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Osmolality of Serum

Published: August 9, 2026Updated: August 16, 2026

This test measures the total concentration of dissolved particles (solutes) in your blood serum, which helps assess your body’s balance of water and electrolytes.

Quick summary

["Osmolality helps diagnose dehydration, kidney issues, or other fluid imbalances."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
37811-7
Short code
XR tomo Should-R
Synonyms
XR tomo Should-R
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
xr-tomography
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why Is This Test Done?

Your doctor may order this test if you have symptoms like extreme thirst, confusion, rapid breathing, or unexplained weight changes. It’s also used to monitor conditions like diabetes, kidney disease, or after certain surgeries or treatments that affect fluid balance.

What to Expect Before the Test

No special preparation is needed. You may have a small needle inserted into a vein in your arm, and a small amount of blood will be drawn. The process usually takes a few minutes.

What the Results Mean

Osmolality is measured in milliosmoles per kilogram (mOsm/kg). High levels may indicate dehydration or conditions like diabetes insipidus, while low levels could suggest overhydration or kidney problems.

Common Conditions Monitored

This test is often used to diagnose or monitor dehydration, kidney function, or fluid imbalances in patients with diabetes, severe infections, or after trauma.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Serum Osmolality

Frequently asked questions

What causes high osmolarity in serum?
High osmolarity can result from dehydration (losing more fluids than you take in), conditions like diabetes insipidus (a hormone disorder affecting thirst and urine production), or excessive use of certain medications (e.g., diuretics).
What causes low osmolarity in serum?
Low osmolarity may occur with overhydration (too much fluid intake), kidney problems that fail to concentrate urine properly, or rare conditions like pseudohyponatremia (where lab errors or fluid shifts in cells give false low sodium readings).
Can osmolarity test be affected by diet?
Dietary factors like high salt intake or certain medications (e.g., laxatives) can influence fluid balance but do not directly affect osmolarity results. Your doctor will consider overall health and symptoms, not just diet, when interpreting the test.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy