Assay of aminoglycoside antibiotics in clinical specimens

Optimal therapeutic use of aminoglycoside antibiotics requires monitoring of levels in serum. Three methods have been developed for clinical use; one yields a specific 14C-labeled product, and two are microbiologic methods in which different indicator strains are used to measure the diffusion of drug into agar. These methods were compared in assays with 200 sera from patients receiving gentamicin or sisomicin. Some specimens also contained either a beta-lactam antibiotic or clindamycin. In the presence of penicillin or cephalosporin, the level of aminoglycoside measured was accurate if the specimens were treated with beta-lactamase. The presence of clindamycin invalidated the results when Bacillus subtilis was used as the indicator strain, but not with the other microbiologic method or the enzymatic method. Under proper circumstances, the results obtained by the various methods were comparable, according to statistical analysis. The enzymatic procedure is the most specific and rapid method, but materials for agar well diffusion methods are more readily available and more economical. The procedures are practical, and their use is recommended in patients being treated with aminoglycoside antibiotics.