Your Natural Anti-Clotting Shield
Antithrombin is your body's built-in blood thinner. It neutralizes thrombin and other clotting factors, preventing clots from forming when they shouldn't. Think of it as the brakes on your coagulation system. Without adequate antithrombin, your blood clots too easily (thrombophilia). Heparin works by supercharging antithrombin.
What is Antithrombin III?
Antithrombin (AT, previously AT-III) is a serine protease inhibitor (serpin) that inactivates thrombin, Factor Xa, and other clotting proteases. It's the primary physiological anticoagulant. Heparin accelerates its activity 1000-fold. Produced by the liver.
↑ What High Antithrombin III Means
Rarely clinically significant. Can occur with warfarin use or acute illness.
Common symptoms:
Generally not clinically significant
↓ What Low Antithrombin III Means
Your natural anticoagulant defense is weakened. Increased risk of blood clots (DVT, PE). Can be inherited (antithrombin deficiency—most thrombogenic inherited thrombophilia) or acquired (liver disease, nephrotic syndrome, DIC, heparin use).
Common symptoms:
Recurrent blood clots (DVT, PE) · Clots at young age or unusual locations · Heparin resistance (heparin doesn't prolong aPTT as expected)
Why It Matters
When normal:
Primary natural anticoagulant
Deficiency is the most thrombogenic inherited thrombophilia
Explains heparin resistance (low AT = heparin doesn't work)
Important in DIC management
Risks if abnormal:
Low: high clot risk (DVT, PE, mesenteric thrombosis)
Inherited deficiency: 50% lifetime clot risk
Heparin resistance if AT very low
What Can Cause Abnormal Levels?
Inherited Deficiency
30% likelyAutosomal dominant. Prevalence ~1:5000. Most thrombogenic inherited thrombophilia. 50% lifetime DVT/PE risk.
Liver Disease (acquired low)
40% likelyLiver produces antithrombin. Cirrhosis reduces production.
Nephrotic Syndrome (acquired low)
Antithrombin is lost in urine due to proteinuria.
DIC (consumed)
Antithrombin is consumed by widespread clotting.
Heparin (consumed)
Heparin uses antithrombin to work—prolonged heparin can deplete it.
Estrogen (decreased)
Oral contraceptives and pregnancy lower antithrombin.
What You Can Do
Don't test during acute clot or heparin therapy (results unreliable)
Impact: Test when stable, off anticoagulation for 2 weeks \u00B7 Timeline: Delayed
If lifestyle changes aren't enough:
If inherited deficiency confirmed: lifelong awareness of clot risk
Impact: Avoid additional risk factors (smoking, OCP, immobility) \u00B7 Timeline: Lifelong
Genetic counseling for family members
Impact: Autosomal dominant—50% chance of passing to children \u00B7 Timeline: One-time
Recommended retest: Confirm with repeat testing 2+ weeks off anticoagulation; test family members
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