The effects of cefamandole and cefoxitin on the assays of serum containing gentamicin, tobramycin, or amikacin by the BACTEC 460 (Johnston Laboratories, Cockeysville, Md.) were studied. The results were analyzed to determine whether the presence of the test substances, cefamandole or cefoxitin, caused a statistically different mean value of aminoglycoside or increase in variance as compared with serum assayed in their absence. The results were then considered in light of medical significance to see whether the differences observed would have any real effect on patient care. The results of such analyses dictate that serum of patients treated with both amikacin and cefamandole be tested in triplicate.Both cefamandole and cefoxitin are relatively new antibiotics and may be used in combination with an aminoglycoside in the treatment of serious gram-negative infections. The necessity of monitoring the serum level of the aminoglycosides due to their toxic effects is well established. We have previously reported the usefulness of the BACTEC 460 instrument (Johnston Laboratories, Cockeysville, Md.) for aminoglycoside assays in laboratories which routinely use the instrument to process blood cultures (8). However, the effects of cefamandole and cefoxitin on aminoglycoside assays using the BACTEC method have not been well established. We report here the results of a study designed to estimate the magnitude and clinical significance of interferences due to cefamandole and cefoxitin in the BACTEC analyses of gentamicin, tobramycin, and amikacin. MATERIALS AND METHODSSamples of pooled serum of healthy laboratory volunteers were individually adjusted to therapeutic concentrations of gentamicin, tobramycin, or amikacin.Ten replicate samples of serum containing the aminoglycoside alone and 20 replicate samples containing aminoglycoside with either 30 or 150 yg of cefamandole or cefoxitin per ml were assayed in the same run by the BACTEC 460 according to the manufacturer's instructions (package inserts, pp-008D, serum aminoglycoside assay-amikacin; pp-003K, serum aminoglycoside assay-gentamicin; and pp-007F, serum aminoglycoside assay-tobramycin; Johnston Laboratories). Aqueous solutions of cefamandole and cefoxitin were freshly prepared before each analysis.Statistical analyses of the data were performed to determine whether the variance (F test [1
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