What are the indications, contraindications, route of administration, and dosages for adults and children for Gentamycin (aminoglycoside antibiotic)?

Gentamicin: Indications, Contraindications, Routes, and Dosing

Indications

Gentamicin is indicated for serious infections caused by susceptible gram-negative bacteria and as combination therapy for specific gram-positive infections, particularly endocarditis. 1

FDA-Approved Indications

• Gram-negative infections: Klebsiella pneumoniae, Escherichia coli, Serratia marcescens, Citrobacter spp., Enterobacteriaceae spp., Pseudomonas spp. 1, 2
• Staphylococcal infections (including skin, bone, and soft tissue) 1
• Bacterial septicemia and sepsis (including neonatal sepsis) 1
• Bacterial meningitis 1
• Infective endocarditis (in combination with beta-lactams) 3
• Respiratory tract infections 1
• Urinary tract infections 1
• Peritonitis (including peritoneal dialysis-associated) 1
• Eye infections (topical formulations) 1

Combination Therapy for Endocarditis

• Staphylococcal endocarditis: Combined with nafcillin/oxacillin (or vancomycin if methicillin-resistant) for 3-5 days to enhance bacterial killing 3
• Enterococcal endocarditis: Combined with ampicillin or penicillin for synergistic effect throughout treatment course 3
• Streptococcal endocarditis: Combined with penicillin or ceftriaxone for highly susceptible strains, allowing shortened 2-week regimens 3

Contraindications

Absolute Contraindications

• Hypersensitivity to gentamicin or other aminoglycosides 1
• Myasthenia gravis (aminoglycosides can worsen neuromuscular blockade) 4

Relative Contraindications (Require Dose Adjustment or Avoidance)

• Pre-existing eighth cranial nerve dysfunction (vestibular or auditory impairment) 3, 5
• Severe renal impairment (creatinine clearance <20 mL/min) without dose adjustment 5, 1
• Concurrent use of other nephrotoxic or ototoxic drugs 3, 1
• Pregnancy (aminoglycosides cross placenta and may cause fetal ototoxicity) 4

Routes of Administration

Gentamicin may be administered intramuscularly (IM) or intravenously (IV); oral administration is ineffective due to poor gastrointestinal absorption. 1, 4

Intravenous Administration

• Preferred route for patients with bacterial septicemia, shock, congestive heart failure, hematologic disorders, severe burns, or reduced muscle mass 1
• Infusion method: Dilute single dose in 50-200 mL sterile isotonic saline or 5% dextrose; infuse over 30 minutes to 2 hours 1
• Pediatric volumes: Use smaller diluent volumes in infants and children 1

Intramuscular Administration

• Rapid absorption with peak serum concentrations at 30-90 minutes post-injection 1, 6
• Equivalent efficacy to IV administration 1

Topical Administration

• Ophthalmic formulations (0.3% eye drops) for bacterial conjunctivitis and keratitis 7

Adult Dosing

Standard Dosing for Normal Renal Function

For serious infections in adults with normal renal function, administer 3 mg/kg/day divided into three equal doses every 8 hours. 1, 5

• Life-threatening infections: May increase to 5 mg/kg/day in 3-4 divided doses initially, then reduce to 3 mg/kg/day as clinically indicated 1
• Once-daily dosing: 7 mg/kg IV once daily is increasingly used for non-endocarditis infections to optimize concentration-dependent killing 8
• Duration: 7-10 days for most infections; longer courses require enhanced monitoring 1, 5

Endocarditis-Specific Dosing

For enterococcal endocarditis, gentamicin should be administered at 3 mg/kg/day in multiple divided doses (every 8 hours), NOT once-daily dosing. 3, 5

• Native valve endocarditis (streptococcal): 3 mg/kg/day divided every 8 hours for 2 weeks when combined with penicillin/ceftriaxone 3
• Prosthetic valve endocarditis (staphylococcal): 3 mg/kg/day divided every 8 hours for 2 weeks, combined with nafcillin/oxacillin plus rifampin (total therapy ≥6 weeks) 3, 5
• Enterococcal endocarditis: 3 mg/kg/day divided every 8 hours for 4-6 weeks depending on symptom duration 3, 5

Therapeutic Drug Monitoring Targets

Target peak concentration of 3-4 μg/mL (measured 30-60 minutes post-dose) and trough concentration <1 μg/mL (preferably <0.5 μg/mL) to balance efficacy and minimize nephrotoxicity. 3, 5, 1

• Peak levels >12 μg/mL should be avoided (increased toxicity risk) 1
• Trough levels >2 μg/mL should be avoided (nephrotoxicity risk) 1, 8

Renal Impairment Adjustments

Dosage must be adjusted in patients with impaired renal function; standard 3 mg/kg/day dosing is only for normal renal function. 1, 5

Interval Extension Method

• Multiply serum creatinine (mg/dL) by 8 to determine dosing interval in hours 1
• Example: Creatinine 2.0 mg/dL → dose every 16 hours (2 × 8) 1

Dose Reduction Method

• Divide normal dose by serum creatinine level for every 8-hour dosing 1
• Example: 60 mg normal dose ÷ 2.0 creatinine = 30 mg every 8 hours 1

Severe Renal Impairment (CrCl <50 mL/min)

• Consult infectious diseases specialist for dosing guidance 3
• Therapeutic drug monitoring is mandatory 5, 8

Hemodialysis

• Post-dialysis dosing: 1-1.7 mg/kg after each dialysis session (approximately 50% removed during 8-hour hemodialysis) 1

Pediatric Dosing

Standard Pediatric Dosing (Normal Renal Function)

Children (>1 month old): 6-7.5 mg/kg/day divided into 2-2.5 mg/kg every 8 hours. 1

• Infants and neonates: 7.5 mg/kg/day (2.5 mg/kg every 8 hours) 1
• Premature or full-term neonates ≤1 week old: 5 mg/kg/day (2.5 mg/kg every 12 hours) 1
• Pediatric doses should not exceed adult doses 3, 1

Endocarditis Dosing in Children

For pediatric endocarditis, gentamicin 3 mg/kg/day divided into three equal doses every 8 hours is recommended. 3

• Staphylococcal endocarditis: Nafcillin/oxacillin 200 mg/kg/day + gentamicin 3 mg/kg/day in 3 divided doses 3
• Streptococcal endocarditis: Penicillin G 200,000-300,000 U/kg/day + gentamicin 3 mg/kg/day in 3 divided doses 3
• Prosthetic valve endocarditis: Gentamicin 3 mg/kg/day in 3 divided doses for 2 weeks (total therapy ≥6 weeks) 3

Pediatric UTI Dosing

For urinary tract infections in children, gentamicin 3 mg/kg/day divided into three equal doses. 9

Critical Safety Considerations

Nephrotoxicity Prevention

• Therapeutic drug monitoring is recommended for all patients receiving >1 dose to optimize trough concentrations and reduce nephrotoxicity 5, 8
• Monitor renal function weekly at minimum during treatment 5
• Avoid concurrent nephrotoxic drugs (NSAIDs, vancomycin, amphotericin B) 3
• Elderly patients (>65 years) are at significantly higher risk and should receive shortest effective duration 5

Ototoxicity Prevention

• Patients with eighth cranial nerve dysfunction should avoid gentamicin or receive shortest possible course 3, 5
• Ototoxicity incidence is approximately 2% with vestibular effects more common than hearing loss 6
• Risk increases with treatment >10 days 1

High-Risk Populations Requiring Enhanced Monitoring

• Critically ill patients (altered pharmacokinetics) 8
• Burn patients (reduced serum concentrations) 1
• Obese patients (dose based on adjusted body weight, not total body weight) 8
• Neonates and premature infants (immature renal function) 4
• Patients on intermittent hemodialysis 8

Duration Limitations

Treatment duration should be limited to 7-10 days for most infections; courses >10 days require enhanced monitoring of renal, auditory, and vestibular function due to increased toxicity risk. 1, 5