What is the dose of gentamicin (an aminoglycoside antibiotic) for a 3-week-old infant weighing 4.12 kg?

Gentamicin Dosing for a 3-Week-Old Infant Weighing 4.12 kg

The appropriate dose of gentamicin for a 3-week-old infant weighing 4.12 kg is 2.5 mg/kg every 8 hours, which equals approximately 10.3 mg per dose or 30.9 mg per day. 1

Dosing Rationale

The FDA-approved gentamicin dosing guidelines provide specific recommendations for neonates and infants:

• For infants and neonates: 7.5 mg/kg/day divided into 3 equal doses (2.5 mg/kg every 8 hours) 1
• For premature or full-term neonates one week of age or less: 5 mg/kg/day (2.5 mg/kg every 12 hours) 1

Since this infant is 3 weeks old, the appropriate dosing regimen is 2.5 mg/kg every 8 hours.

Calculation

• Weight: 4.12 kg
• Dose per administration: 4.12 kg × 2.5 mg/kg = 10.3 mg every 8 hours
• Total daily dose: 10.3 mg × 3 = 30.9 mg/day

Monitoring Requirements

When administering gentamicin to infants, careful monitoring is essential due to the narrow therapeutic window:

• Serum drug levels: Measure both peak and trough concentrations

  • Target peak: 4-6 mcg/mL (30-60 minutes after administration)
  • Avoid prolonged levels above 12 mcg/mL
  • Target trough: below 2 mcg/mL (measured just before next dose) 1

• Renal function: Monitor serum creatinine regularly during treatment

  • Gentamicin can decrease glomerular filtration rate (GFR) and increase fractional excretion of beta-2-microglobulin 2
  • These effects are typically reversible after discontinuation

Important Clinical Considerations

1. Duration of therapy: The usual duration is 7-10 days. Extended treatment beyond 10 days increases risk of toxicity 1

2. Administration method: For intravenous administration in infants, dilute in a smaller volume of sterile isotonic saline or 5% dextrose in water than would be used for adults 1

3. Potential toxicities to monitor:

   • Ototoxicity (auditory and vestibular function)
   • Nephrotoxicity
   • Neuromuscular blockade (rare)

4. Dose adjustment: If renal impairment develops during treatment, dose adjustment will be necessary based on serum creatinine levels and/or measured gentamicin levels 1

Clinical Pitfalls to Avoid

1. Avoid extended treatment courses without monitoring renal, auditory, and vestibular functions, as toxicity risk increases significantly after 10 days 1

2. Don't physically premix gentamicin with other drugs; administer separately 1

3. Be cautious in patients with extensive burns as altered pharmacokinetics may result in reduced serum concentrations, requiring dose adjustment based on measured levels 1

4. Consider the impact of urine pH and osmolality on gentamicin efficacy when treating urinary tract infections, as concentrated acidic urine can inhibit gentamicin activity 3

5. Remember that gentamicin affects mitochondrial function, which may contribute to its ototoxicity profile, particularly with prolonged use 4