Comparison of an immunoassay and a microbiological assay for tobramycin serum concentrations.

An enzyme multiplied immunoassay technique (EMIT) and a microbiological assay were compared as methods for determining the concentration of tobramycin in human serum. Tobramycin sulfate in various concentrations (0.4-20.0 microgram/ml) was dissolved in lyophilized human serum. Tobramycin concentration was determined by EMIT assay and a microbiological agar diffusion assay using Bacillus subtilis ATCC 6633 as the test organism. Serum samples of 162 patients who received tobramycin 60 mg i.m. were also assayed by the two methods. The effect of six antibiotics on the determination of tobramycin concentration by EMIT assay was measured. To test whether freezing affects tobramycin concentration measured by EMIT assay, tobramycin in five concentrations in lyophilized human serum were frozen and analyzed at weekly intervals for seven weeks. The concentration of tobramycin determined by EMIT assay and microbiological assay in both lyophilized human serum and patient samples were highly correlated (r = 0.994 and 0.949) respectively). Dibekacin, an aminoglycoside structurally similar to tobramycin, interfered with the determination of tobramycin concentration. No effect of freezing was observed. The results of this study show that EMIT assay is an acceptable method for routine laboratory analysis of tobramycin serum concentration. It is as accurate as the microbiological assay tested.