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Heparin Anti-Xa

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Also known as: Anti-Xa; Xa Inhibition; Anti-factor Xa Heparin; Anti-FXa; Heparin Level; Heparin Activity
Formal name: Heparin Anti-Xa

At a Glance

Why Get Tested?

To monitor standard, unfractionated heparin (UFH) therapy and sometimes to monitor low molecular weight heparin (LMWH) therapy

When to Get Tested?

When you are being treated with UFH or LMWH and your healthcare provider wants to monitor the amount of heparin in your blood

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


The Test Sample

What is being tested?

Heparin is a drug that inhibits blood clotting (anticoagulant) and is used to treat people who have developed dangerous blood clots (thrombi) or have a high risk of developing them. This test indirectly measures the amount of heparin in a person's blood by measuring its inhibition of factor Xa activity, one of the proteins involved in blood clot formation (anti-Xa activity). The test is used to monitor heparin therapy to ensure that a person is receiving sufficient heparin for anticoagulation without causing excess bleeding.

Blood clotting is a normal response to blood vessel or tissue injury. It is a complex process that involves the activation and clumping of platelets at the site of injury and the initiation of the coagulation cascade - a sequential activation of coagulation factors, proteins that produce clots and regulate their development.

There are a variety of acute and chronic conditions in which excessive clotting may occur. Some of these include surgeries, DVT (deep vein thrombosis), and other excessive clotting disorders (hypercoagulable disorders). They are associated with inappropriate blood clot formation in veins and arteries – especially in the legs. These clots can obstruct blood flow and cause tissue damage in the affected area. Pieces of the blood clot can break off and travel to the lungs – causing pulmonary embolism, or to the heart – causing a heart attack. In pregnant women, blood clot formation can sometimes affect blood flow to the fetus and result in a miscarriage.

Heparin, through its action on the protein antithrombin, interferes with the clotting process by accelerating the inhibition of coagulation factors, particularly factors Xa and IIa (thrombin). There are two types of heparin that may be used in anticoagulation therapy: low molecular weight heparin (LMWH) and unfractionated heparin (UFH), also known as standard heparin. Heparin molecules vary in size and activity. UFH includes a broad range of sizes, while LMWH consists of a narrower range of smaller heparin molecules. UFH is usually given intravenously (I.V.), whereas LMWH is typically given through injections under the skin (subcutaneous). UFH affects both factors Xa and IIa, is more variable in its inhibitory activity, and must be closely monitored. Complications may include clotting (insufficient heparin), excessive bleeding (too much heparin), and sometimes decreased platelets (heparin-associated thrombocytopenia). On the contrary, LMWH primarily affects Xa and its effect is more predictable; therefore routing monitoring is not required.

Heparin is often used for treatment when the condition is acute and is eventually replaced by the use of oral anticoagulants. UFH is usually given in a hospital setting and monitored with the partial thromboplastin time (PTT) test, but it may need to be monitored with the heparin anti-Xa test. High doses of UFH given during surgeries such as cardiopulmonary bypass are monitored using the activated clotting time (ACT) test. LMWH may be given in either an outpatient or hospital setting. If monitoring is required, the anti-Xa test is used.

How is the sample collected for testing?

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

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Harmening D. Clinical Hematology and Fundamentals of Hemostasis, Fifth Edition, F.A. Davis Company, Philadelphia, 2009, Pp 862-863, 584-585, 685-686.

Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Pp 780-781.

(© 1995-2011). Unit Code 80609: Heparin Anti-Xa Assay, Plasma. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at through Accessed November 2011.

Smith, M. and Wheeler, K. (2010 May 6). Weight-based Heparin Protocol using Antifactor Xa Monitoring. Medscape Today News from American Journal of Health-System Pharmacy. 2010;67(5):371-374. [On-line information]. Available online at through Accessed November 2011.

Guervil, D. et. al. (2011 August 9). Activated Partial Thromboplastin Time versus Antifactor Xa Heparin Assay in Monitoring Unfractionated Heparin by Continuous Intravenous Infusion. Medscape Today News from The Annals of Pharmacotherapy. 2011;45(8):861-868 [On-line information]. Available online at through Accessed November 2011.

Shapiro, N. et. al. (2011 August 1). Dosing and Monitoring of Low-molecular-weight Heparin in High-risk Pregnancy Single-Center Experience. Medscape Today News from Pharmacotherapy. 2011;31(7):678-685 [On-line information]. Available online at through Accessed November 2011.

Lehman, C. and Frank, E. (2009 January). Laboratory Monitoring of Heparin Therapy: Partial Thromboplastin Time or Anti-Xa Assay? ASCP LabMedicine, 40, 47-51 [On-line information]. Available online at through Accessed November 2011.

Sources Used in Previous Reviews

Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. Pp 233-234.

Tahir, R. (2007 July 13). A Review of Unfractionated Heparin and Its Monitoring. U.S. Pharm. 2007:32(7) [On-line information]. Available online at through Accessed on 6/1/08.

(2008 January, Updated). Heparin and Anti-Xa Assays]. Clot-ED [On-line information]. Available online at through Accessed on 6/1/08.

Schick, P. and Schick, B. (2007 April 30). Hypercoagulability: Hereditary Thrombophilia and Lupus Anticoagulants Associated with Venous Thrombosis and Emboli. emedicine [On-line information]. Available online at through Accessed on 6/1/08.

O’Donnell, M. et. al. (© 2007). Brief Communication: Preoperative Anticoagulant Activity after Bridging Low-Molecular-Weight Heparin for Temporary Interruption of Warfarin. Annals of Internal Medicine 2007;146:184-187. [On-line information]. Available online at through Accessed on 6/1/08.

Coon, D. and Hamachi-Lopez, M. (2007 January 8). New Test Announcement, Antifactor Xa Assay. CLH Technical Bulletin 07-003 [On-line information]. PDF available for download at through Accessed on 6/1/08.

Baglin, T. et. al. (© 2006) Guidelines on the use and monitoring of heparin. British Society for Haematology v(133) 19-34. [On-line information]. PDF available for download at through Accessed on 6/1/08.

McGlasson, D. (2005 May 25). Using a Single Calibration Curve With the Anti-Xa Chromogenic Assay for Monitoring Heparin Anticoagulation. Lab Med. 2005;36(5):297-299. [On-line information]. Available online at through Accessed on 6/1/08.

Hirsh, J. and Raschke, R. (© 2004). Heparin and Low-Molecular-Weight Heparin: The Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy. CHEST 2004;126;188-203. [On-line information]. Available online at through Accessed on 6/1/08.

Duplaga, B. et. al. (© 2001). Dosing and Monitoring of Low-Molecular-Weight Heparins in Special Populations. Medscape from Pharmacotherapy 21(2):218-234. [On-line information]. Available online at through Accessed on 6/1/08.

Yeager, B. and Matheny, S. (1999 February 15). Low-Molecular-Weight Heparin in Outpatient Treatment of DVT. American Family Physician [On-line information]. Available online at through Accessed on 6/1/08.