Aminoglycosides — Toxicity & Monitoring
Gentamicin can cure a life-threatening infection and permanently destroy hearing or kidney function in the same course of therapy. The difference is how you monitor.
Core Concept
Aminoglycosides (gentamicin, tobramycin, amikacin) carry two dose-limiting toxicities: nephrotoxicity and ototoxicity. Nephrotoxicity is usually reversible if caught early — monitor serum creatinine and BUN at baseline and throughout therapy. A rising creatinine signals accumulation. Ototoxicity can be irreversible and affects both cochlear (hearing loss, tinnitus) and vestibular (vertigo, ataxia) branches of cranial nerve VIII. Hearing loss typically starts in high-frequency ranges before the client notices it, so report any tinnitus or balance changes immediately. Peak levels (drawn 30 minutes after IV infusion ends) confirm the dose reaches therapeutic concentration. Trough levels (drawn just before the next dose) confirm the drug is clearing adequately — an elevated trough is the strongest predictor of toxicity. For gentamicin and tobramycin, the traditional trough target is less than 2 mcg/mL. Maintain hydration to support renal clearance, track intake and output, and avoid stacking nephrotoxic drugs (NSAIDs, vancomycin, amphotericin B, contrast dye) without close monitoring.
Watch Out For
Don't confuse peak and trough timing — peak is drawn after infusion (efficacy check), trough is drawn before the next dose (toxicity check). Students mix up ototoxicity and nephrotoxicity reversibility: nephrotoxicity is often reversible, ototoxicity frequently is not. Vancomycin also causes nephro- and ototoxicity, but aminoglycoside monitoring relies on both peak and trough levels, whereas current vancomycin practice increasingly uses AUC/MIC-guided dosing.
Clinical Pearl
Trough predicts toxicity, peak predicts efficacy. If the trough won't come down, the kidneys can't keep up — hold the dose and call the provider.
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