newborns | physiologically lower; reaches the adult range by ~6 months |
| Pregnancy, estrogen or oral contraceptives | often modestly lower — read with caution |
Sex makes little difference. Heterozygous inherited deficiency usually sits around 40–70%, while heparin, an active clot, liver disease and pregnancy all pull the number down — which is why timing matters (below). Per Mayo Clinic Laboratories, the functional assay is the preferred first-line screen. Reference ranges depend on the lab, sex and age — always read your result against your own report.
Why antithrombin III is low
A low antithrombin is the finding that matters, because it raises the risk of venous clots. By far the commonest reasons are acquired, not inherited:
- Consumption in active clotting: a fresh DVT or PE, and especially disseminated intravascular coagulation (DIC), sepsis, major trauma, burns or cardiopulmonary bypass.
- Reduced production: liver disease such as cirrhosis or acute liver failure, since antithrombin is made there.
- Increased loss: nephrotic syndrome loses it in the urine; protein-losing bowel disease loses it through the gut.
- Drugs and hormones: heparin lowers it modestly; estrogen, the contraceptive pill, pregnancy and L-asparaginase chemotherapy also reduce it.
- Inherited deficiency: rarer (roughly 1 in 2,000–5,000), from a variant in the SERPINC1 gene, inherited in an autosomal-dominant pattern. It is the most thrombogenic of the inherited thrombophilias, often causing clots before age 50, per StatPearls. Type I is a shortage of normal protein (activity and antigen both low); type II is a normal amount of faulty protein (activity low, antigen normal).
When is it urgent? A low antithrombin with a new clot, or with the falling platelets and fibrinogen of suspected DIC, needs same-day care; in pregnancy, inherited deficiency carries a high clot and miscarriage risk. Because heparin works through antithrombin, true deficiency can also cause heparin resistance, needing large doses or an alternative anticoagulant.
Why antithrombin III is high
A high antithrombin has no established disease link and needs no work-up — a reassuring result. Mild elevations can accompany warfarin (vitamin-K-antagonist) treatment and, occasionally, anabolic steroids. Because it neither causes bleeding nor protects against clots, an isolated high value is not chased; this test’s value lies in detecting low levels.
What to test alongside
Antithrombin is rarely read alone:
- D-dimer — flags active clotting and clot breakdown; often raised as antithrombin is consumed.
- Fibrinogen and platelets — fall alongside antithrombin in DIC.
- Prothrombin time and INR — core clotting times and warfarin monitor; also gauge liver synthesis.
- aPTT — monitors heparin, which behaves unreliably when antithrombin is deficient.
- Thrombin time — sensitive to heparin effect.
- Lupus anticoagulant — another cause of unexplained clotting, in the same panel.
- ALT and AST — check liver synthesis when antithrombin is low.
- Creatinine — a kidney check when nephrotic protein loss is suspected.
A full thrombophilia panel also adds protein C, protein S, factor V Leiden and the prothrombin gene mutation.
What to do about an abnormal result
- Don’t self-interpret a single low value. Most low results are acquired and temporary, not an inherited disorder.
- Mind the timing. Heparin, warfarin, pregnancy, the contraceptive pill, an active clot and acute illness all distort the result. Confirming inherited deficiency means repeating the activity test when you are well and off these influences — usually weeks after a clot.
- Confirm and classify. A genuinely low activity is rechecked and paired with an antigen assay to tell type I from type II, alongside your personal and family clot history.
- See the right doctor. Primary care coordinates the first steps; suspected inherited deficiency belongs with a hematologist, and clotting in pregnancy with an obstetric or thrombosis specialist.
- Never start or stop anticoagulants on your own. A diagnosis does not automatically mean lifelong blood thinners — decisions weigh prior clots, family history and high-risk situations such as surgery, immobility and pregnancy.
Mini-FAQ
What is antithrombin III and what does it do?
Antithrombin III (usually just “antithrombin”) is a natural blood thinner made by the liver. It switches off clotting enzymes such as thrombin and factor Xa; low levels tip the balance toward clotting.
What is a normal antithrombin level?
For adults, an activity of about 80–120% of normal. Newborns run lower and catch up by around 6 months. Ranges vary by lab, so read your result against your own report.
Does a low antithrombin mean I will get a blood clot?
Not necessarily. Low antithrombin raises the risk of venous clots (DVT, pulmonary embolism), and inherited deficiency carries the highest risk among the inherited thrombophilias — yet many people with low levels never clot. Risk also depends on family history, past clots and situations like surgery or pregnancy.
Why is my antithrombin low if it isn’t inherited?
Acquired causes are far more common: an active clot, liver disease, nephrotic syndrome (protein lost in urine), DIC, major surgery, pregnancy, estrogen and heparin all lower it. That is why a low value is repeated when you are well.
Should the test be repeated?
Yes. A single low value is often temporary, and heparin, warfarin, pregnancy, an active clot and acute illness all distort it. Inherited deficiency is confirmed by repeat testing when you are well and off those influences, plus family history — usually with a hematologist.
