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walk a tightrope between bleeding and clot-ting—and a hundred things can tip the balance. It’s a difficult drug to use, with a narrow therapeu-tic index, but 60 years after it was introduced it is still the mainstay of oral anticoagulant treatment. More than 2 million North Americans take it, and this number continues to grow with our aging pop-ulation.1 Only a minority of these patients are man-aged by anticoagulation clinics, despite evi-dence that patients managed by anticoagula-tion clinics have fewer bleeding and throm-boembolic events than those who receive usual medical care,2,3 and their international normalized ratios (INRs) are in the therapeu