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Heparin is an injectable anticoagulant, made from the stomach of animals, typically pigs. It is used both as an anticoagulant in people, and in various medical devices such as test tubes and extracorporal circulation[?] devices such as renal dialysis[?] machines.

Heparin works by potentiating the action of antithrombin III[?], as it is similar to the heparan sulfate[?] proteoglycans[?] which are naturally present on the cell membrane of the endothelium[?]. Because antithrombin III inactivates many coagulation proteins, the process of coagulation will, slow down. In case of overdose, a chemical protamine[?] can be given to counteract the heparin action. The effects of heparin are measured in the lab by the APTT[?], (the time it takes the blood plasma to clot).

Heparin has to be adminstered parenterally, it is not absorbed when taken by mouth. It can be injected intravenously, into a muscle, or subcutaneously (under the skin). Because of its short biologic half-life of approximately one hour, heparin must be given frequently or as a continuous infusion.

A serious side effect of heparin is heparin-induced thrombocytopenia[?] (HIT syndrome). HIT is caused by an immunological reaction that makes platelets form clots within the blood vessels, thereby using up coagulation factors. Formation of platelet clots can lead to thrombosis, while the loss of coagulation factors and platelets may result in bleeding. HIT can (rarely) occur shortly after heparin is given, but also when a person has been on heparin for a long while. Immunologic tests are available for the diagnosis of HIT. There is also a benign form of thrombocytopenia associated with heparin use.

Because of the serious side effects and because heparin must be injected, it is often only used to commence anticoagulation therapy until the oral anticoagulant warfarin is working effectively.

Test tubes, vacutainers, and capillary tubes[?], that use lithium heparin as an anticoagulant are usually marked with green stickers and green tops. Heparin has the advantage over EDTA as an anticoagulant, as it does not affect levels of ions (such as calcium). Heparin can interfere with some immunoassays[?], however. As lithium heparin is usually used, a person's lithium levels cannot be obtained from these tubes: for this purpose royal-blue topped vacutainers containing sodium heparin are used.

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