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). A serum osmolality test is done on a blood sample taken from a vein.

Serum osmolality is controlled partly by a hormone called antidiuretic hormone (ADH). 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, ADH is released. This increases the amount of water in the blood and helps restore serum osmolality to normal levels.

When you drink too much water, serum osmolality decreases. When serum osmolality decreases, ADH is suppressed. This increases the amount of water in your urine and prevents too much water from building up in your body (overhydration).

Why It Is Done

Serum osmolality is measured to:

• Check the balance between the water and the chemicals dissolved in blood.
• Find out if severe dehydration or overhydration is present.
• Check to see if the hypothalamus is producing enough antidiuretic hormone (ADH).
• Find the cause of seizures or coma. In severe cases, an imbalance between water and electrolytes in the body can cause seizures or coma.
• Find out if a person has swallowed certain poisons, such as rubbing alcohol (isopropanol), wood alcohol (methanol), or antifreeze (ethylene glycol). Serum osmolality can also help find out how much of these poisons are present in the blood.

How To Prepare

Many medicines may change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will indicate. 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.

Risks

There is very little chance of a problem from having 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. A warm compress can be used several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

Results

A serum osmolality test measures the amount of chemicals dissolved in the liquid part (serum) of the blood.

Normal

Normal values may vary from lab to lab. Results are usually available in about 4 hours.

High values

High serum osmolality levels may be caused by:

• Too little water in the body (dehydration).
• High levels of salt or sugar in the blood, such as poorly controlled diabetes and diabetic coma.
• A buildup of urea in the blood.
• Poisoning with certain substances, such as ethanol (the alcohol in alcoholic beverages), rubbing alcohol (isopropanol), wood alcohol (methanol), or antifreeze (ethylene glycol).
• A rare disease, such as diabetes insipidus, that causes the kidneys to produce large amounts of urine.

Low values

Low serum osmolality levels may be caused by:

• Too much water in the body (overhydration).
• A low level of salt in the blood.
• Overproduction of antidiuretic hormone (ADH) that may occur with some diseases, such as lung cancer.

What Affects the Test

Results from a serum osmolality test may be affected by:

• Drinking alcohol before the test.
• Recently receiving a blood transfusion.

What To Think About

• Serum osmolality can be calculated by measuring the amounts of sodium, glucose, and blood urea nitrogen (BUN) in the blood.
• The osmolality of urine may be measured and compared to the blood osmolality to help evaluate how well the kidneys work to remove water and chemicals from the blood.
• Sometimes antidiuretic hormone (ADH) increases even though the amount of chemicals in the blood is not high. This is called inappropriate antidiuretic hormone secretion (SIADH). It is sometimes seen in people who use some types of medicine or who have lung disease, cancer, or diseases of the brain and spinal cord. Serum osmolality levels can check to see if SIADH is present.

References

Other Works Consulted

• Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2002). Mosby's Manual of Diagnostic and Laboratory Tests, 2nd ed. St. Louis: Mosby.

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