Assaying for osmolality is indicated in the investigation of: acid base disorders; diabetes insipidus (e.g. post head injury); syndrome of inappropriate antidiuretic hormone (SIADH); osmolar gap (caused by an exogenous osmolar substances such as ethanol or salicylate).
Serum and urine osmolality should both be measured when performing a water deprivation test, assessing causes of hyponatraemia and when investigating diabetes insipidus to enable appropriate interpretation.
