A simplified model for adjustment of gentamicin dosage in newborn infants

In a retrospective study of 72 neonates during treatment with gentamicin, poor correlation was found between dosage based on body weight and gentamicin serum concentrations. Calculation of adequate gentamicin dosage regimen during steady-state based on individual pharmacokinetic parameters according to Gibaldi & Perrier was then studied in 35 newborn infants during therapy. Predictions were based on gentamicin serum concentrations taken prior to and 1, 3, and 5 hours after the first (n = 8) or second (n = 12) dose (group A), or only prior to and one hour after dose (group B, n = 15). Half-life (t1/2), apparent volume of distribution (Vd), body clearance (Clbody) and elimination rate constant (beta) were not significantly different when calculated after the first or second dose or during steady-state. The correlation between predicted and observed gentamicin concentration was high in both groups (p less than 0.005) and the slopes congruent with unity. After dose or interval correction, 73% of the observed predose concentrations (mean 2.0 micrograms/ml) were within 1 microgram/ml of predicted values. One hour after dose the predicted (mean 5.7 micrograms/ml) and observed (mean 6.2 micrograms/ml) values were not significantly different. Higher precision was noted when the predictions were based on 4 samples (group A) compared to 2 (group B). Since the calculations may be performed by a simple desk calculator rapid advice may be given to the clinical staff on adequate gentamicin dosage even in small severely ill preterm infants.