Top of the page
A serum osmolality test measures the amount of chemicals dissolved in the liquid part (serum) of the blood. Chemicals that affect serum osmolality include sodium, chloride, bicarbonate, proteins, and sugar (glucose).
This test is done on a blood sample taken from a vein.
A substance called antidiuretic hormone (ADH) partly controls serum osmolality. Water constantly leaves your body as you breathe, sweat, and urinate. If you do not drink enough water, the concentration of chemicals in your blood (serum osmolality) increases. When serum osmolality increases, your body releases ADH. This keeps water from leaving in the urine, and it increases the amount of water in the blood. The ADH helps restore serum osmolality to normal levels.
If you drink too much water, the concentration of chemicals in your blood decreases. When serum osmolality decreases, your body stops releasing ADH. This increases the amount of water in your urine. It keeps too much water from building up in your body (overhydration).
This test may be done to:
Be sure to tell your doctor about all the medicines you take, including over-the-counter ones. Many medicines can change the results of this test.
Talk to your doctor if you have any concerns about the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form .
The health professional taking a sample of your blood will:
The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.
There is very little chance of a problem from having a blood sample taken from a vein.
A serum osmolality test measures the amount of chemicals in the liquid part (serum) of the blood.
Results are usually available in about 4 hours.
These numbers are just a guide. The range for "normal" varies from lab to lab. Your lab may have a different range. Your lab report should show what range your lab uses for "normal." Also, your doctor will evaluate your results based on your health and other factors. So a number that is outside the normal range here may still be normal for you.
278–300 milliosmoles per kilogram (mOsm/kg) of water (278–300 mmol/kg of water)
High levels may be caused by:
Low levels may be caused by:
You may not be able to have the test, or the results may not be helpful, if:
CitationsFischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.Other Works ConsultedChernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Current as of: March 31, 2020
Author: Healthwise StaffMedical Review: E. Gregory Thompson MD - Internal MedicineAdam Husney MD - Family MedicineKathleen Romito MD - Family MedicineAlan C. Dalkin MD - Endocrinology
Current as of: March 31, 2020
Author: Healthwise Staff
Medical Review:E. Gregory Thompson MD - Internal Medicine & Adam Husney MD - Family Medicine & Kathleen Romito MD - Family Medicine & Alan C. Dalkin MD - Endocrinology
To learn more about Healthwise, visit Healthwise.org.
© 1995-2020 Healthwise, Incorporated. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.