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Osmolality of Serum (Plasma)

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

This test measures the concentration of dissolved substances in your blood, helping doctors assess how well your kidneys are balancing fluids and electrolytes.

Quick summary

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

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

Key facts at a glance

LOINC
37874-5
Short code
XR tomo Skull
Synonyms
XR tomo Skull
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, swelling, or unexplained weight changes. It’s also used to check for conditions like diabetes, kidney disease, or severe dehydration.

What to Expect Before the Test

No special preparation is needed. You’ll have a small needle inserted into your arm or vein to draw a blood sample. The process takes a few minutes.

What Does the Test Result Mean?

Osmolality is measured in milliosmoles per kilogram (mOsm/kg). High levels may indicate dehydration or liver/kidney issues, while low levels could suggest overhydration or certain metabolic disorders.

Common Conditions Linked to Osmolality

Abnormal results can signal dehydration (too low), diabetes insipidus (too high), kidney failure, or other fluid/electrolyte imbalances.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Osmolality is a critical marker for fluid balance and electrolyte homeostasis. It helps diagnose conditions that disrupt normal bodily processes, such as dehydration, electrolyte disorders, or metabolic syndromes.

Short name

Serum Osmolality

Frequently asked questions

What causes high osmolarity in blood?
High osmolarity can result from dehydration, diabetes insipidus, severe diarrhea, or conditions that increase blood sugar (like uncontrolled diabetes).
What causes low osmolarity in blood?
Low osmolarity may occur with overhydration, certain kidney disorders, or conditions affecting sodium levels (e.g., excessive water intake).
Can osmolarity test be done at home?
No, this test requires a blood sample drawn by a healthcare professional in a lab setting.
  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