Osmolality, Urine

Test ID: 


CPT code:



Urine Osmolality

Clinical Use:

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

Test Information:

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. With poor concentrating ability the ratio is low but still ≥1.0. In SIADH urine sodium and urine osmolality are high for plasma osmolality.
Low birthweight infants have been reported to have increased serum osmolality with normal urine osmolality.
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

Specimen Type:

Urine (random or 24-hour)

Requested Volume: 

2 mL

Minimum Volume: 

0.2 mL

Container Type: 

Plastic urine container, no preservative


Centrifuge urine sample after collection for five minutes at 4°C to 25°C to remove gross particulate matter.

Storage Instructions:


Stability Requirements:



Room temperature

14 days


14 days


14 days

Freeze/thaw cycles

Stable x3


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