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.

As medications

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 in those who are predisposed.

Vitamin K antagonists

The oral anticoagulants are a class of pharmaceuticals that act by antagonizing the effects of vitamin K. It is important to note that they take at least 48 to 72 hours for the anticoagulant effect to develop fully. In cases when an immediate effect is required, heparin must be given concomitantly. Generally, these anticoagulants are used in prevention of embolization -- in pulmonary embolism, and patients with atrial fibrillation and mechanical prosthetic heart valves.

The most important oral anticoagulants are:

• warfarin
• acenocoumarol
• phenindione

Direct thrombin inhibitors

The new drug class of the direct thrombin inhibitors, that will be introduced in the near future, is expected to replace warfarin in some indications. Its main representative, ximelagatran (Exanta®), has performed well in clinical trials and is being reviewed by the Food and Drug Administration.

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 medical devices. Vacutainers and test tubes containing heparin are usually colored green.

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 less side effects.

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 vacutainers and test tubes. This chemical strongly and irreversibly binds calcium. It is in a powdered form.
• Citrate is usually in blue vacutainers. 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.
• Oxalate has a similar mechanism to citrate. It is the anticoagulant used in fluoride (grey top) tubes.