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Antithrombin Deficiency
Let's continue our thrombophilia series with antithrombin deficiency
What is Antithrombin?
Antithrombin (also known as antithrombin III) is a natural anticoagulant protein produced by the liver. It works by inhibiting several clotting factors, most importantly thrombin (Factor IIa) and Factor Xa. Think of it as one of the body's natural "brakes" on the clotting system. Heparin works by enhancing antithrombin's activity, which is why patients with severe antithrombin deficiency may have heparin resistance.
What is Antithrombin Deficiency?
Antithrombin deficiency is a condition in which there are reduced levels or reduced function of antithrombin. It can be inherited (congenital) or acquired. The inherited form is caused by mutations in the SERPINC1 gene and can present as either quantitative deficiency (low levels of normal protein) or qualitative deficiency (normal levels of dysfunctional protein). It can be heterozygous or homozygous, though homozygous deficiency is extremely rare and often incompatible with life.
Prevalence:
Heterozygous antithrombin deficiency occurs in approximately 0.02-0.2% of the general population (1 in 500 to 5,000 people)
Found in about 1-3% of patients with VTE
One of the least common inherited thrombophilias
No significant ethnic variation in prevalence
Risk of VTE:
Heterozygous antithrombin deficiency carries one of the highest risks among inherited thrombophilias - approximately 10 to 50 fold increased lifetime risk
First VTE often occurs at a young age (before age 50), frequently without obvious provocation
Risk varies depending on the specific mutation type
Acquired causes of low antithrombin include liver disease, nephrotic syndrome, DIC, acute thrombosis, heparin therapy, and L-asparaginase chemotherapy
Management:
1st Acute VTE episode: treat with anticoagulation; may require higher doses of heparin or direct oral anticoagulants. Antithrombin concentrate may be needed in severe cases
Recurrent VTE or unprovoked first VTE: strong consideration for lifelong anticoagulation
Pregnancy: prophylactic anticoagulation usually recommended throughout pregnancy and postpartum period due to high VTE risk
Family members: consider testing first-degree relatives
For surgeries or high-risk situations: prophylactic anticoagulation typically recommended
Due to the significantly elevated VTE risk, antithrombin deficiency often warrants more aggressive prevention and treatment strategies compared to Factor V Leiden or prothrombin gene mutation.
Reference: ASH SAP 8th edition, Patnaik & Moll, Blood Reviews 2008
I hope you found this helpful! In the next post, we'll discuss protein C and protein S deficiency.
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