Gentamicin dosing in elderly patients based on bioelectrical impedance analysis

The relationship between gentamicin pharmacokinetics and measures of bioelectrical impedance (BI) in elderly patients was investigated with the aim of developing a potential noninvasive means of individualising gentamicin dosage. Linear regression analyses identified height/resistance2 as a statistically significant predictor of gentamicin distribution volume, V, [adjusted (adj)r2 = 0.53, coefficient of variation (CV) = 15.2%], and resistance/reactance and creatinine clearance (CLcr) as predictors of total systemic clearance, CL, adj r2 = 0.52, CV = 20.1%. Individualisation of gentamicin dosage regimens based on these relationships to achieve steady-state (SS), peak gentamicin concentrations, Css,max, and SS trough concentrations, Css,min, of 7.0 and 1.0 micrograms/ml, respectively, in an independent group of elderly patients resulted in serum gentamicin levels of 5.9 +/- 0.7 and 0.8 +/- 0.4 micrograms/ml. Mean absolute prediction errors averaged 0.7 +/- 0.5 micrograms/ml for Css,max and 0.5 +/- 0.3 micrograms/ml for Css,min. Measures of BI provided the best predictions of Css,max, whereas models based on CLcr alone were the best predictors of Css,min. This technique provides a means of complementing routine pharmacokinetic monitoring of gentamicin pharmacotherapy in the elderly hospitalised patient with reductions in patient discomfort and potential savings in time and cost.