To monitor standard, unfractionated heparin (UFH) therapy and sometimes to monitor low molecular weight heparin (LMWH) therapy
Heparin Anti-Xa
Heparin Anti-Xa
Also Known As
Anti-Xa chromogenic
Xa Inhibition
Anti-factor Xa Heparin
Anti-FXa
Heparin Level
Heparin Activity
Formal Name
Heparin Anti-Xa
This article was last reviewed
on
This article waslast modified
on November 5, 2017.
At a Glance
Why Get Tested?
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?
None
Accordion Title
Common Questions
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How is it used?
Heparin anti-Xa tests are sometimes used to monitor and adjust standard heparin (unfractionated heparin, UFH) therapy, though the primary monitoring tool for UFH is currently the PTT test. Heparin anti-Xa may be used to monitor some people who have "heparin resistance" who do not respond as expected to UFH or who have an underlying condition such as liver dysfunction or interfering factor(s) such as lupus anticoagulant (LAC) that affects the PTT test result.
Low molecular weight heparin (LMWH) and fondaparinux therapy is usually not monitored, but healthcare practitioners may order heparin anti-Xa tests in some cases. These include for women who are pregnant, people who are obese (more than 100 kg body weight), very young, or elderly and those who have kidney dysfunction. LMWH and fondaparinux are primarily cleared from the body by the kidneys. Any condition that decreases kidney function can decrease their clearance, increasing their concentration in the blood and increasing the potential for bleeding.
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When is it ordered?
The heparin anti-Xa test is not routinely ordered but may be performed whenever a healthcare practitioner wants to evaluate UFH, LMWH, or fondaparinux concentrations in the blood.
It may be ordered periodically to monitor UFH therapy, especially when a person is not responding as expected to UFH or when the PTT is not useful.
When it is used as a LMWH and fondaparinux monitoring tool, heparin anti-Xa is primarily ordered as a "peak" test. It is typically collected about 4 hours and 3 hours after a LMWH and fondaparinux dose is given, respectively, when the level in the blood is expected to be at its highest level. Random and "trough" anti-Xa tests may also be ordered when a healthcare practitioner suspects that someone may not be clearing the drug at a normal rate. Trough tests are collected just prior to the next dose, when levels are expected to be at their lowest.
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What does the test result mean?
Heparin anti-Xa results must be evaluated in the context of the type of heparin that a person is receiving (UFH, LMWH, or fondaparinux), 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 intervals and the heparins that they are based on vary.
In general, for heparin therapy (UFH, LMWH or fondaparinux), if test results are within an established therapeutic range and the person is doing well clinically – not clotting, bleeding excessively, or experiencing other complications – then the dosage is considered appropriate.
If the heparin anti-Xa result is high, then the person 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 heparin anti-Xa result is below the therapeutic range, then the dosage of heparin may need to be increased to prevent excessive clotting.
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Is there anything else I should know?
Because of differences in the methods used for measuring heparin anti-Xa and in the test results generated by various laboratories, samples for repeat anti-Xa testing should be sent to the same laboratory.
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How long will I have to receive heparin treatment?
Unfractionated heparin therapy is 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 are usually used. An exception to this is during pregnancy, when heparin is the preferred anticoagulant, but the need for heparin therapy in this case typically ends around the time of delivery. After delivery, if continued anticoagulation therapy is needed, a healthcare practitioner may consider warfarin or a low molecular weight heparin such as enoxaparin.
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Do I need to tell my other healthcare providers that I am receiving heparin?
Yes, this is information that will be important for healthcare practitioners to know when evaluating you for or determining treatment options for other conditions.
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How are the newly approved oral anticoagulation drugs monitored?
Newly approved oral anticoagulants include direct thrombin inhibitor (e.g., dabigatran) and direct factor Xa inhibitors (e.g., rivaroxaban, apixaban). Routine laboratory monitoring of direct factor Xa inhibitor is not required. If a healthcare practitioner determines that monitoring is necessary, an anti-Xa test that is tailored for these drugs may be used.
