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JOHN E. MURPHY, PHARMD., FCCP, FASHP, is Professor of Pharmacy. Practice and Science and Associate Dean for Academic and Professional Affairs, College of Pharmacy, University of Arizona, Tucson. ANTHONY M. ROETHER, PHARM.D., is Pharmacist, United Drugs, Tucson.

Address correspondence to Dr. Murphy at the College of Pharmacy, University of Arizona, P.O. Box 210202, Tucson, AZ 85721-0202 (murphy{at}pharmacy.arizona.edu).

Purpose. The development of two nomograms to predict dosing intervals for gentamicin in neonates based on one gentamicin concentration is described. Methods. Pooled data from three retrospective studies on neonates age seven days or younger were used to create nomograms that would predict dosing intervals for gentamicin. The population volume of distribution (0.45 L/kg) and a determined half-life were used to create nomogram cutoff concentrations that could select a dosing interval for neonates to achieve steady-state trough concentrations of 0.5 or 1 mg/L. A dose of 4 mg/kg was used to simulate concentration-versus-time profiles for included neonates based on their individual pharmacokinetic data. Predicted concentrations from hours 6 to 22, at one-hour intervals, for each neonate were compared against the nomograms and evaluated for the number of correct interval predictions. The nomograms were considered to have failed at any time point where they indicated an interval that would not have achieved the desired trough concentration of 0.5 or 1 mg/L or if the interval chosen was longer than necessary. Results. The 0.5- and 1-mg/L nomograms predicted correct dosing intervals for 81–92% of neonates for postinfusion hours between 15 to 21 and 86–93% for postinfusion hours of 13 and 21, respectively. Accuracy of the nomograms to predict correct dosing intervals improved as the postinfusion time before the next concentration measurement increased. Conclusion. Using the two nomograms may help predict the correct extended-dosing intervals of gentamicin administration for neonates. Prospective evaluation and validation of the nomograms may be necessary for their wider use as a clinical tool. Index terms: Aminoglycosides; Blood levels; Dosage; Drugs, body distribution; Gentamicin; Half-life; Injections; Methodology; Nomograms; Pediatrics; Pharmacokinetics