Anticoagulant: Any agent used to prevent the formation of blood clots.

Anticoagulants have various uses. Some are used for the prophylaxis (prevention) or the treatment of thromboembolic disorders. Thrombi are clots. Emboli are clots that break free, travel through the bloodstream, and lodge therein. The anticoagulant drugs used for these clinical purposes include:

• Intravenous heparin -- which acts by inactivating thrombin and several other clotting factors required for a clot to form;
• Oral anticoagulants such as warfarin and dicumarol -- which act by inhibiting the liver's production of vitamin K dependent factors crucial to clotting.

Anticoagulant solutions are also used for the preservation of stored whole blood and blood fractions. These anticoagulants include heparin and acid citrate dextrose (commonly called ACD).

Anticoagulants are also used to keep laboratory blood specimens from clotting. These agents include not only heparin but also several agents that make calcium ions unavailable to the clotting process and so prevent the formation of clots; these agents include ethylenediaminetetraacetic acid (commonly called EDTA), citrate, oxalate and fluoride.