Estimation of gentamicin clearance and volume of distribution in neonates and young children.

Gentamicin therapy should be guided by serum level monitoring in all age groups, dosage adjustments depending on age related changes in pharmacokinetics. Population data analysed from two centres (43 infants from Glasgow and 100 infants and children from Manchester) by the computer program NONMEM showed that volume of distribution was related to body weight by a proportionality factor that decreased from the region of 0.41-0.46 l/kg in children less than 3 months to 0.25-0.32 l/kg in older children, a value which merges with that accepted for adults (0.25 l/kg). In both young and older children, clearance was also found to be dependent on body weight. Renal function (creatinine concentrations) provided no further explanatory power. When these results were used prospectively to forecast gentamicin concentrations with a Bayesian kinetic parameter estimation program, trough concentrations were more precisely predicted than peaks when a single concentration measurement was used. In clinical practice, however, two concentration measurements are usually routinely available and these should lead to greater precision of both peak and trough predictions. These results have been incorporated into a simple nomogram which can be used to determine a dose of gentamicin which will achieve target peak concentrations in infants, assuming that troughs should not exceed 2 micrograms/ml.