Also Known As
Xa inhibition
Antifactor Xa heparin
Heparin level
Formal Name
Heparin Anti-Xa (pronounced “anti-ten-a”)
This article was last reviewed on
This article waslast modified on 29 October 2017.
At a Glance
Why Get Tested?

To monitor low molecular weight heparin (LWMH) therapy and sometimes to monitor unfractionated heparin (UFH) therapy

When To Get Tested?

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

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

No test preparation is needed.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you will be able to access your results online.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, gender, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

This test measures the effect of low molecular weight heparin (LMWH) or unfractionated heparin (UFH) in the blood by measuring anti-Xa activity. Heparin is an anticoagulant, a drug that inhibits blood clotting. 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. There are several types of LMWH available and each one is slightly different. Both UFH and LMWH are given intravenously (I.V.) or through a subcutaneous injection to people who have inappropriate blood clots (thrombi) and/or are at an increased risk of developing them.

Blood clotting is a normal response to blood vessel or tissue injury. It is a process that involves the initiation of the coagulation cascade - a sequential activation of coagulation factors, proteins that regulate blood clot development. A variety of acute and chronic conditions including surgery, pregnancy and some oral contraceptives, serious illnesses where patients are bed-bound and other hypercoagulable disorders are associated with inappropriate blood clot (thrombus) formation in veins – 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. In pregnant women, blood clot formation can sometimes affect blood flow to the fetus and result in miscarriages or other complications of pregnancy.

Heparin can also inhibit blood clot formation in diseased arteries, which sometimes cause heart attacks or strokes.

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). UFH affects both Xa and IIa, is more variable in its inhibitory activity, and must be closely monitored. Complications may include clotting, excessive bleeding, and sometimes a serious complication called “HIT” (Heparin Induced Thrombocytopenia) where platelets in the blood clump together and block the circulation. UFH is usually given in a hospital setting and monitored with the activated partial thromboplastin time (PTT) test, but it may also be monitored with the anti-Xa test. High doses of UFH given during surgery such as cardiopulmonary bypass are monitored using the activated clotting time (ACT) test. LMWH has more anti-Xa than anti-IIa activity and the response to it is more predictable. It may be given in either an outpatient or hospital setting. Routine monitoring of LMWH is seldom required but when it is monitored, 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. Some doctors might wish to take two blood tests, one before and one four hours after your heparin injection.

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

No test preparation is needed.

Accordion Title
Common Questions
  • How is it used?

    Anti-Xa tests are sometimes requested to monitor and adjust unfractionated heparin (UFH) concentrations in the blood, though the primary monitoring tool for UFH is currently the PTT test. Anti-Xa may be used to monitor some patients who have “heparin resistance” – do not respond as expected to UFH – or who have an underlying condition or interfering factor(s) that alter the PTT test result.

    Low molecular weight heparin (LMWH) therapy is usually not monitored, but doctors may request anti-Xa tests in some cases. These include patients who are pregnant, obese, very young, elderly and those who have kidney disease or dysfunction. LMWH is primarily cleared from the body by the kidneys. Any condition that decreases kidney function can potentially decrease LMWH clearance, increasing its concentration in the blood and increasing the potential for excessive bleeding.

  • When is it requested?

    The anti-Xa test is not routinely requested but may be performed whenever a doctor wants to evaluate UFH or LMWH concentrations in the blood. It may be used periodically to monitor UFH therapy, especially when a doctor feels that a patient is not responding as expected to UFH or when the PTT is not useful.

    When it is used as a LMWH monitoring tool, anti-Xa is primarily requested as a “peak” test.  It is collected about 4 hours after a LMWH dose is given, when the concentration of LMWH in the blood is expected to be at its highest level. Random and “trough” anti-Xa tests may also be requested when a doctor suspects that a patient may not be clearing the LMWH at a normal rate. Trough tests are collected just prior to the next dose, when heparin concentrations are expected to be at their lowest.

  • What does the test result mean?

    Anti-Xa results must be evaluated in the context of the type of heparin that a person is receiving (UFH or LMWH and type of LMWH), the timing of the sample collection, and the condition that the person is being treated for. Results from different laboratories may not be interchangeable.  Therapeutic reference ranges and the heparins that they are based on vary.

    In general, for UFH and LMWH, if concentrations are within an established therapeutic range and the patient is doing well clinically – not clotting, bleeding excessively, or experiencing other complications – then the dosage is considered appropriate. If the anti-Xa concentration is high, then the patient may be getting an excessive dose and/or not be clearing the drug at an expected rate and may be at an increased risk for excessive bleeding.

    If the anti-Xa concentration is below the therapeutic range, then the dosage of heparin may need to be increased. When a person is not taking heparin, anti-Xa concentrations should be zero or undetectable.

  • Is there anything else I should know?

    Because of differences in the methods used for measuring anti-Xa and in the test results generated by various laboratories, samples for repeat anti-Xa testing should be sent to the same laboratory.

  • How long will I have to take heparin?

    LMWH and UFH therapy are usually used for short periods of time to help treat and prevent inappropriate clotting.  When long-term anticoagulation is required, other drug therapies such as warfarin tablets are usually used.

  • Do I need to tell my other doctors that I am receiving LMWH?

    Yes, this is information that will be important for doctors and other medical professionals to know when evaluating you for or determining treatment options for other conditions.