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Osmolarity of Serum (or Plasma)

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 balance problems in your body fluids."]

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

Key facts at a glance

LOINC
1273-2
Short code
Deprecated O Ab Ser Ql
Synonyms
Deprecated O Ab Ser Ql
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
aggl-rbc
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 excessive thirst, confusion, or unexplained fatigue. 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 blood. The process takes a few minutes, and you may feel a brief pinch.

What the Results Mean

Osmolality is measured in milliosmoles per kilogram (mOsm/kg). High levels may indicate dehydration, diabetes, or liver/kidney problems. Low levels can suggest overhydration or certain metabolic disorders.

Common Conditions Linked to Osmolality

Abnormal results can signal dehydration from vomiting, diarrhea, or excessive fluid loss. It may also help diagnose conditions like diabetes insipidus (a hormone imbalance causing extreme thirst) or severe kidney dysfunction.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Serum Osmolality

Frequently asked questions

How long does it take to get my osmolarity results back?
Results are typically available within 1–3 business days, depending on your lab’s processing time. Your doctor will discuss them with you during your follow-up visit.
Can I drink water before this test?
Yes, but avoid excessive fluid intake 24 hours before testing, as it may affect results. Follow your doctor’s specific instructions if you have a known condition requiring hydration adjustments.
What does a high osmolarity level mean?
A high osmolarity level often points to dehydration or conditions like diabetes, where the body loses too much water. Your doctor will review your symptoms and other test results to determine the cause.
  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