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 antidiuretic hormone (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

In general, there's nothing you have to do before this test, unless your doctor tells you to.

How It Is Done

A health professional uses a needle to take a blood sample, usually from the arm.

How It Feels

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Risks

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Results

Normal

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

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.