Gentamicin Dosing for Complicated UTI: Use Actual Body Weight
For your patient with complicated UTI who is 5'6" and 72 kg, administer gentamicin 5 mg/kg based on actual body weight = 360 mg IM once daily. 1, 2
Dosing Weight Selection
The European Association of Urology guidelines explicitly recommend 5 mg/kg once daily for complicated pyelonephritis requiring hospitalization, and this dosing is based on actual body weight. 1
The FDA label states that "dosage of aminoglycosides in obese patients should be based on an estimate of the lean body mass," but this caveat applies specifically to obese patients, not patients of normal weight. 3
Your patient at 72 kg and 5'6" (167.6 cm) has a BMI of approximately 26 kg/m², which is overweight but not obese (BMI <30). Therefore, actual body weight should be used. 3
Specific Dose Calculation
This once-daily dosing regimen is the standard for complicated UTI/pyelonephritis requiring parenteral therapy 1, 2
Duration: 5-7 days for uncomplicated pyelonephritis; longer if truly complicated with persistent infection 1
Critical Dosing Considerations
Lean body mass dosing is only recommended for genuinely obese patients to minimize nephrotoxicity risk, as aminoglycosides distribute poorly into adipose tissue. 2, 3
Recent evidence supports that a 7 mg/kg loading dose based on total body weight increases probability of target attainment in critically ill patients, but for standard complicated UTI, the guideline-recommended 5 mg/kg is appropriate. 4, 5
The once-daily dosing strategy (5 mg/kg) is superior to divided dosing for UTI with equivalent efficacy and potentially reduced nephrotoxicity. 6
Monitoring Requirements
Measure trough concentration before the second dose, targeting <1 mg/L (preferably <0.5 mg/L) to minimize nephrotoxicity risk. 2, 4
Peak concentration monitoring is less critical for once-daily dosing in UTI compared to endocarditis, but if measured, target approximately 16-20 mg/L for optimal bactericidal activity. 4, 5
Renal function monitoring is mandatory: Check serum creatinine every 2-3 days during therapy, as gentamicin nephrotoxicity is dose-dependent and cumulative. 2, 3
Common Pitfalls to Avoid
Do not use lean body weight for non-obese patients – this will result in underdosing and potential treatment failure. 3
Do not use divided dosing (every 8 hours) for UTI – once-daily dosing is the evidence-based standard with better pharmacodynamics. 1, 6
Do not extend therapy beyond 7 days without clear indication – prolonged aminoglycoside therapy significantly increases nephrotoxicity and ototoxicity risk. 3, 4
Ensure adequate hydration and avoid concurrent nephrotoxic agents (NSAIDs, contrast, vancomycin) when possible. 4