Proceeds from website advertising help sustain Lab Tests Online. AACC is a not-for-profit organization and does not endorse non-AACC products and services.

Heparin Anti-Xa

Print this article
Share this page:
Also known as: Anti-Xa chromogenic; Xa Inhibition; Anti-factor Xa Heparin; Anti-FXa; Heparin Level; Heparin Activity
Formal name: Heparin Anti-Xa

Board approvedAll content on Lab Tests Online has been reviewed and approved by the Editorial Review Board.

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 (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.

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.