Anticoagulant Therapy

Anticoagulants reduce new clot formation and clot extension; they do not dissolve a clot that is already present. The prescribing decision is therefore a balance: enough anticoagulant effect to prevent embolism or recurrent venous thromboembolism, but not so much that bleeding becomes more likely than the thrombotic event being prevented. The indication determines the acceptable level of anticoagulation. Atrial fibrillation therapy prevents left-atrial thrombus from embolising, treatment of DVT or PE prevents propagation and recurrence, and mechanical valves require protection from valve thrombosis. Warfarin remains distinctive because its effect is delayed, its therapeutic range is measured with the INR, and its response changes substantially with diet, interacting drugs, illness, and hepatic function.