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Study Objective. To evaluate the effect of levofloxacin coadministration on the international normalized ratio (INR) in patients receiving warfarin therapy. Design. Prospective analysis. Setting. Outpatient clinic at a Veterans Affairs medical center. Patients. Eighteen adult patients receiving warfarin. Intervention. On the basis of clinical diagnosis and judgment, levofloxacin was prescribed to the 18 patients for treatment of various types of infection. The INR was measured before and at 2-8-day intervals after the coadministration of levofloxacin therapy, and once after completing therapy. Warfarin dosages were adjusted when necessary. Measurements and Main Results. Warfarin dosages were changed in seven patients as a result of the first nontherapeutic INR values obtained after start of levofloxacin therapy. Owing to a concern regarding noncompliance and the adverse effect of bleeding, warfarin dosage was adjusted in one patient even though his first INR value was in the high end of the therapeutic range (2.98, therapeutic range 2-3). One patient withdrew from the study after the first INR measurement after levofloxacin coadministration. Because of a concern about the possible bleeding complication, warfarin dosage was also adjusted in this patient after obtaining his first INR value. Therefore, only the INR values obtained before and the first INR values obtained after levofloxacin administration were compared to evaluate the effect of levofloxacin on INR determination of warfarin therapy. The INR values obtained before levofloxacin administration did not differ significantly from the first INR values obtained after levofloxacin coadministration (mean ± SD 2.61 ± 0.44 vs 2.74 ± 0.83, 95% confidence interval −0.449-0.196, p=0.419). Conclusion. The INR values measured before and after concomitant levofloxacin therapy were not significantly different. However, the ability to detect a significant difference may be affected by the sm... [ABSTRACT FROM AUTHOR]