An anticoagulant is a substance that prevents coagulation; that is, it stops blood from clotting. A group of pharmaceuticals called anticoagulants can be used in vivo as a medication for thrombotic disorders. Some chemical compounds are used in medical equipment, such as test tubes, blood transfusion bags, and renal dialysis equipment. They also have military applications, whereby their introduction into the wounds of enemy soldiers will make their treatment significantly more difficult.
Anticoagulants are given to people to stop thrombosis (blood clotting inappropriately in the blood vessels). This is useful in primary and secondary prevention of deep vein thrombosis, pulmonary embolism, myocardial infarctions and strokes.
Vitamin K antagonists
The oral anticoagulants are a class of pharmaceuticals that act by antagonizing the effects of vitamin K. Examples include warfarin. It is important to note that they take at least 48 to 72 hours for the anticoagulant effect to develop fully. In cases when any immediate effect is required, heparin must be given concomitantly. Generally, these anticoagulants are used to treat patients with deep-vein thrombosis (DVT), pulmonary embolism (PE), atrial fibrillation (AF), and mechanical prosthetic heart valves.
Patients aged 80 years or more may be especially susceptible to bleeding complications with a rate of 13 bleeds per 100 person-years.
The most important oral anticoagulants are:
- Warfarin (Coumadin). This is the main agent used in the U.S. and UK
- Acenocoumarol and phenprocoumon This is used more commonly outside the U.S. and the UK
Heparin and derivative substances
Heparin is a biological substance, usually made from pig intestines. It works by activating antithrombin III, which blocks thrombin from clotting blood. Heparin can be used in vivo (by injection), and also in vitro to prevent blood or plasma clotting in or on medical devices. Vacutainer brand test tubes containing heparin are usually colored green.
Low molecular weight heparin
Low molecular weight heparin is a more highly processed product that is useful as it does not require monitoring of the APTT coagulation parameter (it has more predictable plasma levels) and has fewer side effects.
Ultra low molecular weight heparin
Direct Acting Oral Anticoagulants
- The role of direct acting oral anticoagulants for the treatment of pulmonary embolism has been reviewed by the Cochrane Collaboration.
- Recommendations for safe usage have been made by Thrombosis Canada.
Factor Xa Inhibitors
- Fondaparinux is a synthetic sugar composed of the five sugars (pentasaccharide) in heparin that bind to antithrombin. It is a smaller molecule than low molecular weight heparin.
Factor Xa inhibitors can be reversed by andexanet.
Direct thrombin inhibitors
- Another type of anticoagulant is the direct thrombin inhibitor. Current members of this class include argatroban, lepirudin, and bivalirudin. An oral direct thrombin inhibitor, ximelagatran (Exanta®) was denied approval by the Food and Drug Administration (FDA) in September 2004  and was pulled from the market entirely in February 2006 after reports of severe liver damage and heart attacks. 
Anticoagulants outside the body
Laboratory instruments, test tubes, blood transfusion bags, and medical and surgical equipment will get clogged up and become nonoperational if blood is allowed to clot. Chemicals can be added to stop blood clotting. Apart from heparin, most of these chemicals work by binding calcium ions, preventing the coagulation proteins from using them.
- EDTA is denoted by mauve or purple caps on Vacutainer brand test tubes. This chemical strongly and irreversibly binds calcium. It is in a powdered form.
- Citrate is usually in blue Vacutainer tube. It is in liquid form in the tube and is used for coagulation tests, as well as in blood transfusion bags. It gets rid of the calcium, but not as strongly as EDTA. Correct proportion of this anticoagulant to blood is crucial because of the dilution. It can be in the form of sodium citrate or ACD.
- Oxalate has a similar mechanism to citrate. It is the anticoagulant used in fluoride (grey top) tubes.
For the meaning of more colors, see Vacutainer#including coagulants.
- Hylek EM, Evans-Molina C, Shea C, Henault LE, Regan S (2007). "Major hemorrhage and tolerability of warfarin in the first year of therapy among elderly patients with atrial fibrillation". Circulation 115 (21): 2689-96. doi:10.1161/CIRCULATIONAHA.106.653048. PMID 17515465.
- Robertson L, Kesteven P, McCaslin JE (2015). "Oral direct thrombin inhibitors or oral factor Xa inhibitors for the treatment of pulmonary embolism.". Cochrane Database Syst Rev 12: CD010957. doi:10.1002/14651858.CD010957.pub2. PMID 26636644.
- Gladstone DJ, Geerts WH, Douketis J, Ivers N, Healey JS, Leblanc K (2015). "How to Monitor Patients Receiving Direct Oral Anticoagulants for Stroke Prevention in Atrial Fibrillation: A Practice Tool Endorsed by Thrombosis Canada, the Canadian Stroke Consortium, the Canadian Cardiovascular Pharmacists Network, and the Canadian Cardiovascular Society.". Ann Intern Med 163 (5): 382-5. doi:10.7326/M15-0143. PMID 26121536.
- Siegal DM, Curnutte JT, Connolly SJ, Lu G, Conley PB, Wiens BL et al. (2015). "Andexanet Alfa for the Reversal of Factor Xa Inhibitor Activity.". N Engl J Med. doi:10.1056/NEJMoa1510991. PMID 26559317.
- Di Nisio M, Middeldorp S, Büller HR (2005). "Direct thrombin inhibitors". N. Engl. J. Med. 353 (10): 1028-40. doi:10.1056/NEJMra044440. PMID 16148288.