Comparison of urinary excretion of tobramycin and gentamicin in adults

The urinary excretion of tobramycin and gentamicin was measured after intramuscular injections of 1 mg of each drug/kg in six normal volunteers. The antibiotics were assayed by the adenylyltransferase enzymatic method, which enables urine concentrations to be estimated accurately without dependence on urinary pH. The levels of the antibiotics in urine varied with the urine volume. The concentration of gentamicin was greater than 113 mug/ml and that of tobramycin was greater than 94 mug/ml 1 hr after administration. Urinary excretion of both antibiotics was rapid; 63.5% of the gentamicin and 74% of the tobramycin was excreted with 24 hr, with a large proportion being excreted by 4 hr after injection. There was a striking uniformity of individual values, and the method offers considerable advantages over existing microbiologic assays.