Gentamicin Dosing Guidelines for Adults
For adults with normal renal function, gentamicin should be administered at a dose of 3-5 mg/kg/day divided every 8 hours, with dosing adjusted to achieve a 1-hour serum concentration of approximately 3 μg/mL and a trough concentration of <1 μg/mL. 1, 2
Standard Dosing Recommendations
Normal Renal Function
- Standard dose: 3 mg/kg/day divided every 8 hours for serious infections 1, 2
- Life-threatening infections: Up to 5 mg/kg/day divided every 8 hours (should be reduced to 3 mg/kg/day as soon as clinically indicated) 2
- Target serum levels:
- Duration: Typically 7-10 days; longer courses may require additional monitoring 2
Specific Clinical Scenarios
Endocarditis Treatment
- Enterococcal endocarditis: 3 mg/kg/day divided every 8 hours for 4-6 weeks 3, 1
- Native valve endocarditis (viridans group streptococci): 3 mg/kg/day for 2 weeks 1
- Prosthetic valve endocarditis: 3 mg/kg/day divided every 8 hours for 2 weeks 1
Dosing in Special Populations
Renal Impairment
For patients with impaired renal function, two approaches are recommended:
Extended interval approach: Multiply serum creatinine (mg/dL) by 8 to determine dosing interval in hours 2
- Example: Patient with creatinine of 2 mg/dL would receive dose every 16 hours
Reduced dose approach: Divide normal dose by serum creatinine level 2
- Example: 60 mg ÷ 2 mg/dL = 30 mg every 8 hours
Dosage adjustment guide:
| Serum Creatinine (mg%) | Approximate CrCl (mL/min/1.73m²) | % of Normal Dose |
|---|---|---|
| ≤1 | >100 | 100 |
| 1.1 to 1.3 | 70 to 100 | 80 |
| 1.4 to 1.6 | 55 to 70 | 65 |
| 1.7 to 1.9 | 45 to 55 | 55 |
| 2 to 2.2 | 40 to 45 | 50 |
Elderly Patients
- Elderly patients (>65 years) have altered pharmacokinetics and are at higher risk for nephrotoxicity and ototoxicity 4
- Consider lower initial doses and careful monitoring of renal function 1, 4
Obese Patients
- Dosing should be based on an estimate of lean body mass rather than total body weight 2
Administration Methods
Intravenous Administration
- Dilute single dose in 50-200 mL of sterile isotonic saline or 5% dextrose solution
- Infuse over 30 minutes to 2 hours
- Do not physically premix gentamicin with other drugs 2
Intramuscular Administration
- Peak serum concentrations typically occur between 30-60 minutes after IM administration 2
Therapeutic Drug Monitoring
Therapeutic drug monitoring is essential for:
- Courses longer than 3 days
- Patients with fluctuating renal function
- Elderly patients
- Critically ill patients 1, 5
Monitoring recommendations:
- Measure both peak (30-60 minutes after administration) and trough (just before next dose) levels
- Adjust dosing to maintain peak levels of 4-6 μg/mL and trough levels <2 μg/mL (preferably <1 μg/mL) 1, 2
- Monitor renal function regularly (serum creatinine, BUN, electrolytes) 1
Toxicity Prevention
Nephrotoxicity Prevention
- Ensure adequate hydration before and during therapy 1
- Avoid concurrent use of other nephrotoxic medications when possible 1, 4
- Consider once-daily dosing in appropriate patients to potentially reduce nephrotoxicity 6
- Monitor renal function regularly, especially in high-risk patients 1
Ototoxicity Prevention
- Limit treatment duration when possible
- Monitor for auditory and vestibular symptoms
- Consider audiometric testing for courses >10 days 2
Important Caveats
- Recent evidence suggests that once-daily dosing may reduce nephrotoxicity compared to multiple daily dosing, but for serious infections like endocarditis, multiple daily dosing is still recommended 3, 1, 6
- Therapeutic drug monitoring is crucial for optimizing efficacy while minimizing toxicity, especially in high-risk populations 1, 5
- For pathogens with MIC ≥2 mg/L, gentamicin may not achieve adequate therapeutic levels even at higher doses 7
- Risk of nephrotoxicity increases with treatment duration beyond 10 days, advanced age, and concurrent use of nephrotoxic medications 4, 8
Remember that gentamicin has a narrow therapeutic index, and careful dosing and monitoring are essential to balance efficacy and safety.