## Clinical Context This patient has multiple risk factors for aminoglycoside nephrotoxicity: advanced age, diabetes, and significantly reduced renal function (eGFR 35). The question tests knowledge of aminoglycoside dosing strategies in renal impairment and nephrotoxicity prevention. ## Key Pharmacokinetic Principle **Key Point:** Extended-interval (once-daily) dosing of aminoglycosides is preferred over conventional thrice-daily dosing in modern practice because it achieves superior bactericidal efficacy (concentration-dependent killing) while reducing nephrotoxicity and ototoxicity risk. **High-Yield:** In renal impairment, the dosing interval must be extended (not the dose reduced proportionally). The standard approach is to use the normal dose (5 mg/kg) but space it further apart based on eGFR. For eGFR 30–50 mL/min, gentamicin is typically dosed every 24–36 hours. ## Therapeutic Drug Monitoring (TDM) **Clinical Pearl:** TDM is mandatory for aminoglycosides in renal impairment to avoid accumulation and toxicity. Peak levels (drawn 30 min post-infusion) and trough levels (drawn just before next dose) guide subsequent dosing adjustments. For once-daily dosing, trough levels should be <1 μg/mL. ## Why Once-Daily Dosing? Once-daily dosing achieves: - Higher peak concentrations → enhanced bactericidal activity - Lower trough concentrations → reduced nephrotoxicity - Reduced cellular uptake into proximal tubule cells (the mechanism of nephrotoxicity) ## Dosing Algorithm for This Patient | eGFR (mL/min) | Gentamicin Dosing Strategy | |---|---| | >50 | 5 mg/kg IV q24h | | 30–50 | 5 mg/kg IV q24–36h (monitor levels) | | 10–30 | 5 mg/kg IV q36–48h (monitor levels) | | <10 | Avoid or use dialysis dosing | **Key Point:** This patient (eGFR 35) falls into the 30–50 category and should receive 5 mg/kg once daily with mandatory TDM.
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