Modified enzyme immunoassays for tobramycin using reduced sample and reagent volumes

The accuracy of two modified enzyme immunoassay (EMIT) methods using reduced sample and reagent volumes for determining serum tobramycin concentrations was compared with that of the standard method. The modified EMIT assays used sample and reagent volumes of 25 and 30 microL instead of the standard 50-microL volumes. Solutions of tobramycin sulfate with known concentrations of 1, 2, 4, 8, and 12 mg/L were assayed five times using each of the three methods. Fourteen blood samples containing unknown concentrations of tobramycin were obtained from patients over a four-week period and assayed once by each method. The mean percentage recovery of tobramycin using the standard 50-microL sample-volume method (104.4 +/- 8.8%) was not significantly different from that of the 30-microL (106.6 +/- 9.9%) or the 25-microL (100.4 +/- 11.4%) assay method. There was good correlation between the standard method and the 25-microL (r = 0.993) and the 30-microL (r = 0.988) methods for determining unknown tobramycin concentrations. If the 25- and 30-microL assays were used in place of the standard 50-microL assay, costs would be reduced and the number of assays per kit would increase. The 30-microL and 25-microL sample and reagent volumes used in this study for the tobramycin EMIT assay allow substantial cost savings without a significant loss in accuracy.