Improved specificity in gentamicin bioassay

An optimized bioassay for determination of gentamicin concentrations in serum of patients is presented. It was possible to enhance the exactness and representibility of the bioassay by means of a standardized methodical process, a suitable arrangement of the assay and reading at fifteen-fold enlargement. A systematic arrangement of the assay with randomized blocks is a safeguard against larger errors during the assay. The standard deviations in 5 culture plates for each serum sample amounted to about 0.3 mg/l envolving thus variation coefficients under 10%. The specificity of the bioassay was determined by means of the analytical procedure. The determination of gentamicin in blood serum, as it is generally known, may be masked in depencence on the concentration by heparin, vitamins, sodium chloride and sodium phosphate. Our model assays have revealed that for recognition of trouble factors in determination of gentamicin the evaluation by means of the parallel-line-assay according to the four point method should be performed. Inactivation of gentamicin for example by influence of phosphate cannot be recognized when evaluation is determined by linear regression. The bioassay, as a simple and economic method for control of treatment and pharmacokinetic investigations is available for any clinical and bacteriological laboratory.