Serum concentrations and inhibitory ratios during amikacin therapy of gram-negative infections.

The ratio of the peak serum concentration after a 500-mg dose of amikacin to the pathogen minimum inhibitory concentration was determined for 95 patients under treatment for serious Gram-negative infections. There were 113 such ratios. The relationship of this inhibitory ratio to the clinical effectiveness and side effects of treatment with this new aminoglycoside was also studied. Mean peak serum concentration of drug was 25.8 microgram/ml, and mean inhibitory ratio was 13.0. Ninety-six per cent of inhibitory ratios were greater than or equal to 1.0. Therapy was rated totally effective in 85% of patients clinically evaluated and partially effective in 3%. Signs of renal or eighth cranial nerve impairment attributable to drug administration were confined to only two patients, and there were no other side effects. The reliability of amikacin therapy appears to be related to dependable serum levels and high inhibitory ratios.