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Acute lower urinary tract infection in a patient at high risk of trimethoprim-associated hyperkalaemia — MSRA

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Hardall topics relevant for this examAcute lower urinary tract infection in a patient at high risk of trimethoprim-associated hyperkalaemiaMSRA

A 79-year-old woman presents with 2 days of dysuria, urinary frequency and suprapubic discomfort. She has no fever, rigors, flank pain, nausea, vaginal symptoms or haematuria. She is not catheterised and is haemodynamically stable. She requests immediate treatment because symptoms are preventing sleep. She has HFrEF and CKD G3b. Her regular medicines are ramipril 10 mg once daily, spironolactone 25 mg once daily, bisoprolol and furosemide. Her most recent eGFR, measured 8 days ago, was 42 mL/min/1.73 m² and potassium was 4.8 mmol/L. She has no drug allergies. A urine culture has been sent. A culture during a similar episode 4 months ago grew Escherichia coli susceptible to nitrofurantoin, trimethoprim and pivmecillinam, but resistant to amoxicillin and fosfomycin. She has not received antibiotics within the past 3 months. Which is the most appropriate immediate antimicrobial prescription?

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Correct answer: C — Pivmecillinam 400 mg as an initial dose, then 200 mg three times daily for a total of 3 days

This is a lower UTI without features of pyelonephritis or sepsis, so oral treatment is appropriate. Pivmecillinam is the best option because the usual first-line agents are unsuitable in this specific patient. Nitrofurantoin is generally recommended only when eGFR is at least 45 mL/minute. Although short-course treatment may exceptionally be considered at eGFR 30–44 mL/minute for suspected or proven multidrug-resistant lower UTI, that exception does not apply here: there is a susceptible alternative. Trimethoprim would usually be attractive because the previous isolate was susceptible and she has not used it recently. However, she has several converging risk factors for clinically important hyperkalaemia: CKD, ramipril and spironolactone. The trimethoprim SmPC specifically identifies renal insufficiency, renin–angiotensin-system inhibitors and potassium-sparing diuretics as risk factors; monitoring does not make it the preferred choice when an effective alternative is available. Fosfomycin is a NICE second-choice option when first-choice treatment is unsuitable, but the previous isolate was resistant. Amoxicillin should be used for lower UTI only when current culture confirms susceptibility, and this patient’s recent isolate was resistant. Pivmecillinam therefore provides guideline-concordant treatment while avoiding both reduced nitrofurantoin suitability and avoidable hyperkalaemia risk.

Reference: NICE NG109: Urinary tract infection (lower): antimicrobial prescribing — Recommendations (2018) — https://www.nice.org.uk/guidance/ng109/chapter/Recommendations Trimethoprim 50 mg/5 ml Suspension — Summary of Product Characteristics (Revised 2 June 2026) — https://www.medicines.org.uk/emc/product/4566/smpc Nitrofurantoin 100 mg Tablets — Summary of Product Characteristics (2024) — https://www.medicines.org.uk/emc/product/3602/smpc