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
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
- Check the balance between the water and the
chemicals dissolved in blood.
- Find out if severe
dehydration or overhydration is
- Check to see if the
hypothalamus is producing enough antidiuretic hormone
- 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
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
- 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
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