Test/Range/Collection

Osmolality , urine (Urine Osm)

Random: 100–900 mosm/kg H₂O [mmol/kg H₂O]

Urine container

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Physiologic Basis

Test measures renal tubular concentrating ability.

Urine osmolality and specific gravity usually change in parallel with each other. When large molecules such as glucose and protein are present, however, the results diverge. Specific gravity is increased more, due to the weight of the molecules, whereas urine osmolality is increased less, reflecting the number of molecules.

Interpretation

Increased in: Hypovolemia, syndrome of inappropriate ADH secretion (SIADH). Drugs: anesthetic agents (during surgery), carbamazepine, chlorpropamide, cyclophosphamide, metolazone, vincristine.

Decreased in: Diabetes insipidus, primary polydipsia, exercise, starvation. Drugs: acetohexamide, demeclocycline, glyburide, lithium, tolazamide.

Comments

In the hypoosmolar state (serum osmolality < 280 mosm/kg), urine osmolality is used to determine whether water excretion is normal or impaired. A urine osmolality value of < 100 mosm/kg indicates complete and appropriate suppression of antidiuretic hormone secretion.

With average fluid intake, normal random urine osmolality is 100–900 mosm/kg H₂O.

After 12-hour fluid restriction, normal random urine osmolality is > 850 mosm/kg H₂O.

Capatina C et al. Diabetes insipidus after traumatic brain injury. J Clin Med 2015;4:1448.  [PMID: 26239685]

Oh JY et al. Syndrome of inappropriate antidiuretic hormone secretion and cerebral/renal salt wasting syndrome: similarities and differences. Front Pediatr 2015;2:146.  [PMID: 25657991]

Pasquel FJ et al. Hyperosmolar hyperglycemic state: a historic review of the clinical presentation, diagnosis, and treatment. Diabetes Care 2014;37:3124.  [PMID: 25342831]