skip to main content

Trimethoprim Renal Effects — RACP Adult Medicine MCQ

Instant feedback + full explanation. One question, done properly.

ModerateNephrologyTrimethoprim Renal EffectsRACP Adult Medicine

A 75-year-old man with known chronic kidney disease stage 4 develops acute-on-chronic kidney injury after commencing a new medication. His creatinine rises from 200 to 450 µmol/L over 1 week. He was recently started on trimethoprim for a UTI. His potassium is now 6.5 mmol/L. Beyond stopping trimethoprim, what mechanism should be considered?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: C — Trimethoprim causes only cosmetic creatinine elevation

Trimethoprim inhibits tubular creatinine secretion, causing a rise in serum creatinine (typically 10–20%) without a true decrease in GFR – this is a 'pseudonephrotoxicity'. However, in this case the rise is too large to be explained by secretion inhibition alone. True nephrotoxicity can also occur: trimethoprim causes hyperkalaemia (ENaC blockade), and rarely acute interstitial nephritis. In CKD stage 4, the combination of hyperkalaemia and large creatinine rise warrants drug cessation and investigation. The key teaching point is that not all creatinine rises on trimethoprim represent true AKI.

Reference: AMH – 2025 – Trimethoprim; eTG – 2025 – Nephrology