Consensus guidelines for warfarin therapy - Recommendations from the Australasian Society of Thrombosis and Haemostasis

The anticoagulant effect of warfarin should be kept at an international nor malised ratio (INR) of about 2.5 (desirable range, 2.0-3.0), although a hig her level may be better in a few clinical conditions. The risk of bleeding increases exponentially with INR and becomes clinically unacceptable once t he INR exceeds 5.0. Warfarin therapy should be continued for around six weeks for symptomatic c alf vein thrombosis, and for 3-6 months after proximal deep vein thrombosis (DVT) that occurs after surgery or limited medical illness. Therapy for si x months or longer could be considered for DVT occurring without an obvious precipitating factor, proven recurrent venous thromboembolism (VTE), or if there are continuing risk factors. Oral anticoagulants prevent ischaemic stroke in atrial fibrillation (AF). M aximum efficacy requires an INR > 2.0, but some benefit remains at an INR o f 1.5-1.9. Patients aged over 75 years are at greatest risk of intracranial bleeding during warfarin therapy for AF, and the target INR may be reduced to 2.0-2.5, or perhaps as low as 1.5-2.0, in such patients. Warfarin shoul d be withheld if it is more likely to cause major bleeding than to protect from stroke leg, in young people with isolated AF where the annual baseline risk of stroke is < 1%). In patients with AF, aspirin is less effective th an warfarin (much less effective after such patients have had a stroke or t ransient cerebral ischaemia). In people with prosthetic heart valves, an INR of 2.5-3.5 is probably suffi cient for bileaflet or tilting disc valves, but a higher target INR is nece ssary for caged ball or caged disc valves. The addition of aspirin (100 mg/ day) further decreases the risk of embolism but increases the risk of gastr ointestinal bleeding.