What intramuscular dosing regimen (dose per injection, volume, injection sites) should be used for a 179.4‑lb (81.4 kg) patient receiving tobramycin 10 mg per kilogram per day?

Intramuscular Tobramycin Dosing for 81.4 kg Patient

For an 81.4 kg (179.4 lb) patient receiving tobramycin 10 mg/kg/day intramuscularly, administer 814 mg total daily dose divided into 3-4 equal injections, which translates to approximately 271 mg (6.8 mL) every 8 hours or 203 mg (5.1 mL) every 6 hours using the standard 40 mg/mL concentration. 1

Standard Dosing Calculation

• Total daily dose: 10 mg/kg/day × 81.4 kg = 814 mg/day 1
• Divided dosing options:
  • Every 8 hours (3 times daily): 271 mg per dose (approximately 6.8 mL of 40 mg/mL solution) 1
  • Every 6 hours (4 times daily): 203 mg per dose (approximately 5.1 mL of 40 mg/mL solution) 1

Volume and Injection Site Considerations

• Volume per injection: Using standard tobramycin 40 mg/mL concentration, each injection ranges from 5.1-6.8 mL depending on dosing interval 1
• Injection sites: Rotate between large muscle groups (gluteus maximus, vastus lateralis, deltoid for smaller volumes) to minimize local tissue irritation 1
• Maximum volume per site: Limit to 3-4 mL per injection site; if dose exceeds this volume, divide into two separate injection sites 1

Clinical Context for High-Dose Regimen

This 10 mg/kg/day dosing represents an intensive regimen typically reserved for specific clinical scenarios 1:

• Cystic fibrosis patients: Initial dosing of 10 mg/kg/day in 4 equally divided doses (203 mg every 6 hours for this patient) is recommended due to altered pharmacokinetics 1
• Life-threatening infections: Up to 5 mg/kg/day is standard maximum for serious infections in patients with normal renal function, but higher doses may be used with serum level monitoring 1

Critical Monitoring Requirements

• Serum concentration monitoring is mandatory at this dosing level to prevent toxicity 1
• Peak levels: Draw 30-60 minutes after IM injection; target 4-10 mcg/mL 1
• Trough levels: Draw immediately before next dose; maintain <2 mcg/mL to minimize nephrotoxicity and ototoxicity 1
• Renal function: Monitor serum creatinine and creatinine clearance at baseline and during therapy 1
• Auditory/vestibular function: Assess for hearing loss, tinnitus, or vertigo, particularly if treatment exceeds 10 days 1

Duration and Dose Adjustment

• Usual treatment duration: 7-10 days; extended courses require enhanced monitoring for neurotoxicity 1
• Dose reduction: Decrease to 3 mg/kg/day (244 mg/day for this patient) as soon as clinically indicated to minimize toxicity risk 1
• Renal impairment: If creatinine clearance decreases during therapy, adjust dosing interval by multiplying serum creatinine by 6 to determine new interval in hours 2

Common Pitfalls to Avoid

• Exceeding 5 mg/kg/day without serum monitoring significantly increases risk of irreversible ototoxicity and nephrotoxicity 1
• Inadequate injection site rotation can cause sterile abscesses and tissue necrosis 1
• Failure to obtain loading dose: Always administer full calculated dose initially, even in renal impairment, then adjust subsequent doses 1, 2
• Concurrent nephrotoxic agents: Avoid combining with other aminoglycosides, loop diuretics, or vancomycin when possible 1