Comparison of gentamicin pharmacokinetic parameters determined by fluorescence polarization immunoassay and latex agglutination methods

Fluorescence polarization immunoassay (FPIA) (TDx, Abbott Laboratories Diagnostics, Irvin, TX, U.S.A.) is commonly utilized for quantitative determination of gentamicin serum concentrations. Recently, automated homogeneous latex agglutination (LA) (Technicon Immuno-1, Miles Diagnostics Division, Tarrytown, NY, U.S.A.) for the quantitative determination of gentamicin serum concentrations has been approved for commercial use. The purpose of this study was to determine whether gentamicin serum concentration-time data from the same patients assayed by FPIA and LA would produce the same estimates for half-life, elimination rate constant, distribution volume, drug clearance, dosage interval, and dose. A total of 70 pre- and postinfusion serum samples were obtained from 19 patients. Each sample was divided into two aliquots; one was assayed by FPIA and the other by LA. The correlation coefficient between the two assay methods was 0.99 (y = 1.03x – 0.05). The mean differences for half-life, volume of distribution, elimination rate constant, total body clearance, and gentamicin dosage were 0.13, 0.32, 0.66, -0.99, and 0.03%, respectively. No statistically significant differences were seen in calculated gentamicin pharmacokinetic parameters (p < 0.05). Pharmacokinetic parameters and dosage recommendations derived from FPIA and the LA assay using pre- and postinfusion serum concentration-time data appear interchangeable and do not result in differences between gentamicin dosing regimens.