Serum gentamicin assay by a radiometric procedure.

A new radiometric (BACTEC) microbiologic procedure, using a 14C-urea substrate and a Proteus species culture, was compared with three microbiologic assays and a radioimmunoassay (RIA) method for determination of gentamicin levels in serum. The non-radiometric microbiologic assays did not differ significantly from the RIA assay, but the BACTEC method showed significant differences with specimens containing greater than 4 microgram/ml gentamicin. Overall, the BACTEC assay was found to be simple to run, fast and reproducible. At concentrations of gentamicin less than 4 microgram/ml, it was just as accurate as were the microbiologic assays. However, at concentrations greater than 4 microgram/ml, the BACTEC assay read consistently less than RIA and the other assays. Because of the BACTEC assay's high cost per single test, it cannot approach the utility of the rapid, easy, reliable, and comparatively inexpensive microbiologic assays. The BACTEC assay is, however, a useful alternative to the RIA method in laboratories that already have radiometric equipment and in which batching of serum samples for gentamicin assay is necessary.