Clinical Biochemistry

Osmolality

Test Background
Measurement of osmolality aids interpretation of electrolyte disturbance in the context of SIADH (especially following head injury/surgery), diabetes insipidus, confirmation of laboratory-derived electrolyte findings, and diagnosis of alcohol/ethylene glycol abuse via determination of the osmolar gap. Measurement of serum osmolality is best performed in conjunction with that of urine osmolality to assess the appropriateness of the renal response.

Clinical Indications
Differential diagnosis of hyponatraemia, eg. cerebral salt wasting vs. SIADH (via performing water loading test) Diagnosis of diabetes insipidus (via performing water deprivation test) Suspected alcohol/ethylene glycol ingestion (via calculation of osmolar gap)

Reference Range

Sample Required
SST (gold top) / Red Top and/or random urine

Sample Volume
0.5 mL

Turnaround Time
1 day