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Reference Range

133 – 146 mmol/L

Test Usage

How should I investigate a patient with low serum sodium?

Answer: We recommend:

  • Establish history of fluid intake and current treatments
  • Assess fluid status, to identify whether hypovolaemia or hypervolaemia is present
  • Repeat to confirm and establish whether acute and changing or chronic and stable. Changes of up to 5 mmol/L can reflect non-significant variation

Depending on result:

  • Persistent and stable serum Na 132-135 mmol/L in a clinically well patient may reflect a statistical population outlier and may not require investigation unless large recent fall
  • Serum Na 125-131 mmol/L
  • Check serum potassium, urea, creatinine and plasma glucose.
    • If cause not clinically apparent check
    • Urine Na and osmolality if SIADH suspected
      • If Urine Na > 30 mmol/ L and urine osmolality significantly higher than serum osmolality this suggests SIADH
    • Consider Addison’s disease and hypothyroidism
    • Consider effects of drug therapy
    • Consider reset osmostat syndrome in patients with chronic illness and stable hyponatraemia
    • Consider artefactual causes: Hyperproteinaemia (eg myeloma) or severe hyperlipidaemia
  • Serum Na 115-124 mmol/L
    • Check as above
    • Seek specialist advice unless long term stable and cause established
    • Consider immediate admission if Na falling rapidly or neurological signs or symptoms present
  • Serum Na <115 mmol/L
    • Immediate admission usually indicated

How should I investigate a patient with raised serum sodium?

Answer: Hypernatraemia can be defined as a serum sodium > 145 mmol/L

We recommend:

  • Repeat to confirm and establish whether acute and changing or chronic and stable. Changes of up to 5 mmol/L can reflect non significant variation
  • Establish history of thirst, fluid intake and losses and current treatments
  • Check for clinical features of dehydration and/or hypovolaemia

Depending on result:

Persistent serum Na 146-148 mmol/L without clinical features of hypovolaemia may reflect a statistical population outlier.

Serum Na 149 – 154 mmol/L

  • Request serum potassium, urea, creatinine, calcium, and plasma glucose to evaluate further hydration status and renal function and exclude diabetes mellitus and hypercalcaemia as causes for dehydration
  • Request serum lithium in patients treated with lithium
  • Request urine and serum osmolality if diabetes insipidus suspected (In DI high serum osmolality (>300 mOsm/Kg) and inappropriately dilute urine (less than serum)
  • Consider specialist advice if clinical cause not apparent or oral rehydration is indicated but is not realistically practical.

Serum Na > 155 mmol/L

  • Seek specialist advice or admission in addition to above


Local test

Turn around Time

1 day

Can be added on to an existing request up to 4 days following sample receipt

Specimen Labelling Procedure
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