Observed differences in gentamicin pharmacokinetic parameters and dosage recommendations determined by fluorescent polarization immunoassay and radioimmunoassay methods

Radioimmunoassay (RIA) and fluorescent polarization immunoassay (FPI) methods for quantitative gentamicin serum concentration assay have been shown to be comparable. The purpose of this study was to determine if serum concentration-time data from the same patients assayed by RIA and FPI would provide the same estimates for half-life, elimination rate constant, distribution volume, drug clearance, and gentamicin dose. A total of 99 pre-and postinfusion serum samples were obtained from 30 patients. Samples were divided and assayed by RIA or FPI, and the resultant serum concentration-time data were fitted to a standard one-compartment model. The correlation between the two assay methods was 0.99 (p < 0.005). A mean difference of 10% was seen in distribution volume, gentamicin clearance, and gentamicin dose from quantitative data from the two methods. These differences were significant (p < 0.01). Although the two methods appear to be interchangeable, based on in vitro comparison, differences in calculated pharmacokinetic parameters resulted in significant differences in dose recommendations.