Deceptive potassium and magnesium measurements

Potassium and magnesium are important circulating cations and are predominantly intracellular elements. Only a small fraction of these elements is present in extracellular fluids including blood (∼1%). Measurement of the concentration of such small fractions in blood is commonly used to assess and reflect their body content levels. However, some of these measurements can be flawed and a failure to recognise the limitations of these tests may result in misdiagnosis and/or unnecessary follow-up investigations and/or expensive hospital admissions. The focus of this note is three-fold (a) to highlight and discuss separately the less appreciated pitfalls of potassium and magnesium measurements per se, (b) suggestions to identify and rectify these snags and to improve their clinical interpretation, and finally