Gentamicin Dosing for Gustillo Type 3 Open Fracture
For a 70-80kg patient with a Gustillo Type 3 open fracture, administer gentamicin 5 mg/kg IV once daily (approximately 350-400mg as a single dose), in addition to your 2g IV cefazolin. 1, 2
Specific Dosing Calculation
- For 70kg patient: 350mg IV gentamicin once daily 1
- For 75kg patient: 375mg IV gentamicin once daily 1
- For 80kg patient: 400mg IV gentamicin once daily 1
The FDA label specifies that for life-threatening infections (which Type 3 open fractures qualify as), the dose is 1.7 mg/kg every 8 hours (5 mg/kg/day), but once-daily dosing at 5-6 mg/kg has become the preferred approach for open fractures 1, 2, 3.
Rationale for Once-Daily High-Dose Regimen
- Once-daily dosing at 5-6 mg/kg is superior to divided dosing for open fractures because it optimizes peak concentration-dependent killing of gram-negative organisms while potentially reducing nephrotoxicity 2, 3
- Research specifically in Gustillo Type II and III fractures demonstrated that gentamicin 5-6 mg/kg once daily is safe, effective, and achieves adequate tissue penetration without increased toxicity 2, 3
- The Surviving Sepsis Campaign guidelines emphasize that aminoglycosides require high peak concentrations (optimally 5-7 mg/kg daily) to maximize efficacy, particularly in critically ill patients with expanded volume of distribution 4
Duration and Monitoring
- Continue gentamicin for 24-48 hours after definitive wound closure, not to exceed 72 hours total unless there is established infection 5, 6
- Monitor trough levels before the second dose (target <2 mcg/mL) and check baseline creatinine 1
- Peak levels should reach 4-6 mcg/mL, though with once-daily dosing, trough monitoring is more critical to prevent toxicity 1
Critical Timing Consideration
- Administer both antibiotics within 1 hour of patient arrival to the emergency department, as delays beyond this significantly increase infection risk 7
- The median time to gentamicin administration in trauma centers is often delayed (180 minutes) compared to cefazolin (17 minutes) because gentamicin requires weight-based dosing and may not be stocked in the ER 7
Common Pitfall to Avoid
Do not use divided dosing (e.g., 1.7 mg/kg every 8 hours) for open fractures—this outdated approach provides suboptimal peak concentrations and may increase nephrotoxicity risk compared to once-daily dosing 2, 3. The once-daily regimen has been specifically validated in Type III open fractures and is now considered standard of care 2, 3.