Study subjects were provided two strengths of warfarin tablets (1 mg and 2.5 mg) or matching placebos. Subjects measured their INR with encrypted meters (INRatio®, Alere, San Diego, CA) at least weekly. Home monitoring was validated by plasma INR measurement at the week 1 and 16 visits. Individual INR meters and test strips were replaced and subjects were reinstructed if meter INR readings varied by more than 30% from the laboratory INR. Efficacy of home INR measures were determined by time-in-target INR range of all patients, calculated on the basis of linear interpolation, 12 after excluding readings taken at baseline, during initial warfarin titration (until INR ≥ 2.0), study drug interruption, or following the discontinuation of study drug.