Interpretation in primary care: Hypokalaemia is serum potassium below 3.5 mmol/L; classify it as mild (3.0–3.4 mmol/L), moderate (2.5–2.9 mmol/L), or severe (below 2.5 mmol/L). NICE CKS Biochemical severity may not reflect clinical severity, which depends on symptoms and ECG abnormalities. NICE CKS Although poor intake, fasting, hypocaloric or unbalanced diets, gastrointestinal illness, eating disorders, malnutrition, and alcohol misuse can contribute, hypokalaemia is more commonly due to potassium loss or redistribution and is rarely due to low dietary intake alone. NICE CKS Causes may coexist, so assess diet alongside medicines, especially thiazide or loop diuretics, beta-agonists, insulin and corticosteroids, vomiting or diarrhoea, hypomagnesaemia, acid–base disturbance, endocrine causes, and cardiac, renal or hepatic comorbidity. NICE CKS
Immediate assessment and escalation: Take a detailed history including medication, dietary and alcohol history, examine blood pressure and volume status, and assess for weakness, gastrointestinal symptoms, dehydration, malnutrition and features of thyrotoxicosis. NICE CKS Admit immediately for potassium below 2.5 mmol/L, symptoms such as dysrhythmia, paralysis, respiratory failure or severe weakness, or signs of hypovolaemia, major acid–base disturbance, hyperosmolar hyperglycaemic state, diabetic ketoacidosis, or severe or symptomatic hypomagnesaemia. NICE CKS Discuss specialist referral for asymptomatic moderate hypokalaemia, an uncertain cause, or coexisting cardiac, renal or hepatic disease that increases complication risk. NICE CKS
Initial follow-up tests: Repeat serum potassium at a clinically appropriate interval to exclude a spurious result and detect a rapidly falling concentration. NICE CKS Check urea and electrolytes including potassium, sodium, chloride and magnesium, plus glucose, creatinine and bicarbonate. NICE CKS Obtain an ECG when appropriate to detect changes or arrhythmias associated with hypokalaemia. NICE CKS Identify and correct hypomagnesaemia, as potassium may be difficult to correct until magnesium is corrected. NICE CKS
Tests when the cause remains unclear: Consider calcium and/or phosphate, venous blood gas, digoxin concentration when relevant, aldosterone and renin, thyroid-stimulating hormone, and urinary potassium, chloride, creatinine and sodium to help distinguish renal from non-renal potassium loss. NICE CKS
Management and repeat testing by likely cause: Mild or moderate hypokalaemia not requiring referral can usually be managed in primary care by treating the suspected cause, stopping or changing a causative medicine where appropriate, considering oral potassium replacement, and giving oral magnesium if magnesium is low. NICE CKS If poor diet is the likely cause, promote increased dietary potassium intake and recheck serum potassium after 2 weeks, or sooner if clinical judgement indicates. NICE CKS If ongoing vomiting, diarrhoea, medication-related loss, alcohol misuse, malnutrition, or an acid–base or endocrine disorder is suspected, address that cause and arrange earlier reassessment where clinical risk or potassium trajectory warrants it. NICE CKS Arrange further investigation or referral if potassium does not correct, falls, symptoms develop, or the underlying cause remains uncertain. NICE CKS