Comparison of Gentamicin Bioassay Values Utilizing Three Different Indicator Organisms

Gentamicin serum concentrations were monitored via bioassay with three indicators, to determine if assay results were influenced by the testing organism. When mean assay values, from seven samples, were compared to mean target values, there was no significant differences with Staphylococcus epidermidis, Bacillus subtilis or Klebsiella pneumoniae. However, levels from one of the seven samples yield dramatically lower concentrations, for all three indicators, than the target. This suggests the presence of a bioassay inhibitor substance. Additional testing of the organisms, versus serum without gentamicin, revealed Bacillus subtilis to be much more serum-sensitive than the others.