Heparin Anti-Xa

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 (PTT) 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 (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 not required but when it is monitored, the anti-Xa test is used.

A blood sample is obtained by inserting a needle into a vein in the arm.

No test preparation is needed.