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Osmolality of Plasma (measured in milliosmoles per kilogram, mOsm/kg)

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

This test measures the concentration of dissolved substances (like water, salts, and sugars) in your blood plasma. It helps assess how well your body is balancing fluids and electrolytes.

Quick summary

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

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

Key facts at a glance

LOINC
89468-3
Short code
OB Pre-op note
Synonyms
OB Pre-op note
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
obstetrics
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 Before Surgery?

Your healthcare team may order this test to check if your body is properly hydrated and balanced before major surgery. Abnormal results could signal dehydration, kidney problems, or other conditions that need attention before the procedure.

What Does Osmolality Tell Us About Your Health?

Osmolality helps identify whether your blood is too dilute (low osmolality) or too concentrated (high osmolality). For example, dehydration can raise osmolality, while overhydration or kidney issues may lower it. This information helps doctors plan your care safely.

How Is This Test Performed?

A small blood sample is drawn from a vein, usually in your arm. The plasma portion of the blood is then tested in a lab to measure its osmolality using specialized equipment.

What Are Common Causes of Abnormal Osmolality?

High osmolality might be due to dehydration, diabetes, or kidney problems, while low osmolality could result from overhydration or certain medications. Your doctor will review your medical history and other test results to determine the cause.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Plasma Osmolality

Frequently asked questions

Will I Feel Pain or Discomfort During the Blood Draw?
You may feel a slight pinch when the needle is inserted, but most people describe it as mild. A band or ice can help ease any discomfort afterward.
How Long Does It Take to Get Results?
Results are typically available within 1 to 2 business days, depending on your lab’s processing time. Your doctor will discuss the findings with you during your follow-up.
Can I Eat or Drink Normally Before This Test?
You can eat and drink as usual unless your doctor specifies otherwise. This test doesn’t require fasting, but check with your provider for any specific instructions.
  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