Expansion
Check osmolality, then volume status, then urine sodium
Expansion
Step 1: plasma osmolality
- Normal: pseudohyponatraemia (hyperlipidaemia, paraproteinaemia)
- High: hyperglycaemia or mannitol drawing water out of cells
- Low: true hypotonic hyponatraemia, so continue
Step 2: volume status
- Hypovolaemic: vomiting, diarrhoea, diuretics, third spacing, adrenal insufficiency
- Euvolaemic: SIADH, hypothyroidism, glucocorticoid deficiency, primary polydipsia
- Hypervolaemic: heart failure, cirrhosis, nephrotic syndrome, renal failure
Step 3: urine sodium and osmolality
- Urine sodium under 20: appropriate renal conservation, so a non-renal loss or an oedematous state
- Urine sodium over 30 with concentrated urine in a euvolaemic patient: SIADH
- Urine osmolality under 100: primary polydipsia or low solute intake
Correction must be slow, no more than 8 to 10 mmol/l in 24 hours, to avoid osmotic demyelination, and slower still in chronic hyponatraemia, alcoholism, malnutrition and liver disease.