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  • Title: Retrospective evaluation of a possible interaction between warfarin and levofloxacin.
    Author: McCall KL, Scott JC, Anderson HG.
    Journal: Pharmacotherapy; 2005 Jan; 25(1):67-73. PubMed ID: 15767222.
    Abstract:
    STUDY OBJECTIVES: In order to clarify the clinical significance of a suspected drug interaction, we sought to determine if the international normalized ratio (INR) is affected when levofloxacin is administered in patients receiving long-term warfarin therapy. DESIGN: Retrospective cohort study using pharmacy and medical records. SETTING: Outpatient clinic. PATIENTS: Forty-three patients receiving long-term warfarin therapy who subsequently were prescribed either levofloxacin (22 patients) or felodipine (21 controls); felodipine was chosen as it has been shown not to interact with warfarin. Patients in both groups were required to have a documented INR before the start of levofloxacin or felodipine (pre-INR) and either during levofloxacin or felodipine therapy or within 15 days after the drug had been started (post-INR). MEASUREMENTS AND MAIN RESULTS: Patient demographics were similar between the two treatment groups. The mean +/- SD age of the patients in the levofloxacin and control groups was 59.5 +/- 8.7 and 65.3 +/- 11.5 years, respectively (p=0.07). The mean change between the pre- and post-INR (primary outcome measure) was 0.31 +/- 0.82 (pre-INR 2.46, post-INR 2.76) and 0.21 +/- 0.54 (pre-INR 2.46, post-INR 2.67) in the levofloxacin and felodipine groups, respectively (p=0.65). A post hoc power analysis, based on a sample-derived, weighted standard deviation of 0.68, revealed that the study had 66% power to detect a change of 0.5 in the INR value. The percentage of patients who required a warfarin dosage adjustment based on the post-INR (secondary outcome measure) was 41% (9 of 22 patients) in the levofloxacin group and 33% (7 of 21 patients) in the felodipine group. CONCLUSION: Although our primary analysis did not detect a warfarin-levofloxacin interaction, the potential for such an interaction, especially in idiosyncratic cases, cannot be ruled out. Clinicians should closely monitor INR values when levofloxacin is administered jointly with warfarin.
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