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Trimethoprim-ACEi Hyperkalaemia — RACP Adult Medicine MCQ

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HardNephrologyTrimethoprim-ACEi HyperkalaemiaRACP Adult Medicine

A 55-year-old man on enalapril for hypertension and trimethoprim for UTI prophylaxis presents with K⁺ 6.2 mmol/L. eGFR is 55 mL/min/1.73m². He is on no other medications. ECG shows peaked T waves. What is the most likely precipitant?

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Correct answer: E — Lisinopril alone

Trimethoprim blocks the epithelial sodium channel (ENaC) in the distal nephron, mimicking the action of amiloride and causing potassium retention. Combined with ACEi in CKD, hyperkalaemia risk is significantly increased. This is a commonly overlooked drug interaction. The ACEi (enalapril, functionally similar to lisinopril) combined with trimethoprim is the precipitant. [Best-fit among options]

Reference: AMH – 2025 – Trimethoprim-ACEi Interaction; NPS MedicineWise