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Iowa Neonatology Handbook: Pharmacology
Monitoring Gentamicin Therapy by Single Serum Sample
Determination
Edward F. Bell, MD and Jeffrey L. Segar, MD
Peer Review Status: Internally Peer Reviewed
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:
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Zone I:
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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.
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Zone II:
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If 8 to 12 hour level is above the Normal Zone, perform
the following calculation (%RDR = percent required dose
reduction):
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%RDR =
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observed conc. - maximum acceptable conc.
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x 100
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observed conc.
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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.
Reference:
Leff RD, Andersen RD, Roberts RJ. Simplified gentamicin dosing in
neonates: a time- and cost-efficient approach. Pediatr Infect Dis
1984;3:208-212.
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