Animal Health Diagnostic Center

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Clinical Pathology

Urine Osmolality

Urine osmolality is measured by freezing point depression in our laboratory. Osmolality is affected by the number of osmotically active particles in solution and is unaffected by their molecular weight and size. For this reason, osmolality is superior to specific gravity, which is affected by particle weight and size. Isosthenuric urine has an osmolality similar to plasma, approximately 300 to 320 mOsm/kg. Urine osmolality is useful for evaluating urine concentrating ability, for example in water deprivation tests, and is more accurate than measurement of urine specific gravity in this regard.