title: Retrospective evaluation of a possible interaction between warfarin and levofloxacin.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  Retrospective cohort study using pharmacy and medical records.
  
  
  METHODS
  Outpatient clinic.
  
  
  METHODS
  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).
  
  
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: School of Pharmacy, Texas Tech University Health Sciences Center, Amarillo, Texas 79106-1712, USA. ken.mccall@ttuhsc.edu
authors.forename: Kenneth L
authors.initials: KL
authors.lastname: McCall
authors.forename: John C
authors.initials: JC
authors.lastname: Scott
authors.forename: H Glenn
authors.initials: HG
authors.lastname: Anderson
mesh_terms: D000368:Aged; D001831:Body Temperature; D003625:Data Collection; D004334:Drug Administration Schedule; D004347:Drug Interactions; D011307:Drug Prescriptions; D004359:Drug Therapy, Combination; D015736:Felodipine / Q000008:administration & dosage / Q000493:pharmacokinetics; D005260:Female; D006801:Humans; D019934:International Normalized Ratio / Q000706:statistics & numerical data; D064704:Levofloxacin; D008297:Male; D008499:Medical Records; D008875:Middle Aged; D015242:Ofloxacin / Q000008:administration & dosage / Q000493:pharmacokinetics / Q000627:therapeutic use*; D010045:Outpatients / Q000706:statistics & numerical data; D016032:Randomized Controlled Trials as Topic; D012189:Retrospective Studies; D013997:Time Factors; D016896:Treatment Outcome; D014859:Warfarin / Q000008:administration & dosage / Q000493:pharmacokinetics / Q000627:therapeutic use*
chemical_list: D014859:Warfarin; D064704:Levofloxacin; D015242:Ofloxacin; D015736:Felodipine