Evaluation of a bioassay method for serum amikacin concentrations

The bioassay system from American Diagnostic for amikacin was compared to the same company's radioimmunoassay (RIA) technique used in this hospital. The bioassay is performed by adding serum from a patient to wells cut in agar which have been seeded with a susceptible fast growing microorganism (Enterobacter). After incubation of five hours at 37 degrees, the zones of inhibition for four standards are measured and plotted. The amount of amikacin in the serum sample is then determined from the linear graph. Ten replicates of a serum with 8 microgram per ml of amikacin and 10 replicates of a serum with 20 microgram per ml of amikacin were assayed on each of four days. Aliquots of the same samples were assayed in a similar manner by RIA. The coefficients of variation (CVs) of the within run assays by the bioassay and RIA methods for the low amikacin test sample ranged from 4.7 to 7.2 percent and 6.2 to 13.0 percent, respectively. The within run CVs for the high amikacin test sample for the bioassay and RIA methods ranged from 5.2 to 6.9 percent and 5.0 to 13.9 percent, respectively. The day to day and overall CVs for the bioassay and RIA methods for the low amikacin test sample were 1.5 and 5.8 percent and 7.4 and 11.8 percent, respectively. For the high amikacin test sample, they were 0.4 and 6.0 percent and 4.0 and 9.6 percent, respectively. The correlation coefficient for 49 sera from patients on amikacin was 94 percent between the two methods.