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 keeps water from leaving in the urine and increases the amount of water in the blood. And it 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).

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 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.