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Osmolality of Serum or Plasma (measured in milliosmoles per kilogram)

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

This test measures the total concentration of dissolved particles (solutes) in your blood, helping assess hydration and kidney function.

Quick summary

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

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

Key facts at a glance

LOINC
35972-9
Short code
XR tomo LE
Synonyms
XR tomo LE
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 healthcare provider may order this test if you have symptoms like extreme thirst, confusion, or signs of dehydration (e.g., dry mouth, low urine output). It’s also used to check kidney function, especially in cases of high or low blood sodium levels, or after certain medications or illnesses.

What to Expect Before the Test

No special preparation is needed. You may have a small needle inserted into a vein (usually in your arm) to draw blood. If you’re tested for urine osmolality, you might be asked to collect a sample at home or in the lab.

What the Results Mean

Osmolality results help determine whether your body is retaining too much or too little water. Abnormal levels may indicate conditions like diabetes insipidus, kidney disease, or excessive sweating.

Common Conditions Affecting Results

High osmolality (hyperosmolality) can occur with dehydration, diabetes, or severe infections. Low osmolality (hypoosmolality) may suggest overhydration, kidney problems, or certain medications.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

This test is critical for diagnosing and managing fluid and electrolyte imbalances, guiding treatment for dehydration, kidney disorders, or metabolic conditions like diabetes.

Short name

Serum/Plasma Osmolality

Frequently asked questions

What causes abnormal osmolality levels?
Abnormal osmolality can result from dehydration (losing too much water), kidney disease, diabetes (especially diabetes insipidus), excessive sweating, or certain medications that affect fluid balance.
How long does it take to get results?
Results are typically available within 1 to 3 business days, depending on your lab’s processing time. Your healthcare provider will discuss the findings with you during your follow-up visit.
Can I eat or drink before this test?
For blood osmolality, you can eat and drink normally. For urine osmolality, avoid excessive fluids 24 hours before testing if instructed by your provider, as it may affect results.
  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