Urine Osmolality
How concentrated your urine is, counted as dissolved particles per kilogram of water. It mostly reflects how much you have had to drink and how your kidneys and hormones balance water.
AI-verified against cited sources. Awaiting expert reviewAlso called Uosm, U OsmLOINC 2695-5
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What it is
Urine osmolality counts the dissolved particles in urine, such as salts, urea and sugar. When you are short on fluid, your kidneys hold on to water and make concentrated urine, so the number is high. After you drink a lot, they release water, the urine is dilute and the number is low. Labs usually measure it with a freezing-point osmometer. It is often read together with a blood osmolality or blood sodium result.
Why it matters
Urine osmolality shows how well your kidneys and the water-control hormone vasopressin (also called ADH) are working for the amount of fluid in your body. Clinicians use it to look into a low or high blood sodium, passing a lot of urine, or signs of dehydration. One random result mostly reflects the last few hours of drinking, so it is read with blood tests and your symptoms.
If it’s low
A low result means dilute urine. Most often you simply drank a lot of fluid. Dilute urine together with passing a large amount of urine can lead your clinician to look at vasopressin or at how the kidneys respond to it. Dilute urine can also be the body's normal way of getting rid of extra water.
Common causes
- Drinking a lot of fluid, which is very common
- Primary polydipsia, which means drinking very large amounts of water
- Diabetes insipidus, where the body lacks vasopressin or the kidneys do not respond to it
- Kidney conditions that limit the ability to concentrate urine
- A normal response to a low blood sodium caused by too much water, when the body is trying to get rid of extra water
What can help
- Keep a simple record of how much you drink and how often you pass urine, and share it with your clinician
- Ask whether a blood sodium and blood osmolality should be checked at the same time
If it’s high
A high result means concentrated urine. Most often this is normal and reflects less to drink, sweating or a first-morning sample. Sugar or other dissolved substances in the urine, and hormone effects that make the body hold on to water, can also push it up. Your clinician reads it with your blood sodium and osmolality.
Common causes
- Not drinking enough, or losing fluid through sweat, fever, vomiting or diarrhea
- A first-morning sample, which is usually the most concentrated
- Sugar in the urine, as with high blood sugar, or other dissolved substances such as mannitol
- Hormone-driven water retention, such as SIADH (too much vasopressin activity)
- Heart failure or liver disease, which can make the body hold on to extra water (MedlinePlus lists heart, liver and kidney disease among the reasons for an abnormal result)
What can help
- Ask your clinician how much fluid is right for you, especially if you have a heart, kidney or liver condition
- If you were thirsty or had little to drink before the test, ask whether a repeat after normal drinking would be more informative
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
The usual range depends on age. Choose an age to see the one that applies.
- 17+50–1,400 mOsm/kg
Source: Labcorp
Children and teens 2 ranges
- under 0.083350–645 mOsm/kg
Source: ARUP Laboratories
- 0.0833–1650–1,500 mOsm/kg
Source: ARUP Laboratories
See Urine Osmolality by age and sex, with the U.S. survey numbers where they exist.
What can affect your result
- Fluid intake in the hours before the test
- Time of day: first-morning urine is usually the most concentrated
- Sweating, fever, vomiting or diarrhea
- Sugar or other dissolved substances in the urine
- Medicines that affect water balance, such as desmopressin
- Type of sample: random, 24-hour or after limiting fluids, each with a different expected range
- Age: ARUP prints different intervals for newborns, children and adults
- How much you have had to drink changes this result a lot, which is why labs print a very wide range.
- Sugar (glucose), salt, urea and osmotic medicines such as mannitol add dissolved particles to urine and push the number up (a solute diuresis). Desmopressin raises it too.
- Some providers ask you to fast for about 6 hours or to limit fluids for 12 to 14 hours before the test (MedlinePlus). Follow the instructions you were given.
- Sample handling differs by lab. ARUP does not accept urine collected with preservatives and ships it frozen, while Mayo's 24-hour test accepts refrigerated or diazolidinyl urea-preserved collections.
A random (any time) urine sample for the usual test. A first-morning sample is usually the most concentrated, and a sample after drinking a lot is the most dilute. Some orders specify a 24-hour collection or a sample after limiting fluids. Hours. It follows how much you drink, sweat and urinate on the day of the test.
When to talk to a clinician
A urine osmolality result is rarely an emergency on its own. Contact a clinician promptly if you have intense thirst with passing very large amounts of urine, or signs of dehydration such as dizziness, very dark urine or passing little urine. Low blood sodium can cause headache, confusion or seizures. If someone is confused, has a seizure or faints, call 911 or your local emergency number.
Questions you could ask
- Was this a random sample, a 24-hour collection or a sample after limiting fluids?
- Does this result fit with how much I have been drinking?
- Should we compare it with my blood sodium and blood osmolality?
- Could how much I drink explain this, or should we look further?
- Is there a test that would help sort this out?
Questions people ask
Why do labs print such a wide range for urine osmolality?
Healthy kidneys can make urine from very dilute to very concentrated depending on how much you drink, so the range is wide. Labs also differ. For a random sample, Labcorp prints 50 to 1,400 and ARUP prints 50 to 800 for adults. Use the range on your own report.
What is the difference between urine osmolality and specific gravity?
Both show how concentrated your urine is. Osmolality counts the dissolved particles directly. Specific gravity compares the weight of urine with water, so heavy substances such as sugar or protein can raise it more than they raise osmolality.
Do I need to fast or limit fluids first?
Sometimes. MedlinePlus says you may be asked to fast for about 6 hours or to limit fluids for 12 to 14 hours. A plain random sample needs no preparation. Follow the instructions you were given.
Show the science and sources
Units
- 1 mOsm/kg = 1 mOsm/kg
- 1 mOsm/kg = 1 mmol/kg
Is a change real?
Urine Osmolality naturally varies about 28.3% from test to test in the same healthy person. Between two results from the same lab, a rise of 137% or more, or a fall of 58% or more, is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Desirable Biological Variation Database specifications, 2014 update (Minchinela J, Ricos C, et al.; the 'Ricos database', a compilation of median published CVI/CVG).
For clinicians and researchers
Osmolality is measured by freezing-point depression (ARUP, Mayo) and reported in mOsm/kg of water, which equals mmol/kg. Single-urine intervals differ by lab: Labcorp 50 to 1,400 (random sample), the MedlinePlus encyclopedia 50 to 1,200 (random sample), and ARUP 50 to 800 for ages 17 and older (50 to 645 for 0 to 30 days and 50 to 1,500 from 1 month to 16 years; ARUP does not name a collection type). Timed collections have their own intervals: Labcorp 300 to 900 for 24 hours and above 850 after 12 hours of fluid restriction, and Mayo 150 to 1,150 for 24 hours from 12 months (50 to 750 under 12 months). Interpretation depends on the question. In hyponatremia, euvolemic hypo-osmolality should produce a large volume of dilute urine (osmolality below 100 mOsm/kg with specific gravity below 1.003), and urine that is less than maximally dilute in the presence of plasma hypo-osmolality is the definition of SIADH (Merck Manual). In suspected hypotonic polyuria, a 24-hour urine osmolality at or below 300 mOsm/kg confirms hypotonic urine, a value at or above 800 mOsm/kg points to a solute diuresis from glucose, sodium, urea or mannitol, and values between 300 and 800 can occur with partial AVP deficiency or resistance (JCEM 2025). Published criteria to exclude AVP deficiency use a morning fasted urine osmolality above 800 mOsm/kg, although above 600 generally excludes clinically significant deficiency in practice. Those cutoffs came from a single study of 36 patients and were never prospectively validated. In the classic water-deprivation test, urine osmolality that stays below 300 mOsm/kg and then rises by more than 50% after desmopressin points to complete AVP deficiency (JCEM 2025). What was once called central diabetes insipidus is now named arginine vasopressin deficiency (AVP-D), and nephrogenic diabetes insipidus is AVP resistance (AVP-R). Read it with serum osmolality and sodium.
LOINC 2695-5 (primary). Specimen: urine.
Sources
- Osmolality, Urine (test 003442): sample patient report with reference intervals, Labcorp
- Osmolality, Urine (test 0020228), ARUP Laboratories
- Osmolality, 24 Hour, Urine (OSM24), Mayo Clinic Laboratories (catalog mirror at testcatalog.org)
- Osmolality Tests, MedlinePlus (U.S. National Library of Medicine)
- Osmolality urine test, MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine)
- Hyponatremia, Merck Manual Professional Version
- Approach to the Patient With Suspected Hypotonic Polyuria, The Journal of Clinical Endocrinology & Metabolism 2025;110:e506-e514 (2025)
- LOINC codes verified with the NLM Clinical Tables LOINC search, U.S. National Library of Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.