Why Get Tested?
To help investigate the cause of recurrent inappropriate blood clotting; to help diagnose an antithrombin deficiency
When To Get Tested?
A couple of months after getting a blood clot (thrombotic episode) or when you are not responding as expected to heparin anticoagulation therapy.
What is being tested?
Antithrombin is a protein produced by the liver that helps regulate blood clot formation (i.e., a naturally-occurring mild blood thinner). Antithrombin testing measures the activity (function) and the amount (quantity) of antithrombin in an individual’s blood and is used to evaluate the person for excessive blood clotting.
Normally, when a blood vessel is injured, the body initiates a complex process called hemostasis to form a blood clot and prevent further blood loss. Part of this complex process involves the activation of several proteins called coagulation factors in a series of steps referred to as the coagulation cascade. Antithrombin helps to regulate this process by inhibiting the action of several activated coagulation factors, including thrombin and factors Xa, IXa, and XIa, to slow down the process and prevent excessive or inappropriate clotting (thrombosis).
People with an excessive clotting disorder due to an inherited or acquired antithrombin deficiency are at increased risk of developing blood clots, especially in deep veins such as in the legs (known as deep venous thrombosis or DVT). Inherited deficiencies are rare, affecting about 1 in 5,000 people. For people who inherit one defective gene and one normal gene (heterozygous), episodes of inappropriate blood clot formation typically start at about 20 to 30 years of age. Very rarely, a person may inherit two defective antithrombin genes, resulting in severe clotting problems soon after birth.
Acquired antithrombin deficiencies may occur at any age. They are associated with a variety of conditions that cause decreased production, excessive consumption, or loss of antithrombin. These conditions include liver disease, extensive thrombosis, disseminated intravascular coagulation (DIC), blood loss, cancer, and nephrotic syndrome – a form of kidney disease.
There are two types of antithrombin deficiency. With type 1, antithrombin functions normally, but the quantity is insufficient. With type 2, there is a sufficient quantity of antithrombin produced, but it is dysfunctional. These types can be differentiated and assessed by testing