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
This test is done on a blood sample taken from a
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
- 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
- Put the needle into the vein. More than one needle stick
may be needed.
- Attach a tube to the needle to fill it with
- Remove the band from your arm when enough blood is
- 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