Heparin Anti-Xa
- Also Known As:
- Anti-Xa chromogenic
- Xa Inhibition
- Anti-factor Xa Heparin
- Anti-FXa
- Heparin Level
- Heparin Activity

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.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 health care 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?
None
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The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.
If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.
Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called “normal” values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.
While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are “within normal limits.”
For more information, please read the article Reference Ranges and What They Mean.
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 (known as heparin 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. Since the test involves a chemical reaction color change (colorimetric), it is also known as chromogenic anti-Xa assay or anti-Xa assay, chromogenic.
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 conditions that can cause excessive clotting and lead to the formation of blood clots within veins and arteries. Some examples include surgeries, DVT (deep vein thrombosis), and other excessive clotting disorders (hypercoagulable disorders). These clots can obstruct blood flow and cause tissue damage. Pieces of the blood clot can also break off and travel to the lungs, causing pulmonary embolism. In pregnant women, blood clot formation can sometimes occur in the placenta and affect blood flow to the developing baby (fetus) and result in a miscarriage.
Heparin may be used to prevent or treat these excessive clotting conditions (anticoagulation therapy). Through its binding to the protein antithrombin, heparin interferes with the clotting process by inhibiting clotting factors, particularly factors Xa and IIa (thrombin).
Heparin molecules vary in size and activity and there are three types of heparin that may be used for treatment:
- Standard heparin, commonly known as unfractionated heparin (UFH), is usually given through injections into a vein (intravenously, IV). UFH affects both factors Xa and IIa and has varying effects among patients, so it must be closely monitored. Complications may include clotting (insufficient heparin), excessive bleeding (too much heparin), and sometimes decrease in platelets and thrombosis (heparin-induced thrombocytopenia and thrombosis).
- Low molecular weight heparin (LMWH) consists of a narrower range of smaller heparin molecules and is typically given through injections under the skin (subcutaneous). LMWH primarily affects Xa and its effect is more predictable; therefore, routine laboratory monitoring is not required.
- Fondaparinux consists of only a small fraction of UFH (a sequence of five monomeric sugar units of heparin) and only inhibits factor Xa. It does not inhibit thrombin. Like LMWH, fondaparinux is given subcutaneously and routine laboratory monitoring is not required.
Unfractionated heparin is often used for initial treatment when excessive clotting conditions are acute. It is eventually replaced by the use of oral anticoagulants or LMWH for longer-term treatment to lower the risk of blood clots. 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 typically monitored using the activated clotting time (ACT) test.
LMWH and fondaparinux may be given in either an outpatient or hospital setting. Neither LMWH nor fondaparinux significantly prolong PTT at therapeutic dosage. If monitoring is required, the anti-Xa test is used.
Common Questions
Related Content
On This Site
Tests: PTT, ACT, Platelet Count, HIT Antibody, Antithrombin, Lupus Anticoagulant Testing
Elsewhere On The Web
National Heart, Lung and Blood Institute (NHLBI): What is Deep Vein Thrombosis?
American Academy of Orthopaedic Surgeons: Deep Vein Thrombosis
March of Dimes: Thrombophilias
NHLBI: What is Antiphospholipid Antibody Syndrome?
MedlinePlus Drug Information: Heparin Injection
View Sources
Sources Used in Current Review
Wintrobe’s Clinical Hematology. 12th ed. Greer J, Foerster J, Rodgers G, Paraskevas F, Glader B, Arber D, Means R, eds. Philadelphia, PA: Lippincott Williams & Wilkins: 2009, Pp 1483-1484.
Henry’s Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2011, Pp 839-840.
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Sources Used in Previous Reviews
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