Non-clinical sciences/Lab medicine · Electrolyte interpretation

Hyponatraemia — the volume-status approach

What it means

Assess volume status first. Hypovolaemic: D&V, diuretics, Addison's (urine Na >20 = renal loss). Euvolaemic: SIADH (urine osm >100, urine Na >30, low serum osm), hypothyroidism, primary polydipsia. Hypervolaemic: heart failure, cirrhosis, nephrotic syndrome.

Worked example

Euvolaemic, low serum osmolality, concentrated urine (osm >100) with urine Na >30 and normal cortisol/thyroid → SIADH.

In the exam

A euvolaemic patient on carbamazepine has Na 122, low serum osmolality, urine osmolality 400 and urine sodium 45.

What settles it

SIADH is EUVOLAEMIC with inappropriately concentrated urine; hypovolaemic hyponatraemia has clinical fluid depletion and (if renal loss) urine Na >20.

Classically confused with

Metabolic alkalosis

Source: NICE CKS — Hyponatraemia