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
A blood sample drawn from a vein in your arm
Test Preparation Needed?
The Test Sample
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 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. There are a variety of acute and chronic conditions including surgeries, DVT (deep vein thrombosis, 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 embolism, 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 thrombocytopenia. 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 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 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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.