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Potassium (K⁺)

Electrolytes · also called K, Serum potassium

What it measures

Potassium is the main intracellular cation and is vital for heart rhythm and nerve and muscle function. It is monitored in kidney disease, with diuretics, ACE inhibitors, and in any acutely ill patient.

Specimen and preparation

Specimen: Serum or lithium-heparin plasma
Tube: Gold top
Preparation: Avoid haemolysis, fist clenching, prolonged tourniquet and delayed separation. Never draw after an EDTA tube (K2EDTA contamination).

Reference range

Group	Range
Adult (serum)	3.5–5.1 mmol/L
Adult (plasma)	3.4–4.8 mmol/L
Newborn	3.7–5.9 mmol/L

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

Kidney failure
ACE inhibitors, ARBs, spironolactone
Acidosis and diabetic ketoacidosis
Tissue breakdown (rhabdomyolysis, tumour lysis)
Addison disease
Haemolysed or delayed sample (spurious)

Causes of low values

Diuretics
Vomiting and diarrhoea
Insulin or beta-agonist therapy
Alkalosis
Hyperaldosteronism or hypomagnesaemia

Method

Ion-selective electrode

Good to know

Critical values are usually <2.8 or >6.2 mmol/L. Check for haemolysis and EDTA contamination (very low calcium) before reporting a high result.

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