Urine osmolality is used to measure the number of dissolved particles per unit of water in the urine. As a measure of urine concentration, it is more accurate than specific gravity. Urine osmolality is useful in diagnosing disorders of urinary concentration such as diabetes insipidus and in assessing hydration status. Often, the assessment of any disorder involving antidiuretic hormone (ADH) will require both serum and urine osmolality to assess the concentrating ability of the kidney.
The normal 24-hour urine osmolality is, on average, 500-800 mOsm/kg of water. Random urine osmolality should average 300-900 mOsm/kg of water. After 12-14 hours of fluid intake restriction, the urine osmolality should exceed 850 mOsm/kg of water.