A 72-year-old male CKD patient with UTI is treated with gentamicin. With Cr 140 μmol/L and eGFR 30, which adjustment is most appropriate?

• A. Increase the dose and shorten interval
• B. Use the same dose and monitor
• C. Reduce the dose and increase the interval
• D. Switch to an oral antibiotic

Answer: (C)

The main idea is how renal impairment affects dosing of renally cleared antibiotics like gentamicin. Gentamicin is eliminated mainly by the kidneys, so when kidney function is reduced, the drug clears more slowly and can accumulate, increasing the risk of nephrotoxicity and ototoxicity. In practice, dosing for aminoglycosides in CKD is adjusted by either reducing the dose, extending the dosing interval, or both, to keep drug exposure effective but not toxic.

With a Cr around 140 μmol/L and an eGFR near 30 mL/min, the patient’s gentamicin clearance is substantially reduced. Giving the usual dose at the usual interval would raise trough levels and promote accumulation. Reducing the amount given per dose lowers peak exposure, and lengthening the interval allows more time for the drug to be cleared between doses, reducing troughs and overall exposure while still providing antibacterial activity. This balance helps maintain efficacy (via the drug’s concentration-dependent killing and the post-antibiotic effect) and minimizes toxicity.

Choosing to increase the dose or shorten the interval would raise exposure and toxicity risk. Keeping the same dose and simply monitoring could let troughs remain high and accumulation occur. Switching to an oral antibiotic isn’t appropriate here because gentamicin has poor oral bioavailability and systemic activity for this infection when given by mouth.