In vitro inactivation of aminoglycosides by cephalosporin antibiotics.

The in vitro inactivation of aminoglycoside antibiotics by semisynthetic penicillins complicates antibiotic assays. Due to the increasing number of new cephalosporins and use of aminoglycoside-cephalosporin combinations, we determined the in vitro stability of 28 aminoglycoside-cephalosporin combinations (gentamicin sulfate, tobramycin sulfate, netilmicin sulfate [10 micrograms/mL], and amikacin [20 micrograms/mL] in combination with cefazolin sodium, cefoxitin sodium, cefoperazone sodium, cefotaxime sodium, ceftazidime acid pentahydrate, cefsulodin sodium, or cefpiramide sodium at 100, 200, and 300 micrograms/mL). These mixtures were incubated at 37 degrees C and sampled at 0, 8, and 24 hours. Amikacin and tobramycin were most stable and netilmicin was the least stable of the aminoglycosides. Cefoxitin, ceftazidime, and cefotaxime were the least inactivating of the cephalosporins. When combined with first- and second-generation cephalosporins, aminoglycosides are relatively stable, but some laboratory precautions may be necessary when determining aminoglycoside levels in the presence of third-generation cephalosporin compounds.