Osmolality, Serum
Serum osmolality shows how concentrated your blood is: how many dissolved particles, mostly sodium, glucose and urea, it holds per kilogram of fluid.
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What it is
Osmolality is the concentration of particles dissolved in your body fluids. Sodium is the main contributor, with smaller contributions from glucose and urea. Your kidneys and hormones, including antidiuretic hormone (ADH), keep it within a narrow range by adjusting how much water you hold onto. The lab measures it directly, usually by freezing-point depression. This is different from the calculated osmolality that some panels print, which is an estimate from sodium, glucose and urea nitrogen.
Why it matters
Blood osmolality shows whether the balance between water and dissolved particles in your blood is off. It is used to look into dehydration, overhydration, low or high sodium, diabetes insipidus and suspected poisoning with substances such as antifreeze or rubbing alcohol. Comparing the measured value with the calculated one gives the osmolal gap, a clue that something besides sodium, glucose and urea is adding to the total. A result is read together with sodium, glucose, BUN and your symptoms.
If it’s low
A result below your lab's range means your blood is more diluted than usual. This is most often tied to low blood sodium (hyponatremia), which can come from taking in more water than the body clears, from conditions that make the body hold onto water, from some medicines, or from the syndrome of inappropriate ADH secretion (SIADH). Mild lows may cause no symptoms. Larger drops can cause headache, confusion or tiredness. Your clinician can put your result in context.
Common causes
- Low blood sodium (hyponatremia)
- Overhydration, from drinking or receiving more fluid than the body clears
- Syndrome of inappropriate ADH secretion (SIADH), where the body makes too much antidiuretic hormone and holds onto water
- Heart failure, liver disease or kidney disease
- Some medicines, including thiazide water pills
What can help
- Ask your clinician whether sodium, urine tests and your fluid intake should be reviewed
- Ask whether any of your medicines could be involved
If it’s high
A result above your lab's range means your blood is more concentrated than usual. Common reasons are not enough water in the body (dehydration), high sodium and high blood sugar. Reduced kidney function can raise it too, through a build-up of urea. Substances that are not normally in the blood, such as ethanol, methanol, ethylene glycol or mannitol, can also raise it. Your clinician can put your result in context.
Common causes
- Dehydration, for example from not drinking enough, fever, vomiting or diarrhea
- High blood sodium (hypernatremia)
- High blood sugar, as in poorly controlled diabetes
- Reduced kidney function with a build-up of urea
- Alcohols such as ethanol, methanol or ethylene glycol (antifreeze)
- Mannitol, a medicine given in hospital
- Diabetes insipidus, where the body loses too much water in the urine
What can help
- Ask your clinician whether sodium, glucose and kidney function should be checked or repeated
- Ask whether your fluid intake or a medicine could be a factor
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.
- 18–60275–295 mOsm/kg
Sources: Mayo Clinic Laboratories (catalog copy hosted by Schneck Medical Center), Labcorp, ARUP Laboratories, Merck Manual Professional Version, MedlinePlus (US National Library of Medicine)
- 61+280–301 mOsm/kg
Source: Labcorp
Children and teens 1 ranges
- under 18275–295 mOsm/kg
Sources: Mayo Clinic Laboratories (catalog copy hosted by Schneck Medical Center), Labcorp
See Osmolality by age and sex, with the U.S. survey numbers where they exist.
What can affect your result
- Sodium, the largest contributor
- Glucose and urea (BUN)
- Alcohols, mannitol and other substances that add dissolved particles
- Hydration and how much fluid you drink
- Age: Labcorp lists a higher range for people 61 and older, and ARUP lists a different range for children
- Lab differences: adult upper limits run from 295 to 303 mOsm/kg
- Sample handling: hemolyzed samples and EDTA plasma are rejected by Labcorp
- Alcohols (ethanol, methanol, ethylene glycol) and mannitol in the blood raise the measured value. That is a real effect on the number rather than a lab error, and it is what makes osmolality useful when poisoning is suspected.
- Labcorp rejects hemolyzed samples and EDTA plasma. Mayo Clinic Laboratories accepts grossly hemolyzed or lipemic samples.
- Serum or plasma should be separated from the cells promptly: as soon as possible after the clot forms at Labcorp and within 2 hours at ARUP.
- 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.
Hours to days, as your water, sodium and blood sugar levels change.
When to talk to a clinician
Contact the clinician who ordered the test soon if your osmolality is outside your lab's range, especially with confusion, severe thirst, headache, vomiting or unusual drowsiness. Houston Methodist's critical-value list calls results of 250 mOsm/kg or lower and 320 mOsm/kg or higher. Call 911 or your local emergency number for a seizure, fainting, trouble breathing, or if you may have swallowed antifreeze, rubbing alcohol or another toxic alcohol.
Questions you could ask
- What prompted this test, and what other tests go with it?
- Could dehydration, my blood sugar or my kidney function explain a higher result?
- Should we compare it with my calculated osmolality?
- Could my sodium level, fluid intake or a medicine explain a lower result?
- Should we repeat it along with sodium and other electrolytes?
Questions people ask
What is the difference between measured and calculated osmolality?
Measured osmolality is a direct lab test of all the dissolved particles in your blood. Calculated osmolality is an estimate from sodium, glucose and urea nitrogen. If the two differ by about 10 or more, something else, such as an alcohol, may be in the blood. That difference is called the osmolal gap.
Do I need to fast or limit fluids before the test?
The test pages from Mayo Clinic Laboratories, Labcorp and ARUP don't list a fasting rule, but some providers ask you to fast for about 6 hours or limit fluids for 12 to 14 hours. Follow the instructions you were given.
Do I need to convert the units?
No. mOsm/kg and mmol/kg are the same number, so a result of 285 mOsm/kg is 285 mmol/kg.
Show the science and sources
Units
- 1 mOsm/kg = 1 mOsm/kg
- 1 mOsm/kg = 1 mmol/kg
Is a change real?
Osmolality naturally varies about 1.3% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 4% 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
Serum osmolality is measured by freezing-point depression (Labcorp, ARUP). Sodium and its accompanying anions account for most of it, and urea crosses cell membranes freely, so it adds to osmolality but not to tonicity (Merck). The Merck Manual gives a normal osmolality of 275–290 mOsm/kg (mmol/kg). Lab intervals: Mayo Clinic Laboratories 275–295 mOsm/kg; Labcorp 275–295 for ages 0–60 and 280–301 for ages 61 and older, with newborns possibly as low as 266; ARUP 271–296 for ages 0–16 and 280–303 for ages 17 and older. Estimated osmolality is calculated from sodium, glucose and urea nitrogen (see the calculated osmolality marker). An osmolal gap of 10 mOsm/kg or more (measured minus estimated) suggests unmeasured osmotically active substances such as ethanol, methanol, isopropanol, ethylene glycol, mannitol or glycine. Severe dyslipidemia or hyperproteinemia can make indirect sodium measurement read falsely low and so lower the estimated value, whereas direct ion-selective electrodes avoid this (Merck). Hyponatremia is classified by volume status, and SIADH, thiazide diuretics, heart failure, cirrhosis and kidney disease are common causes. Critical limits: Houston Methodist 250 or lower and 320 or higher mOsm/kg.
LOINC 2692-2 (primary). Specimen: serum-or-plasma.
Sources
- Osmolality, Serum (test ID UOSMS), Mayo Clinic Laboratories (catalog copy hosted by Schneck Medical Center)
- Osmolality (test 002071), Labcorp
- Osmolality (test 002071), sample patient report, Labcorp
- Osmolality, Serum or Plasma (test 0020046), ARUP Laboratories
- Water and Sodium Balance, Merck Manual Professional Version
- Hyponatremia, Merck Manual Professional Version
- Osmolality Tests, MedlinePlus (US National Library of Medicine)
- Diagnostic Laboratories: Critical Values, Houston Methodist
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.