Osmolality, Urine

Urine (random or 24-hour)

2 mL

0.2 mL (Note: This volume does not allow for repeat testing.)

Plastic urine container, no preservative

Evaluate concentrating ability of the kidneys (eg, in acute and chronic renal failure); evaluate electrolyte and water balance; used in work-up for renal disease, syndrome of inappropriate antidiuretic hormone secretion (SIADH), and diabetes insipidus; may be used with urinalysis when patient has had radiopaque substances, has glycosuria or proteinuria;¹ evaluate dehydration, amyloidosis. Osmolality is desirable in examination of neonatal urine when protein or glucose are present.²

Freezing point depression

• 24-hour urine: 300−900 mOsm/kg

• Random urine: 50−1400 mOsm/kg

• After 12-hour fluid restriction: >850 mOsm/kg

Osmolality is a better measurement than specific gravity. Osmolality is a measure of renal tubular concentration, depending on the state of hydration.

Simultaneous determination of urine and serum osmolalities facilitates interpretation of results.

High urinary:plasma ratio is seen in concentrated urine. Normal ranges for the U:P ratio are given by Weisberg as approximately 0.2−4.7, and >3.0 with overnight dehydration.¹ With poor concentrating ability the ratio is low but still ≥1.0. In SIADH urine sodium and urine osmolality are high for plasma osmolality.³

Low birthweight infants have been reported to have increased serum osmolality with normal urine osmolality.⁴

The urine osmolar gap is described as the sum of urinary concentrations of sodium, potassium, bicarbonate, chloride, glucose, and urea compared to measured urine osmolality. The gap is normally 80−100 mOsm/kg (SI: 80−100 mmol/kg) H₂O. Determination of the urine osmolal gap is used to characterize metabolic acidosis.