Anticoagulant

An anticoagulant is a substance that stops blood from clotting, or slows the process, by interfering with the clotting factors, a cascade of plasma proteins that ends with thrombin turning fibrinogen into a mesh of fibrin. Anticoagulant medicines do not dissolve a clot that has already formed, but they stop it growing and prevent new ones while the body breaks the old clot down. They differ from antiplatelet drugs such as aspirin and clopidogrel, which keep platelets from clumping.

Several kinds are in use. Heparin, given by injection, boosts antithrombin, a natural inhibitor of thrombin and factor Xa. Warfarin, a tablet, blocks the recycling of vitamin K, which the liver needs to make working clotting factors, and its effect is checked with a blood test called the INR. Newer direct oral anticoagulants, such as apixaban, rivaroxaban and dabigatran, block factor Xa or thrombin directly. These drugs treat and prevent deep vein thrombosis and pulmonary embolism, lower the risk of stroke in atrial fibrillation and protect people after some operations or with mechanical heart valves. Bleeding is the main risk.

In the laboratory, anticoagulants such as EDTA and citrate are added to blood collection tubes; they work by binding the calcium ions that clotting needs. Leeches and many other bloodsucking animals make natural anticoagulants in their saliva.