Evaluation of serum gentamicin assay procedures for a clinical microbiology laboratory

Four methods for the measurement of serum gentamicin concentration were evaluated with respect to cost-effectiveness, accuracy, and precision. Gentamicin concentration was determined in 112 clinical samples by the Staphylococcus epidermidis agar diffusion bioassay procedure in routine service in our laboratory at the time this study was initiated. Appropriate portions of these clinical samples were frozen and later thawed for remeasurement of gentamicin by bioassay or for measurement of gentamicin in one of three other systems. These included the Enzymatic Radiochemical Assay, the Diagnostic Products Corporation Radioimmunoassay and the New England Nuclear Corporation Radioimmunoassay. In addition, gentamicin dissolved in horse serum at 2, 4, 6, 8, 10, and 12 μg/ml was aliquoted, frozen, and later thawed for assay in each of the above systems. The data were analyzed for evidence of constant and proportional bias as well as for accuracy and precision.