Urine Osmolality

General Information

Determination of plasma and urine osmolality are used in the assessment of electrolyte and acid-base disorders. Comparison of plasma and urine osmolalities determines the appropriateness and status of water regulation by the kidneys in settings of severe electrolyte disturbances, as might occur in diabetes insipidus or the syndrome of inappropriate secretion of antidiuretic hormone (SIADH).

For the differential diagnosis of hypo and hypernatraemia, urine electrolytes should be requested along with urine osmolality.