Trimethoprim-Induced Hyperkalaemia — RACP Adult Medicine MCQ

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

ModerateNephrologyTrimethoprim-Induced HyperkalaemiaRACP Adult Medicine

A 65-year-old man with stable CKD stage 4 (eGFR 22 mL/min/1.73 m²) on ramipril 10 mg daily has a potassium of 5.8 mmol/L. He has no ECG changes. Which of the following medications could be contributing to his hyperkalaemia?

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

Reveal the answer and explanation

Correct answer: C — Trimethoprim (co-trimoxazole for UTI prophylaxis)

Trimethoprim blocks the epithelial sodium channel (ENaC) in the collecting duct, causing a potassium-sparing effect similar to amiloride. Combined with ACE inhibitor and CKD stage 4, this creates a significant risk of hyperkalaemia. This interaction is under-recognised but clinically important. Other drugs causing hyperkalaemia include NSAIDs, potassium-sparing diuretics, calcineurin inhibitors, and heparin. Trimethoprim should be used cautiously in CKD and avoided if possible when K+ is already elevated.

Reference: AMH – 2025 – Drug Interactions; eTG – 2025 – Nephrology