Sodium (Na⁺)

What it measures

Sodium is the main cation of extracellular fluid and largely controls water distribution and plasma osmolality. It is measured to assess fluid balance, confusion, seizures and many drug effects.

Specimen and preparation

• Specimen: Serum or lithium-heparin plasma
• Tube: Gold top
• Preparation: Do not collect from an arm with an IV infusion running. Severe lipaemia or high protein can cause pseudohyponatraemia with indirect ISE.

Reference range

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

Causes of high values

• Dehydration and inadequate water intake
• Diabetes insipidus
• Hypertonic saline or sodium bicarbonate
• Hyperaldosteronism and Cushing syndrome
• Osmotic diuresis

Causes of low values

• SIADH
• Diuretics (especially thiazides)
• Heart failure, cirrhosis, nephrotic syndrome
• Adrenal insufficiency and hypothyroidism
• Excess water intake

Method

Ion-selective electrode (direct or indirect)

Good to know

Critical values are commonly <120 or >160 mmol/L. Correct chronic hyponatraemia slowly to avoid osmotic demyelination.

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