A 58-year-old man with chronic kidney disease (eGFR 25 mL/min/1.73 m²) is prescribed gentamicin 80 mg IV once daily for a suspected gram-negative infection. His serum creatinine is 3.2 mg/dL. After the first dose, peak serum concentration is 8 µg/mL (therapeutic: 4–10 µg/mL). However, the trough level 24 hours later is 2.5 µg/mL (normal: <1 µg/mL). Which pharmacokinetic parameter is MOST significantly altered in this patient, and what is the primary consequence?

Explanation

## Pharmacokinetic Alterations in Renal Impairment ### Pathophysiology of Gentamicin Clearance Gentamicin is an aminoglycoside that is **eliminated almost entirely by glomerular filtration** — it undergoes minimal hepatic metabolism and is not protein-bound. In chronic kidney disease with eGFR 25 mL/min/1.73 m², the renal clearance of gentamicin is reduced to approximately 25% of normal. **Key Point:** Gentamicin clearance is directly proportional to creatinine clearance. When renal function declines, drug accumulation becomes inevitable with standard dosing intervals. ### Clinical Interpretation of Trough Level The elevated trough concentration (2.5 µg/mL vs. normal <1 µg/mL) is the hallmark of **reduced renal clearance**. This indicates: 1. Insufficient time for renal elimination between doses 2. Progressive drug accumulation in plasma and tissues 3. Risk of nephrotoxicity and ototoxicity with continued standard dosing ### Consequence: Nephrotoxicity and Ototoxicity Risk **High-Yield:** Aminoglycosides are concentration-dependent antibiotics, but their toxicity is **time-dependent**. Prolonged exposure (high trough levels) increases the risk of: - Acute tubular necrosis (nephrotoxicity) - Permanent sensorineural hearing loss (ototoxicity) - Vestibular dysfunction ### Management Principle In renal impairment, gentamicin dosing must be adjusted by **extending the dosing interval** (e.g., 80 mg IV every 48–72 hours instead of every 24 hours) or reducing the dose. Extended-interval dosing is preferred because it maintains the concentration-dependent killing advantage while minimizing toxicity. **Clinical Pearl:** The peak level (8 µg/mL) is within therapeutic range, but the **elevated trough** is the red flag for dose adjustment needed. ### Why Other Parameters Are NOT Primarily Altered - **Volume of distribution (Vd):** Gentamicin distributes mainly in extracellular fluid; Vd is relatively stable in renal disease and does not explain the trough elevation. - **Hepatic metabolism:** Gentamicin is NOT hepatically metabolized; liver function is irrelevant. - **Protein binding:** Aminoglycosides are poorly protein-bound (<10%); changes in protein binding do not significantly affect clearance.