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Serum Osmolality

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 osmolality1
 
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.

What Affects the Test

Results from a serum osmolality test may be affected by:

  • Drinking alcohol right 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 blood osmolality. This can help determine how well the kidneys are working.
  • 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). 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.

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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