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Osmolality test

Also called: Serum osmolality, Urine osmolality

Osmolality measures how concentrated your blood or urine is. It helps explain low sodium, excess thirst, and some poisonings.

SampleBlood and/or urine
Fasting?Usually not needed
ResultsOften the same day
Good to knowOne result rarely tells the whole story. Your provider looks at your results together with your symptoms, exam, and other tests.

What the test is

Blood and urine tests are often done together.

Why it's done

  • Low or high sodium
  • Peeing very large amounts
  • Possible methanol or antifreeze poisoning

Getting ready and what happens

  • Follow any fluid instructions from your provider.

How the test is done

  • A health worker cleans the skin, puts a band around your arm, and draws blood from a vein with a small needle. It takes a few minutes.
  • A urine sample may also be collected.

What the results mean

ResultTypical range for adults
Blood osmolalityAbout 275 to 295 mOsm/kg

Normal ranges vary from lab to lab and can differ by age, sex, and pregnancy. Your report lists your lab's own range next to each result. A result a little outside the range is often not a problem.

Results

Help sort out causes of abnormal sodium, such as too much water, SIADH, or diabetes insipidus.

Questions to ask your provider

  • What do my results mean for me?
  • Do I need to repeat this test or have other tests?

Risks

A blood draw is very safe. You may have slight pain or a small bruise where the needle went in.

Related conditions

Related tests

Sources

Last updated 2026-11-13.