What is the appropriate gentamicin dosing (mg/kg), route, duration, and monitoring for a newborn, distinguishing term (≥37 weeks gestation) and preterm (<37 weeks gestation) infants?

Gentamicin Dosing for Newborns

For term newborns (≥37 weeks gestation), administer gentamicin 4-5 mg/kg IV/IM once daily; for preterm newborns (<37 weeks gestation), use 3-4 mg/kg once daily with dosing intervals adjusted by gestational age and weight. 1, 2

Term Newborns (≥37 weeks gestation, >2000g)

Recommended Dose:

• 4-5 mg/kg IV/IM once every 24 hours 2, 3, 4
• The FDA label specifies 2.5 mg/kg every 8 hours (7.5 mg/kg/day total), but once-daily dosing at 4-5 mg/kg is supported by more recent evidence showing superior pharmacokinetics 2, 3
• Research demonstrates that 4 mg/kg once daily achieves therapeutic peak levels (>4 mcg/mL) in 97% of term neonates with appropriate trough levels (<2 mcg/mL) in 93% 4

Preterm Newborns (<37 weeks gestation)

Dosing by Gestational Age and Weight:

Very Preterm (<32 weeks or <1000g):

• 3.5 mg/kg IV/IM every 24 hours 1, 2
• The IDSA guidelines specify 3.5 mg/kg every 24 hours for premature neonates <1000g 1

Moderate Preterm (32-34 weeks or 1000-2000g):

• 3 mg/kg IV/IM every 24 hours 3
• Alternative FDA-approved regimen: 2.5 mg/kg every 18-24 hours 1, 2
• Research shows 3 mg/kg once daily yields therapeutic levels in 79% of infants <35 weeks gestation 3

Late Preterm (35-36 weeks, >2000g):

• 4 mg/kg IV/IM every 24 hours 3, 4
• This bridges the gap between preterm and term dosing 3

Route and Administration

• Intravenous or intramuscular administration are both acceptable 2
• For IV administration, may dilute in 50-200 mL sterile isotonic saline or 5% dextrose (use smaller volumes for infants) and infuse over 30 minutes to 2 hours 2
• Should not be physically premixed with other drugs 2

Duration of Therapy

• Typical duration: 7-10 days for most infections 2
• 48-72 hours for empiric therapy pending negative cultures (common in neonatal sepsis workups) 3
• Extended courses beyond 10 days require enhanced monitoring due to increased toxicity risk 2

Therapeutic Drug Monitoring

Target Levels:

• Peak (30-60 minutes post-dose): 6-12 mcg/mL (some sources accept 5-12 mcg/mL) 2, 3, 4
• Trough (just before next dose): <2 mcg/mL 2, 3, 4
• Prolonged levels >12 mcg/mL should be avoided 2

Timing of Level Checks:

• Measure peak and trough at the third dose (around 48-72 hours) 5, 3, 4
• This allows steady-state to be reached while still providing early feedback for dose adjustment 3
• More frequent monitoring needed if renal function is impaired or treatment extends beyond 10 days 2

Safety Monitoring

Renal Function:

• Baseline and day 3 serum creatinine 4
• Monitor more frequently if creatinine rises or if concurrent nephrotoxic drugs are used 2
• Dose adjustment required if creatinine clearance decreases 2

Ototoxicity:

• Hearing screening before discharge (or at completion of therapy if prolonged course) 4
• Auditory and vestibular function monitoring recommended for courses >10 days 2

Dosing in Renal Impairment

• Increase dosing interval rather than decrease dose to maintain therapeutic peaks 2
• Approximate interval (hours) = serum creatinine (mg/dL) × 8 2
• Serum level monitoring is essential in renal impairment 2

Common Pitfalls and Caveats

Weight-Based Dosing:

• Use actual body weight for dosing calculations 2
• In obese patients (rare in neonates), use lean body mass 2

Gestational Age Matters More Than Postnatal Age:

• A 2-week-old infant born at 32 weeks gestation should still be dosed as a preterm infant, not a term infant 1, 3
• Renal maturation correlates with gestational age, not postnatal age 6

Once-Daily vs. Multiple Daily Dosing:

• Once-daily dosing is preferred due to concentration-dependent killing, reduced toxicity, and convenience 3, 4, 7
• The older FDA-approved regimen of 2.5 mg/kg every 8-12 hours results in subtherapeutic peaks and toxic troughs in many neonates 5, 7
• Research from 2003-2009 consistently demonstrates superiority of once-daily dosing 5, 3, 4

High Trough Levels:

• If trough >2 mcg/mL, extend dosing interval (e.g., from every 24 hours to every 36 hours) rather than reducing dose 2
• This maintains therapeutic peaks while allowing more time for drug clearance 2

Low Peak Levels:

• If peak <5-6 mcg/mL, increase the dose rather than shortening the interval 2
• Inadequate peaks may result in treatment failure 3