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Serum Osmolality

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.


There 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.

Bleeding can be a problem for people who have bleeding disorders or take blood-thinning medicines such as aspirin or warfarin (Coumadin). If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.


A serum osmolality test measures the amount of chemicals in the liquid part (serum) of the blood.

Results are usually available in about 4 hours.


These 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.

Serum osmolality 1

278-300 milliosmoles per kilogram (mOsm/kg) of water (278-300 mmol/kg of water)

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.

WebMD Medical Reference from Healthwise

Last Updated: November 14, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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