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Trimethoprim hyperkalaemia risk — GPhC CRA MCQ

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HardInfection TherapeuticsTrimethoprim hyperkalaemia riskGPhC CRA

A 55-year-old man with type 2 diabetes and an eGFR of 25 mL/min/1.73m² presents a prescription for trimethoprim 200 mg twice daily for a lower urinary tract infection. He is also taking ramipril and spironolactone. What is the most important concern?

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Correct answer: D — Trimethoprim may cause life-threatening hyperkalaemia in this patient

Trimethoprim blocks the epithelial sodium channel in the distal nephron, causing potassium retention similar to amiloride. In a patient already taking an ACE inhibitor (ramipril) and a potassium-sparing diuretic (spironolactone) with significant renal impairment (eGFR 25), the risk of life-threatening hyperkalaemia is very high. The prescriber should be contacted to discuss alternatives such as nitrofurantoin (if eGFR >45) or a different antibiotic.

Reference: BNF; NICE CKS UTI