Gentamicin Dosing for UTI in a 240-Pound Patient
For a 240-pound (109 kg) patient with a UTI and normal renal function, administer gentamicin 327 mg IM every 8 hours (total 981 mg/day divided into three doses), not twice daily. 1
Critical Dosing Error to Avoid
The FDA-approved dosing for gentamicin is 3 mg/kg/day divided into three equal doses every 8 hours for serious infections in patients with normal renal function, not twice daily. 1 The American Heart Association guidelines consistently recommend multiple divided doses (every 8 hours) rather than twice-daily administration for serious infections. 2, 3
Calculation for This Patient
- Patient weight: 240 pounds = 109 kg
- Standard dose: 3 mg/kg/day = 327 mg/day 1
- Divided dosing: 327 mg ÷ 3 = 109 mg IM every 8 hours 1
For UTI specifically, this three-times-daily regimen is appropriate for 7-10 days of treatment. 1
Why Not Twice Daily?
Twice-daily gentamicin dosing is not recommended in current guidelines. 2, 3 The American Heart Association explicitly states that gentamicin should be administered in multiple divided doses totaling 3 mg/kg/day, with three-times-daily dosing being the standard approach. 2 Single daily dosing (once-daily extended interval dosing) is an alternative for some infections, but twice-daily dosing lacks supporting evidence and falls between established regimens. 4
Alternative: Once-Daily Extended Interval Dosing
If considering once-daily dosing instead (which has evidence for UTI), the dose would be 7 mg/kg = 763 mg IM once daily. 4 Historical studies support single daily dosing of 160 mg for uncomplicated UTI, though this was a fixed dose not adjusted for weight. 5, 6 However, recent evidence suggests 7 mg/kg once daily provides better target attainment. 4
Mandatory Monitoring
- Peak concentration (30-60 minutes post-injection): Target 4-6 mcg/mL, never exceed 12 mcg/mL 1
- Trough concentration (just before next dose): Target <1 mcg/mL, never exceed 2 mcg/mL 1
- Renal function monitoring: Check serum creatinine before treatment and during therapy, as nephrotoxicity occurs in approximately 4% of patients (irreversible in 1% overall) 7
- Duration: Limit to 7-10 days; toxicity increases significantly beyond 10 days 1
Renal Function Considerations
This dosing assumes normal renal function. If creatinine clearance is <50 mL/min, dose reduction and extended intervals are mandatory. 2, 8 For patients with CrCl 40-59 mL/min, consider 36-hour intervals; for CrCl 20-39 mL/min, consider 48-hour intervals. 7