What is the recommended dose of gentamicin (aminoglycoside antibiotic) for an open fracture?

Gentamicin Dosing for Open Fractures

For open fractures, administer gentamicin 5-6 mg/kg IV once daily in combination with cefazolin 1-2 g every 8 hours, continuing for 48 hours after definitive wound closure. 1, 2

Recommended Dosing Regimen

• Once-daily dosing: 5-6 mg/kg IV as a single daily dose 1, 2

  • This regimen has been specifically validated in Gustilo Grade II and III open fractures 1
  • A prospective randomized trial demonstrated that 6 mg/kg once daily was equally effective as divided dosing (5 mg/kg divided twice daily) for infection prevention 1

• Duration: Continue until 48 hours after definitive wound closure 2

  • Typical duration is 2-5 days depending on timing of wound closure 2

• Combination therapy is mandatory: Always administer with cefazolin 1 g every 8 hours 1, 2

  • Gentamicin should never be used as monotherapy for open fractures 1

Dosing in Patients with Normal Renal Function

• Standard FDA-approved dosing for serious infections: 3 mg/kg/day divided every 8 hours 3
• For life-threatening infections: Up to 5 mg/kg/day in divided doses 3
• However, the orthopedic trauma literature supports higher once-daily dosing (5-6 mg/kg) specifically for open fractures 1, 2

Critical Monitoring Parameters

• Peak levels (30-60 minutes post-infusion): Target 4-6 mcg/mL, never exceed 12 mcg/mL 3
• Trough levels (just before next dose): Target <1-2 mcg/mL 3
• Monitor BUN and creatinine daily during therapy 2
• Query patient daily for signs of ototoxicity (tinnitus, vertigo, hearing loss) 2

Dose Adjustments for Renal Impairment

Critical caveat: Serum creatinine alone may not accurately reflect renal function in certain populations 4

• If creatinine clearance <50 mL/min: Reduce dose and extend dosing interval 5
• If creatinine clearance <20 mL/min: Avoid once-daily regimens entirely 5
• For patients with impaired renal function, calculate estimated creatinine clearance based on age, sex, weight, and actual creatinine 5, 4
• Even the first dose should be adjusted in high-risk patients 4

Evidence Quality and Clinical Context

The once-daily high-dose regimen (5-6 mg/kg) is supported by Level II evidence from prospective randomized trials specifically in open fractures 1. A study of 75 Gustilo Grade II and III fractures found no difference in infection rates between once-daily (6 mg/kg) versus divided dosing (5 mg/kg divided twice daily), with the added benefits of cost-efficiency and simplified administration 1. A subsequent study of 16 patients with open tibial shaft fractures using 5 mg/kg once daily showed no nephrotoxicity or ototoxicity 2.

Important distinction: The FDA label recommends lower divided dosing (3 mg/kg/day) for general serious infections 3, but orthopedic trauma-specific evidence supports higher once-daily dosing for open fractures 1, 2. This reflects the unique pharmacokinetic advantages of once-daily aminoglycoside dosing and the specific bacterial pathogens encountered in open fractures.

Common Pitfalls to Avoid

• Do not use gentamicin monotherapy - always combine with a cephalosporin for adequate gram-positive coverage 1, 2
• Do not rely on serum creatinine alone in special populations (spinal cord injury, elderly, low muscle mass) - calculate creatinine clearance 4
• Do not continue beyond 48 hours after wound closure unless treating established infection 2
• Do not use once-daily dosing for endocarditis - that indication requires divided dosing every 8 hours 6
• Do not dose obese patients on total body weight - use lean body mass 3