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Serum Osmolality - Forrest Health
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    Serum Osmolality

    Test Overview

    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).

    Why It Is Done

    This test may be done to:

    • Check the balance between the water and the chemicals in your blood.
    • Find out if you have severe dehydration or overhydration.
    • Check to see if your body is making enough ADH.
    • Find the cause of seizures or coma. In severe cases, these can be caused by an imbalance between water and electrolytes in the body.
    • Find out if you have swallowed a poison, such as rubbing alcohol, wood alcohol, or antifreeze.

    How To Prepare

    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( What is a PDF document? ).

    How It Is Done

    The health professional taking a sample of your blood will:

    • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
    • Clean the needle site with alcohol.
    • Put the needle into the vein. More than one needle stick may be needed.
    • Attach a tube to the needle to fill it with blood.
    • Remove the band from your arm when enough blood is collected.
    • Put a gauze pad or cotton ball over the needle site as the needle is removed.
    • Put pressure on the site and then put on a bandage.

    How It Feels

    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.

    • You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
    • In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. You can treat this by using a warm compress several times a day.


    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.

    Serum osmolalityfootnote 1


    278–300 milliosmoles per kilogram (mOsm/kg) of water (278–300 mmol/kg of water)

    High values

    High levels may be caused by:

    • Too little water in the body (dehydration).
    • High levels of salt or sugar in the blood. This may be caused by problems such as poorly controlled diabetes.
    • Damage to the kidneys. This can cause a buildup of urea in the blood.
    • Poisoning with certain substances. These include ethanol (the alcohol in alcoholic drinks), rubbing alcohol (isopropanol), wood alcohol (methanol), and antifreeze (ethylene glycol).
    • A rare disease, such as diabetes insipidus, that causes the kidneys to lose water and produce large amounts of urine.

    Low values

    Low levels may be caused by:

    • Too much water in the body.
    • A low level of salt in the blood. This can be caused by some medicines, including diuretics and certain blood pressure medicines.
    • A condition called syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH sometimes occurs with lung disease, cancer, diseases of the central nervous system, or the use of certain medicines.

    What Affects the Test

    You may not be able to have the test, or the results may not be helpful, if:

    • You drink alcohol right before the test.
    • You have recently had a blood transfusion.

    What To Think About

    • The concentration of chemicals in your urine (urine osmolality) may be measured and compared to your serum osmolality. This can help your doctor see how well the kidneys are working and find what's causing an imbalance of water and electrolytes in your body.
    • Sometimes the level of antidiuretic hormone (ADH) increases even though the amount of chemicals in the blood is normal. This is called syndrome of inappropriate antidiuretic hormone secretion (SIADH). Serum osmolality can check for SIADH.



    1. Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

    Other Works Consulted

    • Chernecky 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: November 6, 2018

    Author: Healthwise Staff
    Medical Review: E. Gregory Thompson, MD - Internal Medicine
    Kathleen Romito, MD - Family Medicine
    Alan C. Dalkin, MD - Endocrinology and Metabolism



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