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The Heparin Assay Test

written by: Lashan Clarke•edited by: Diana Cooper•updated: 7/27/2010

Heparin is mainly used to treat anyone condition in which the person has blood clots, or is at risk of forming a blood clot. The heparin assay test measures the level of heparin during treatment of medical conditions such as deep vein thrombosis and pulmonary embolism.

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    What Is Heparin?

    Heparin is a substance used to decrease the formation of blood clots within the body. It is given as an injection or IV infusion mainly as treatment for blood clots, certain clotting diseases such as pulmonary embolism. It is a substance that is produced naturally in the body by the cells of the immune system called the mast cells and basophils. Not only does heparin help will anti-coagulation, this substance is also used to improve the body’s defense. In this way, it does not breakdown the clots, but stimulates the body’s defense to act on it.

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    What Is Heparin?

    Heparin is mainly used to treat anyone condition in which the person has blood clots, or is at risk of forming a blood clot. The main medical conditions in which heparin is used include deep vein thrombosis and pulmonary embolism. It can also be used to treat the reaction to blood transfusions, prevent clotting during heart surgery, and during dialysis.

    Deep vein thrombosis occurs when the decreased rate of blood flow forms a blood clot within the legs and pelvis. Deep vein thrombosis can result from many diseases such as cancer, heart failure, or stroke. It can result in swelling in the area around the clot, but also accompanied by pain and redness.

    Pulmonary embolism is a condition in which a clot that has found in another area such as the leg becomes dislodged from the vessel and travels to the lungs. This is a medical emergency as the embolism becomes trapped in the lungs, and oxygen exchange is hampered. The person will have signs and symptoms of difficulty breathing, rapid breathing, coughing, chest pain, and in severe cases sudden death.

    The heparin assay test is used to determine the concentration of heparin currently within the body’s circulation. When heparin is used to treat someone with deep vein thrombosis who has kidney failure, it will be necessary to assay these concentrations to determine if an effective dose is reaching the tissues.

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    How Is The Test Performed?

    The test is performed by taking a sample of the person’s blood through venipuncture. The blood is drawn four hours after the previous does of heparin. The sample of blood is collected in a tube and the sample is added to a mixture of anti-thrombin and the enzyme Factor Xa. The amount of heparin is measured to determine if the dosing is correct.

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    What Does The Test Results Mean?

    The enzyme Factor Xa in the heparin assay test is used in the formation of thrombin. Thrombin is involved in the clotting process. If the person has heparin in his or her blood, it will bind to anti-thrombin, and not to Factor Xa. The amount of unattached Factor Xa left as a residue in the blood sample will be measured, and this is directly proportional to the person’s heparin concentration.

    Normally the concentration of heparin in a person being treated for deep vein thrombosis should be 0.3 to 0.7 units per milliliter of blood. The physician can adjust the dosing if too little or too much heparin is available in the blood.

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    Web Source: RCPA Manual. Australian Government Department of Health and Ageing. "Anti Xa (Anti factor Xa; Heparin assay) - plasma. 2007. Available:

    Web Source: Massachusetts General Hospital Pathology Service. "Heparin Antifactor Xa Assay." 2010. Available: