OBJECTIVES: A 10% increase in international normalized ratio (INR) was reported when a statin and warfarin were co-administered; the effect of such an increase for patients with INR values close to the upper therapeutic range can be clinically problematic and may accrue increased healthcare costs. METHODS: A retrospective cohort study was conducted using a managed care claim database. The index INR test was defined as the first test occurring between 7/1/2008 to 6/30/2009. Stable chronic warfarin users were defined as having continuous warfarin use 60 days before and 6 months after the index INR test. The percentage of these patients with at least one INR value within 10% of the upper bound of therapeutic range (2 to 3) was identified. The number of office visits for INR  monitoring and associated costs were reported. RESULTS: Of the 3097 patients identified, 18527 INR values with a mean of 2.49 (SD=0.99) were identified. Among these patients, 35% had at least one INR value between 2.75 and 3.0. For those with all the INR values between 2 and 3 during the study period, the mean office visit for INR monitoring was 1.79 times (N=588, SD=1.49); the associated average charge was $78.26 per patient. This increased to 8.72 times (N=1523, SD=7.08) and $400.20 for those who had at least one INR greater than 3. Each office visit for INR monitoring requires a co-payment of $5 to $50. When the diagnosis of atrial fibrillation was added into the patient selection criteria, similar patterns of INR value distribution and associated costs were observed. CONCLUSIONS: A significant portion of chronic warfarin users had at least one INR value reported within 10% of the upper bound of the therapeutic range. A 10% increase of INR can result in abnormally high INR values, more office visits, and increased health care costs.