Gentamicin monitoring in low-birth-weight newborns

The pharmacokinetics of gentamicin in premature newborns is influenced by many noncalculable variables. Nevertheless, and despite its narrow therapeutic range, this aminoglycoside antibiotic is usually prescribed according to a fixed schedule. Gentamicin was monitored in 22 severely sick, low-birth-weight newborns on combination therapy of various antibiotics including gentamicin. Dosage regimen of gentamicin was prescribed according to Eichenwald and McCracken [1978]. Serum gentamicin concentrations were measured by radioimmunoassay before infusion, 30 min after, and 3 h after infusion of the antibiotic. In 22 prematures desirable serum gentamicin concentrations with a mean elimination half-life (t 1/2) of 5.3 h; in 2 prematures, levels with a mean t 1/2 of 2.3 h; in 7 prematures, possible toxic levels with a mean t 1/2 of 8.3 h; and in 2 prematures, definite toxic levels with a prolonged t 1/2 up to 17 h were observed. Most of the prematures with possible or definite toxic gentamicin levels were not older than 1 week. This is explained by the immaturity of the kidneys and the retention of aminoglycosides. A satisfactory correlation between postnatal age and gentamicin elimination half-life was found, which might be caused by postnatal maturation of the glomerular function. In prematures in the 1st postnatal week we recommend reducing the daily dose by prolonging the dosage interval, e.g., from 12 to 18 h, or reducing the single gentamicin doses. Gentamicin monitoring seems advisable for detecting toxic serum concentrations and accumulation and for revealing an insufficient dosage of this aminoglycoside antibiotic.