Antithrombin is a natural anticoagulant that inhibits the activated coagulation factors thrombin (factor IIa), factor Xa, and, to a lesser extent, factor XIa and factor IXa.
The reference range of antithrombin varies by age, as follows:
Children and adults: 80%-120%
Plasma concentration: 0.15-0.2 mg/mL
An increased antithrombin level is not a clinical problem.
Inherited types of antithrombin deficiency are as follows:
Type I - Decreased functional and immunological antithrombin level
Type II - Decreased functional antithrombin activity, while protein concentration is normal
Acquired antithrombin deficiency may be associated with the following:
Unfractionated heparin therapy
Collection and Panels
Specimen: Citrated plasma
Collection: Blue-top tube with 3.2% sodium citrate
Centrifugation: 2000-2500 g for 15 minutes or similar regimen to produce platelet-poor plasma
Storage: Up to 8 hours at room temperature; plasma sample should be frozen within 1-2 hours; specimen is stable for one month at -20°C or 6-9 months at -80°C
Antithrombin activity is measured by synthetic chromogenic activity in the presence of excess heparin levels.
A high concentration of hemoglobin, bilirubin, and triglycerides might affect antithrombin measurement.
Treatment with thrombin inhibitors may lead to overestimation of the antithrombin level in plasma.
Antithrombin is glycoprotein (molecular weight, 58 kDa) that is synthesized in the liver. It circulates in the blood, with a half-life of 2-3 days.
Antithrombin is a natural anticoagulant that inhibits the activated coagulation factors thrombin (factor IIa), factor Xa, and, to a lesser extent, factor XIa and factor IXa. Heparin significantly increases the inhibition rate.
The antithrombin level does not influence the results of screening coagulation tests such as partial thromboplastin time (PTT), prothrombin time (PT), and thrombin time. The measurement of antithrombin activity (functional antithrombin level) is a widely used laboratory test in clinical practice, while the antithrombin antigen (immunological antithrombin level) assay, which is used to confirm inherited antithrombin deficiency only, is rarely used.
Antithrombin testing is indicated in the following: 
Monitoring of antithrombin substitution therapy in the treatment of inherited or acquired antithrombin deficiency
Resistance of unfractionated heparin therapy 
Inherited antithrombin deficiency is less common than acquired deficiency.  The diagnosis of hereditary deficiency requires testing of both antithrombin activity and antithrombin antigen, and repeated testing and family studies may be needed.
Antithrombin deficiency can increase the risk of recurrent miscarriage. 
Administration of full-dose unfractionated heparin (but not LMWH) can cause a reversible reduction in antithrombin levels of up to 30% within several days. A falsely normal antithrombin concentration might be determined in antithrombin-deficient patients treated with thrombin inhibitors, but not with anti-Xa inhibitors.