Intended for healthcare professionals

Practice Rational Testing

Investigating hypokalaemia

BMJ 2013; 347 doi: (Published 24 September 2013) Cite this as: BMJ 2013;347:f5137
  1. Richard A Oram, specialist registrar and Diabetes UK clinical training fellow14,
  2. Timothy J McDonald, principal clinical scientist and National Institute for Health Research CSO fellow24,
  3. Bijay Vaidya, consultant endocrinologist and honorary associate professor (reader)3
  1. 1Department of Renal Medicine, Royal Devon and Exeter Hospital, Exeter, UK
  2. 2Department of Blood Sciences, Royal Devon and Exeter Hospital, Exeter, UK
  3. 3Department of Endocrinology, Royal Devon and Exeter Hospital and University of Exeter Medical School, Exeter EX2 5DW, UK
  4. 4NIHR Exeter Clinical Research Facility, University of Exeter Medical School, Exeter, UK
  1. Correspondence to: B Vaidya b.vaidya{at}

Learning points

  • The cause of hypokalaemia is usually obvious from the history and is most often diuretic drugs or gastrointestinal loss

  • Severe hypokalaemia (<2.5 mmol/L) is associated with life threatening arrhythmias and should be treated in an acute facility with electrocardiographic monitoring

  • If hypokalaemia is moderate or severe, or its cause unclear, consider checking serum and urine electrolytes, renal function, and acid base balance

  • Refer patients with no obvious explanation for their hypokalaemia for endocrine or renal assessment

A 22 year old woman had a blood test to investigate lethargy at her general practice surgery and was found to have an isolated low serum potassium (2.6 mmol/L; 1 mmol/L=1mEq/L). Her blood pressure was 110/70 mm Hg, and physical examination showed no abnormalities. She was taking no regular drugs and denied taking liquorice or laxatives. She had no recent history of vomiting or diarrhoea.

What is the next investigation?


Hypokalaemia is defined as a serum potassium below 3.5 mmol/L and is commonly graded as mild (3.1-3.5 mmol/L), moderate (2.5-3.0 mmol/L), and severe (<2.5 mmol/L).1 Hypokalaemia is a common finding in the general population; a community based cohort study of 5200 adults over 55 years in the Netherlands showed a 3% prevalence of mild hypokalaemia.2 It is much more common in hospital populations, with one episode of severe hypokalaemia per week in an observational study at a UK secondary care hospital with a catchment population of 150 000.3

About 98% of the body’s potassium is intracellular,4 and the intracellular-extracellular potassium gradient is crucial to maintaining resting membrane potential and normal nerve and muscle function. Small decreases in extracellular potassium can have serious effects on the heart and skeletal muscles. Mild hypokalaemia is often asymptomatic and picked up on routine blood tests, but severe hypokalaemia is associated with life threatening arrhythmias and sudden cardiac death. In a …

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