Serum osmolality is a useful preliminary investigation for identifying the cause of hyponatraemia. If a patient with significant hyponatraemia (serum sodium < 130 mmol/L) has a normal plasma osmolality, the patient may have pseudohyponatraemia due to excess lipids or proteins, or the sample may have been collected from a drip arm containing dextrose.
If the patient has an increased osmolality it is likely the patient has reactive hyponatraemia due to an excess of solute pulling water out of cells. Examples of this include glucose in diabetes mellitus or hyperglycinaemia after trans-urethral resection of the prostate. The finding of a hypo-osmolar hyponatraemia ("true hyponatraemia") then leads to further investigation of the cause.
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