Assay of gentamicin and tobramycin in sera of patients by gas-liquid chromatography.

Therapeutic levels of gentamicin and tobramycin in the sera of patients were measured by gas-liquid chromatography. Concentration-response curves for both drugs were linear over an expected therapeutic range of 1.3 to 12.5 mug/ml (coefficient of determination was >0.97). Coefficients of variation for chromatographic response to gentamicin varied from 6.3 to 9.6%, and to tobramycin from 3.8 to 13.5%. Paired gas-liquid chromatography and microbiological assays for patient serum aminoglycoside levels were performed on 106 gentamicin and 40 tobramycin sera. At levels <2.0 mug/ml, the average difference of estimates between the two assay techniques for gentamicin and tobramycin were, respectively, 38 and 29%. At drug concentrations >2.0 mug/ml, the mean difference between paired estimates was near 20% for both aminoglycosides. The speed, precision, and accuracy of the gas-liquid chromatography assay indicate that it can be a useful alternative to the microbiological procedure for the determination of gentamicin and tobramycin levels in human serum.