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The in vitro inactivation of gentamicin and tobramycin by four cephalosporins (cefotetan, cefuroxime, cefodizime, cefotiam) in human serum was investigated. Each cephalosporin was added to human serum samples containing gentamicin sulfate or tobramycin sulfate. Blank samples containing only aminoglycosides were used as controls. Samples were stored at -20, 4 and 25^{circ}C and were analyzed for aminoglycoside concentrations by fluorescence polarization immunoassay (TDxFLx^{TM} system) at 0, 2, 4, 8, 12, 24, 48 and 72 hours after mixing. The serum containing cefotiam stored at 25^{circ}C showed significant inactivation of gentamicin by 12% at 72 hours. The results indicate that cefotitan, cefuroxime and cefodizime do not inactivate gentamicin and tobramycin while cefotiam inactivates gentamicin.
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