A 62-year-old man with chronic kidney disease (eGFR 28 mL/min/1.73m²) is prescribed gentamicin 80 mg IV for a suspected gram-negative infection. His serum creatinine is 2.8 mg/dL and he weighs 70 kg. Which is the most appropriate immediate action?

## Pharmacokinetic Adjustment in Renal Impairment **Key Point:** Gentamicin is an aminoglycoside with significant renal excretion (>90% unchanged). In moderate-to-severe renal impairment (eGFR 15–29 mL/min/1.73m²), dose reduction and interval extension are mandatory to prevent accumulation and nephrotoxicity. ### Gentamicin Dosing in CKD | eGFR (mL/min/1.73m²) | Renal Category | Dosing Strategy | |---|---|---| | >60 | Normal | 80 mg IV every 8 h | | 30–60 | Mild–Moderate | 80 mg IV every 12 h | | 15–29 | Moderate–Severe | 80 mg IV every 24–36 h | | <15 | Severe/ESRD | 80 mg IV every 48 h or dialysis dosing | **High-Yield:** This patient's eGFR of 28 mL/min/1.73m² falls into the moderate-to-severe category. Standard every-8-hour dosing would lead to drug accumulation, rising trough levels, and acute kidney injury (AKI) or ototoxicity. ### Monitoring Strategy 1. **Serum level monitoring** is essential with extended-interval dosing (target trough <5 µg/mL, peak 15–30 µg/mL). 2. **Baseline and serial creatinine** to detect further renal decline. 3. **Audiometry** if therapy extends beyond 5–7 days (aminoglycosides are ototoxic). **Clinical Pearl:** Extended-interval dosing (once daily or once every 36 h) is preferred in renal impairment because it maintains adequate peak concentration (bactericidal effect) while allowing trough levels to fall, reducing nephrotoxicity risk compared to dose reduction with standard intervals. **Tip:** Always check the Cockcroft–Gault or MDRD eGFR before prescribing aminoglycosides, and consult institutional renal dosing guidelines or pharmacy.