Edward F. Bell, MD and Jeffrey L. Segar, MD
Peer Review Status: Internally Peer Reviewed
Monitoring Gentamicin Therapy

This nomogram is for use only in term neonates (<30 days old) receiving gentamicin or tobramycin 2.5 mg/kg given IV every 12 hours.

Illustration of the upper and lower range of acceptable gentamicin serum concentrations (heavy dashed and solid parallel lines). The mean + S.D. for the peak serum gentamicin concentrations of 81 neonates is shown as well as individual gentamicin disappearance curves for 31 neonates who required dosing adjustment (Zone I = required increased dosage; Zone II = required decreased dosage). A single blood sample is drawn between 8 and 12 hours after the start of drug infusion. Any value falling within the Normal Zone is bordered by the following: upper 8 and 12 hour serum values = 3.5 and 2 mcg/ml; lower 8 and 12 hour serum values = 1.8 and 1 mcg/ml.

Dosage adjustment

Zone I: If 8 to 12 hour level is below the Normal Zone, the dosing interval should be shortened to every 8 hours. Recheck the drug level 1 to 2 hours prior to dosing. Acceptable level would be between 1 and 2 mcg/ml.
Zone II: If 8 to 12 hour level is above the Normal Zone, perform the following calculation (%RDR = percent required dose reduction):
%RDR= observed conc. - maximum acceptable conc. x 100
  observed conc.

If the %RDR calculated is <25%, then reduce the dose by the calculated percent and continue dosing every 12 hours. If the %RDR calculated is > 25%, then increase the dosing interval to every 24 hours while keeping the dose at 2.5 mg/kg per dose. Recheck drug levels 1 to 2 hours prior to dosing. Acceptable levels would be between 1 and 2 mcg/ml.


Leff RD, Andersen RD, Roberts RJ. Simplified gentamicin dosing in neonates: a time- and cost-efficient approach. Pediatr Infect Dis 1984;3:208-212.