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Current guidelines from the American College of Chest Physicians (Chest 2008; 133:suppl:1695) recommend warfarin monitoring every 4 weeks. In this single-center, Canadian study, 250 patients whose maintenance warfarin dose had been stable for at least 6 months were randomized to international normalized ratio (INR) assessments every 4 or 12 weeks. Patients randomized to the 12-week interval continued to undergo INR checks every 4 weeks; however, sham INRs (in a range that would not require dosing changes) were reported out unless the actual values were extreme (<1.5 or >4.5).
At 12 months, the percentage of time INRs were in the therapeutic range was similar in the two groups (about 70%), and fewer patients in the 12-week group had dosing ch…