Gentamicin dosage recommendations for neonates based on half-life predictions from birthweight

The appropriate dosing of gentamicin in the newborn was evaluated. Gentamicin was administered intramuscularly to 151 neonates ranging in birthweight from 0.66 to 4.7 kg (gestational age, 26-42 weeks) during a 7-month period. A dosage of 2.5 mg/kg of birthweight every 12 hours was initiated at birth until gentamicin serum levels could be determined. These infants were divided into three groups based on birthweight: 1) less than or equal to 1000 gm; 2) 1001-2000 gm; 3) greater than 2000 gm. From gentamicin serum level data, these three groups had significantly different (P less than 0.05) gentamicin half-lives. Based on the desirability of maintaining a trough gentamicin serum level less than 2, 100, 95.5, and 39% of infants in groups 1, 2, and 3, respectively, required initial dosing interval adjustments. A second group of 74 infants whose birthweight ranged from 0.82 to 3.9 kg (gestational age, 26-42 weeks) required aminoglycoside therapy and were prospectively placed on gentamicin 2.5 mg/kg of birthweight on the following schedule: 1) less than or equal to 1000 gm every 24 hours; 2) 1001-2000 gm every 18 hours; 3) greater than 2000 gm every 12 hours. Using this method, only 15, 58, and 33% of infants in groups 1, 2, and 3, respectively, required dosing interval changes. We conclude that a marked reduction in undesirably high trough gentamicin serum levels can be affected by a simple change in dosing interval based on birthweight, although gentamicin serum levels are still required due to a significant number of infants with high trough levels.