What is the recommended dose of gentamicin (an aminoglycoside antibiotic) for a 78-year-old patient with normal renal function (Glomerular Filtration Rate (GFR) of 88) and weighing 59 kg?

Gentamicin Dosing for a 78-Year-Old Patient with GFR 88 mL/min and Weight 59 kg

For this 78-year-old patient with normal renal function (GFR 88 mL/min) weighing 59 kg, the recommended gentamicin dose is 177 mg (3 mg/kg) divided into three doses of 59 mg every 8 hours if treating endocarditis, OR 413 mg (7 mg/kg) once daily if treating sepsis or serious Gram-negative infection.

Critical Decision Point: Clinical Indication Determines Dosing Strategy

The dosing regimen fundamentally differs based on whether this is for:

For Endocarditis (Synergistic Therapy)

• Dose: 177 mg total daily (59 mg every 8 hours) 1, 2
• The American Heart Association explicitly states that gentamicin for endocarditis must be given in multiple divided doses (3 mg/kg/day divided every 8 hours), never once daily, because synergistic bactericidal activity with beta-lactams requires sustained drug exposure 1, 2, 3
• Target peak levels of 3-4 μg/mL and trough <1 μg/mL 1, 2
• Duration limited to 3-5 days for native valve endocarditis 1

For Sepsis or Serious Gram-Negative Infection

• Dose: 413 mg (7 mg/kg) once daily 3, 4, 5
• The Surviving Sepsis Campaign recommends 5-7 mg/kg once daily to optimize concentration-dependent killing 3
• Higher doses (7 mg/kg) are specifically recommended for critically ill patients due to increased volume of distribution from fluid resuscitation 3, 4, 6
• Target peak levels of 16-20 μg/mL and trough <1 μg/mL 3, 5

Age-Related Dosing Adjustment Required

This patient requires dose reduction due to advanced age, despite normal GFR:

• The American Thoracic Society explicitly states that for persons greater than 59 years of age, the dose should be reduced to 10 mg/kg per day (750 mg maximum) for streptomycin, and similar principles apply to gentamicin 1
• The FDA label confirms that elderly patients have reduced gentamicin clearance even with preserved creatinine clearance 7
• Research demonstrates that gentamicin clearance decreases significantly with age (r² = 0.178, p = 0.016), independent of measured renal function 6

Adjusted Dosing for This 78-Year-Old Patient:

For endocarditis: Reduce to approximately 50 mg every 8 hours (2.5 mg/kg/day instead of 3 mg/kg/day) 1, 2

For sepsis: Start with 354 mg once daily (6 mg/kg instead of 7 mg/kg), recognizing the balance between achieving adequate peaks and avoiding accumulation 3, 5, 8

Mandatory Therapeutic Drug Monitoring

All patients receiving gentamicin require serum level monitoring:

• Peak levels: Measure 30-60 minutes after infusion completion 1, 2, 7
  • Endocarditis target: 3-4 μg/mL (never >12 μg/mL) 1, 2
  • Sepsis target: 16-20 μg/mL 3, 5
• Trough levels: Measure immediately before next dose, target <1 μg/mL (never >2 μg/mL) 1, 2, 7
• Serum creatinine: Monitor at least weekly during therapy 1, 2

The American Heart Association recommends mandatory consultation with infectious disease, clinical pharmacy, or nephrology for elderly patients requiring gentamicin therapy 2

Critical Safety Considerations for This Elderly Patient

Risk factors for toxicity are significantly elevated:

• Age >60 years independently increases risk of ototoxicity 1
• Elderly patients show higher rates of nephrotoxicity even with appropriate dosing 5, 8
• Low body weight (59 kg) increases risk of accumulation 8
• The combination of advanced age and borderline body weight necessitates conservative initial dosing with aggressive monitoring 6, 8

Common Pitfalls to Avoid

• Never use once-daily dosing for endocarditis - this is explicitly contraindicated and may cause treatment failure 2, 3
• Never use standard 3 mg/kg dosing for sepsis - this underdoses critically ill patients and is only appropriate for endocarditis synergy 3
• Never ignore age-related dose reduction - elderly patients accumulate gentamicin even with normal measured GFR 1, 6, 8
• Never extend gentamicin beyond 3-5 days without compelling indication, as toxicity risk increases substantially with duration >10 days 1, 7, 4

Renal Function Monitoring Strategy

Despite current GFR of 88 mL/min:

• Measure serum creatinine before each dose for the first 3 days 2
• If creatinine rises by ≥45 μmol/L (0.5 mg/dL), reduce dose or extend interval immediately 1, 2
• Elderly patients can develop rapidly declining renal function requiring urgent dose adjustment 2, 9