At a glance
Also known as
Anti-Xa; Xa inhibition; antifactor Xa heparin; heparin; Heparin anti-Xa; Unfractionated heparin
Why get tested?
To monitor low molecular weight heparin (LWMH) therapy and sometimes to monitor unfractionated heparin (UFH) therapy. Rarely they may also be used to check the blood levels of the new anti-factor Xa levels for apixaban and rivaroxaban.
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. Rarely they may also be used to check the blood levels of the new anti-factor Xa levels for apixaban and rivaroxaban – particularly if the drug levels are important for determining if safe to proceed with surgery or in the case of bleeding in patients taking these drugs.
A blood sample drawn from a vein in your arm
Test preparation needed?
What is being tested?
This test measures the amount of low molecular weight heparin (LMWH) or unfractionated heparin (UFH) in the blood by measuring anti-Xa activity. Heparin is an , 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 or through a injection to people who have inappropriate blood clots (thrombi) and/or are at an increased risk of developing them.
The oral anti-factor Xa inhibitors apixaban and rivaroxaban are tablets containing small components that reduce blood clotting by directly blocking the activity of factor X and therefore reducing the production of a stable blood clot by the clotting system.
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. There are a variety of and conditions including surgeries, DVT (deep vein , and other hypercoagulable disorders that are associated with inappropriate blood clot (thrombus) 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 , 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). 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 . UFH is usually given in a hospital setting and monitored with the activated partial thromboplastin time (APTT) test, but it may also be monitored with the anti Xa levels test. High doses of UFH given during surgeries such as cardiopulmonary bypass are monitored using the activated clotting time ATTP 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 not required but when it is monitored, the anti Xa levels test is used.
Apixaban and rivaroxaban do not specifically require monitoring, however in the patients who have excessive bleeding, or who require urgent interruption of these anticoagulants for a procedure or surgery with high bleeding risk, this test may be useful to clarify if it is safe to proceed with the procedure.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.