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Latest Edit: Hector 2014-03-24 (EDT)

See Also Lab Tests

The osmolality test is used to help evaluate the body's water and electrolyte balance. It is also used to investigate hyponatremia (sodium loss) and increased or decreased urine production, to detect the ingestion of toxins such as methanol, to monitor the effectiveness of treatment for conditions affecting osmolality, and to help determine the cause of chronic diarrhea.[1], [2]


  • Osmolality is a measure of the number of dissolved particles in a fluid.
  • The osmolality test measures the concentration of sodium, potassium, chloride, glucose, and urea in a sample of blood, urine, or sometimes stool.
  • Serum osmolality is mainly ordered to investigate hyponatremia that may be due to sodium loss through the urine or a result of increased fluid in the bloodstream
  • Urine osmolality is frequently ordered along with serum osmolality and is used to help evaluate the body's water balance and investigate increased and decreased urine output.
  • Stool osmolality may be ordered to evaluate chronic diarrhea that does not appear to be due to a bacterial or parasitic infection or to another identifiable cause, such as intestinal inflammation or damage.

Patient Preparation

  • No preparation is required.

Clinical Implications

High serum levels indicate:

High urine levels indicate:

Low serum levels indicate:

Low urine levels indicate:

Associated Tests


  1. Pagana Kathleen D, Pagana Timothy J (2006) Mosby's Manual of Diagnostic and Laboratory Tests, Mosby.
  2. Weatherby Dicken, Ferguson Scott (2002) Blood Chemistry and CBC Analysis: Clinical Laboratory Testing from a Functional Perspective, Bear Mountain.