Functional Antithrombin III

This is a test used to help investigate the cause of recurrent inappropriate blood clotting; to help diagnose an antithrombin deficiency.

Antithrombin testing measures the quantity of antithrombin and how well it works. Antithrombin is a protein that helps control blood clotting. When a blood vessel is injured, the body activates a series of coagulation factors to create a blood clot and prevent further blood loss. Antithrombin helps regulate this process. It does this by slowing down the action of several clotting factors. If someone has an inherited or acquired antithrombin deficiency, they are at an increased risk of forming clots. There are two types of antithrombin deficiency. With type 1, not enough thrombin is made. With type 2, it does not work normally.


A blood sample is obtained by inserting a needle into a vein in the arm.


Plasma: Greater than 50% of control value

Serum: 15%-34% lower than plasma value

Immunologic: 17-30 mg/dL

Functional: 80%-120%

Values vary according to laboratory methods.

Ranges for normal findings may vary among different laboratories and hospitals. You should always check with your doctor after having lab work or other tests done to discuss the meaning of your test results and whether your values are considered within normal limits.


Your caregiver will go over the test results with you and discuss the importance and meaning of your results, as well as treatment options and the need for additional tests if necessary.


It is your responsibility to obtain your test results. Ask the lab or department performing the test when and how you will get your results.