“You’ve rung me at work — is it that serious?”

A potassium of 6.6 mmol/L is severe hyperkalaemia and a risk of life-threatening arrhythmia — and the patient feeling completely well does NOT make it safe or deferrable; hyperkalaemia is frequently silent until it causes a fatal rhythm. The action is urgent same-day hospital assessment (UK Kidney Association 2023: potassium 6.5 mmol/L or more in the community): an ECG (peaked T waves, widened QRS, etc.), an urgent repeat potassium/renal function and emergency treatment as needed — not a practice ECG first. The examinable clinical skill is finding the stacked, partly-hidden drivers: a recently increased ACE inhibitor and spironolactone (hold per plan), plus the contributors the record misses and the patient won’t volunteer — OTC NSAIDs (ibuprofen for his knee) and a potassium-based "low-salt" substitute (both stopped). The hidden agenda is deferral driven by stoicism and hospital fear (a brother who "never came home"), and the belief that OTC items "don’t count" as medicines. Convey urgency without panic, take an active OTC/salt-substitute history, give explicit stop/hold instructions and dietary advice, arrange same-day ECG/assessment, and safety-net cardiac symptoms to 999. He "feels fine" — hyperkalaemia is often asymptomatic until a fatal arrhythmia; muscle weakness/palpitations may or may not be present. Reassuring symptoms do NOT make a potassium of 6.6 safe. The challenge is conveying urgency to a well-feeling man who wants to defer.