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). 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.
Risks
There is very little chance of a problem from
having 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. A warm compress can be used
several times a day to treat this.
- Ongoing bleeding can be a problem for people
with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning
medicines can make bleeding more likely. If you have bleeding or clotting
problems, or if you take blood-thinning medicine, tell your doctor before your
blood sample is taken.
Results
A serum osmolality test measures the
amount of chemicals dissolved in the liquid part (serum) of the blood.
Normal
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what’s normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
Results are usually available in about 4 hours.
High values
High serum osmolality levels may be
caused by:
- Too little water in the body (dehydration).
- High levels of salt or sugar
in the blood, such as poorly controlled
diabetes and diabetic coma.
- Damage to the kidneys, which can cause a buildup of
urea in the blood.
- Poisoning with certain
substances, such as ethanol (the alcohol in alcoholic beverages), rubbing
alcohol (isopropanol), wood alcohol (methanol), or 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 serum osmolality levels may be
caused by:
- Too much water in the body
(overhydration).
- A low level of salt in the blood, which can be caused by some medicines, including diuretics and some medicines used to treat high blood pressure.
- 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 Test
Results from a serum osmolality
test may be affected by:
- Drinking alcohol before the
test.
- Recently receiving a blood transfusion.
What To Think About
- Serum osmolality can be calculated by measuring
the amounts of sodium, glucose, and
blood urea nitrogen (BUN) in the blood.
- The osmolality of urine may be measured and compared to the blood
osmolality to help evaluate how well the kidneys work to remove water and
chemicals from the blood.
- Sometimes antidiuretic hormone (ADH)
increases even though the amount of chemicals in the blood is not high. This is
called syndrome of inappropriate antidiuretic hormone secretion (SIADH). It is sometimes
seen in people who use some types of medicine or who have lung disease, cancer,
or diseases of the brain and spinal cord. Serum osmolality levels can check to
see if SIADH is present.
References
Citations
-
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.
Credits
|
By
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Healthwise Staff |
|
Primary Medical Reviewer
|
E. Gregory Thompson, MD - Internal Medicine |
|
Specialist Medical Reviewer
|
Alan C. Dalkin, MD - Endocrinology |
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Last Revised
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September 30, 2011 |