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📋 Reference sheet · Kidney, electrolytes and bone

Potassium out of range: high or low

Potassium out of range. The number that means today, and the one that means a repeat.

One page, every threshold with its number and unit. Written and reviewed by practising UK GPs. Last reviewed 29 Jul 2026.

Potassium out of range: high or low A primary care flowchart for a serum potassium outside the reference range in an adult: the reference ranges and severity bands with units, the levels that mean same-day emergency assessment, the artefact check on an unexpected result, then three bands each ending in an action a GP takes today, the shared drug review, the potassium numbers that govern starting and stopping an ACE inhibitor or an angiotensin receptor blocker, and two teaching points. Potassium out of range: how urgent, and is it real? What normal looks like in an adult Potassium 3.5 to 5.3 mmol/L, some laboratories 3.5 to 5.0 High 5.5 to 5.9 mild · 6.0 to 6.4 moderate · ≥ 6.5 severe Low 3.1 to 3.4 mild · 2.6 to 3.0 moderate · ≤ 2.5 severe Magnesium 0.7 to 1.0 mmol/L All values serum. Same day, whatever else is going on · K ≥ 6.5 mmol/L, symptoms or none · K 6.0 to 6.4 mmol/L with AKI* or acutely unwell · K ≤ 2.5 mmol/L · Any level with ECG change, weakness or syncope Yes Admit for immediate assessment No Not an emergency? Then ask whether the number is real · Read the laboratory's haemolysis comment first · Long transit to the laboratory, or a cold sample · Difficult bleed, clenched fist, wrong order of draw · Repeat an unexpected result before acting on it Confirmed on repeat, and clinically well: which band? Mild high K 5.5 to 5.9 mmol/L Repeat within 3 days Check kidney function Review the drug list Moderate high K 6.0 to 6.4 mmol/L ECG today Repeat within 1 day Unwell or AKI*: admit Low K 2.6 to 3.4 mmol/L ECG if K ≤ 3.0 mmol/L Replace potassium orally Recheck in 2 to 3 days The drug review, both directions · Stop potassium supplements and salt substitutes · High: NSAID*, trimethoprim, ACEi*, ARB*, spironolactone · Low: loop and thiazide diuretics, laxatives · Recheck potassium 1 to 2 weeks after any dose change Starting or stopping an ACE inhibitor or ARB* · Potassium and kidney function before you start one · Recheck 1 to 2 weeks after starting and each dose rise · Do not routinely start if potassium > 5.0 mmol/L · Stop if potassium ≥ 6.0 mmol/L, other drivers stopped Two that catch people out · A haemolysed sample is the commonest high potassium · Low potassium will not correct until magnesium does * AKI: acute kidney injury · ACEi: angiotensin converting enzyme inhibitor * ARB: angiotensin receptor blocker · NSAID: non-steroidal anti-inflammatory drug Adult thresholds. Education, not advice, and not about any individual patient. GPAtlas · UKKA hyperkalaemia · CKS hypokalaemia · NG203

Related Ocean Article: Hyperkalaemia · also Acute Kidney Injury (AKI), Hypokalaemia. Part of the GP reference sheets, result interpretation one page per test or finding.

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