Osmolality
Concentration of all dissolved particles in blood, urine or stool, a measure of fluid balance
Also known as
Definition
Osmolality describes how many dissolved particles a body fluid contains, mainly sodium, chloride, potassium, glucose and urea. It is measured in blood, urine or stool. When it rises in the blood, the body releases antidiuretic hormone (ADH), the kidneys hold on to water and the urine becomes more concentrated; when it falls, more dilute urine is passed. The test thus shows how fluid balance is regulated and how the kidneys are working.
| Parameter | Value |
|---|---|
| Unit | mOsm/kg (mmol/kg) |
| Reference Range | Blood (serum): typically 275–295 mOsm/kg in adults. Urine, random sample: 50–1200 mOsm/kg; after 12 to 14 hours without fluids, above 850 mOsm/kg. Labs use different methods and samples and therefore differ; the range printed on your report is what counts, and your doctor interprets the result. |
Agree on your personal target range with your doctor.
1 mOsm/kg equals 1 mmol/kg; both notations describe the same value.
↓ What a low value means
Low blood osmolality can be linked to drinking a great deal of fluid (overhydration), a low sodium level, excess ADH release (SIADH, for example with certain lung tumours or due to medicines), an adrenal gland disorder or an underactive thyroid. Low urine osmolality can be linked to high fluid intake, diabetes insipidus, kidney failure, damage to the kidney tubules or a severe kidney infection (pyelonephritis). Discuss deviations with your doctor.
↑ What a high value means
High blood osmolality can be linked to fluid loss (dehydration), a high sodium or blood sugar level, a build-up of nitrogen waste products (uremia), diabetes insipidus or reduced ADH release after a stroke or head injury. Poisoning, for example with methanol, ethylene glycol (antifreeze) or alcohol, can also change the value. High urine osmolality can be linked to fluid loss, heart failure, SIADH, adrenal insufficiency or glucose in the urine, among other things. Discuss deviations with your doctor.
When to test
With signs of a fluid imbalance such as excessive thirst, nausea, headache, confusion or fatigue, when you pass unusually much or little urine, or when diabetes insipidus or poisoning is suspected. Depending on the test, a six-hour fast or 12 to 14 hours without fluids may be required. Tell your practice about your medicines, for example water pills (diuretics), but do not stop them on your own.
Frequently asked questions
Why are blood and urine often tested together? +
Blood osmolality shows the concentration of dissolved particles in the body; urine osmolality shows how strongly the kidneys concentrate or dilute the urine. Together, both values help to assess fluid balance and to find the reasons for passing more or less urine.
What does ADH have to do with osmolality? +
When blood osmolality rises, the body releases antidiuretic hormone (ADH). It makes the kidneys hold on to water, the blood is diluted and the value falls again. When osmolality is low, less ADH is released and more water is passed.
What is stool osmolality measured for? +
Mainly for chronic diarrhea that cannot be explained by a bacterial or parasitic infection. An abnormal value can be linked to overuse of laxatives or to poor absorption of nutrients in the gut (malabsorption).
Which values are often measured along with osmolality? +
Electrolytes such as sodium, potassium and chloride, blood sugar, urea, albumin and a blood count often complement the measurement. Your doctor takes your symptoms, history and these values into account when interpreting the result.
Osmolality
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.