Comparison of an HPLC and bioassay method to determine antimicrobial concentrations after intravenous and oral administration of enrofloxacin in four dogs.

Plasma samples of dogs given 5 mg kg-1 bodyweight of a broad spectrum antimicrobial with the active ingredient enrofloxacin were assayed by two different methods (bioassay: Escherichia coli 14 ICB 4004 on ISO sensitest agar; high performance liquid chromatography with a RP C18 column). At concentrations up to 1000 ng ml-1 a linear correlation between values obtained by the two assay methods was found. At concentrations above 1000 ng ml-1 no correlation could be determined. The main metabolite of enrofloxacin, ciprofloxacin, is an important determinant of overall antimicrobial activity and hence influences bioassay results.