Serum Osmolality
Test OverviewA 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 DoneThis 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 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 PrepareBe 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 DoneThe 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 FeelsThe 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. RisksThere is very little chance of a problem from
having a 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. You can treat this by using a warm compress several times a day.
ResultsA serum osmolality test measures the
amount of chemicals in the liquid part (serum) of the blood. Results are usually available in about 4 hours. NormalThese numbers are just a guide. The range for "normal" varies from lab to lab. Your lab may have a different range. Your lab report should show what range your lab uses for "normal." Also, your doctor will evaluate your results based on your health and other factors. So a number that is outside the normal range here may still be normal for you. High valuesHigh 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 valuesLow 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.
What Affects the TestYou may not be able to have the test, or the results may not be helpful, if: - You drink alcohol right before the
test.
- You have recently had a blood transfusion.
What To Think About- The concentration of chemicals in your urine (urine osmolality) may be measured and compared to your serum osmolality. This can help your doctor see how well the kidneys are working and find what's causing an imbalance of water and
electrolytes in your body.
- Sometimes the level of antidiuretic hormone (ADH)
increases even though the amount of chemicals in the blood is normal. This is
called syndrome of inappropriate antidiuretic hormone secretion (SIADH). Serum osmolality can check for SIADH.
ReferencesCitations- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Other Works Consulted- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
CreditsByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerAlan C. Dalkin, MD - Endocrinology Current as of:
May 3, 2017 Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins. Last modified on: 8 September 2017
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