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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.

Risks

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. 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.

Serum osmolality 1
 
Normal:

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

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.

WebMD Medical Reference from Healthwise

Last Updated: March 18, 2013
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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