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 (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.
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 acute and chronic conditions in which excessive clotting may occur. Some of these include surgeries, DVT (deep vein thrombosis), and other excessive clotting disorders (hypercoagulable disorders). They are associated with inappropriate blood clot 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). There are two types of heparin that may be used in anticoagulation therapy: low molecular weight heparin (LMWH) and unfractionated heparin (UFH), also known as standard heparin. 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. UFH is usually given intravenously (I.V.), whereas LMWH is typically given through injections under the skin (subcutaneous). UFH affects both factors Xa and IIa, is more variable in its inhibitory activity, and must be closely monitored. Complications may include clotting (insufficient heparin), excessive bleeding (too much heparin), and sometimes decreased platelets (heparin-associated thrombocytopenia). On the contrary, LMWH primarily affects Xa and its effect is more predictable; therefore routing monitoring is not required.
Heparin is often used for treatment when the condition is acute and is eventually replaced by the use of oral anticoagulants. 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 monitored using the activated clotting time (ACT) test. LMWH may be given in either an outpatient or hospital setting. 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.