A 58-year-old man with diabetes mellitus type 2 presents with fever (38.5°C), dysuria, and flank pain for 3 days. Urine culture grows Escherichia coli (>10⁵ CFU/mL). Serum creatinine is 1.8 mg/dL (baseline 1.2 mg/dL). He is started on gentamicin 5 mg/kg IV once daily. On day 5 of therapy, his serum creatinine rises to 2.4 mg/dL, urine output drops to 400 mL/24 h, and serum gentamicin trough level is 2.8 mcg/mL (target <1 mcg/mL). What is the most appropriate next step in management?

Explanation

## Clinical Assessment This patient has developed **aminoglycoside-induced acute kidney injury (AKI)** with evidence of drug accumulation: - Rising serum creatinine (1.8 → 2.4 mg/dL) - Oliguria (400 mL/24 h) - Elevated trough gentamicin level (2.8 mcg/mL vs. target <1 mcg/mL) **Key Point:** Aminoglycosides are nephrotoxic and renally cleared. In renal impairment, standard dosing leads to accumulation and further kidney damage. ## Pathophysiology of Aminoglycoside Nephrotoxicity 1. Filtered at glomerulus → reabsorbed via megalin-mediated endocytosis in proximal tubule 2. Accumulates in lysosomes → generates reactive oxygen species (ROS) 3. Causes proximal tubular necrosis → acute tubular necrosis (ATN) 4. Risk factors: pre-existing renal disease, diabetes, volume depletion, prolonged therapy **High-Yield:** Once-daily dosing (5 mg/kg) reduces nephrotoxicity compared to multiple daily dosing, BUT requires dose adjustment in renal impairment. ## Management of Aminoglycoside-Induced AKI | Step | Action | Rationale | |------|--------|----------| | 1 | Assess renal function (Cr, eGFR) | Determine degree of renal impairment | | 2 | Measure serum aminoglycoside levels | Confirm drug accumulation (trough >1 mcg/mL = toxicity risk) | | 3 | Extend dosing interval (e.g., q36h, q48h) | Reduce peak levels and allow clearance | | 4 | Reduce dose if necessary | Maintain therapeutic efficacy while preventing accumulation | | 5 | Recheck levels before next dose | Ensure trough <1 mcg/mL | | 6 | Monitor urine output, Cr daily | Detect worsening AKI early | **Clinical Pearl:** Aminoglycosides have **concentration-dependent bactericidal activity** — efficacy depends on peak level (target peak 15–30 mcg/mL for gentamicin), NOT trough. Once-daily dosing achieves high peaks with low troughs, reducing toxicity while maintaining efficacy. **Mnemonic: ONCE-DAILY DOSING BENEFITS — OD Reduces Nephrotoxicity (ORN)** - **O**nce-daily = higher peak (bactericidal) - **D**ose interval extended = lower trough (less accumulation) - **R**enal function must be monitored - **N**ephrotoxicity risk ↓ vs. multiple daily dosing ## Why Not Discontinue? E. coli urosepsis with rising creatinine and fever warrants continuation of effective therapy. Fluoroquinolones are alternative agents but discontinuing an effective drug without dose adjustment is premature. **Warning:** Do NOT increase dose in renal impairment — this worsens accumulation and AKI. [cite:KD Tripathi 8e Ch 49]