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Osmolality 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 how well your body balances fluids and electrolytes.

Quick summary

["Osmolality helps identify dehydration, overhydration, or kidney issues by checking your blood’s solute balance."]

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

Key facts at a glance

LOINC
26885-4
Short code
O+P Spec Conc
Synonyms
O+P Spec Conc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
concentration
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 monitor 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 a vein (usually in your arm), and a small amount of blood will be drawn. The test is quick and painless.

Your blood sample is sent to a lab where it’s analyzed to measure the total dissolved particles (solutes) in your serum or plasma.

The test uses a method called **osmometry**, which measures the freezing point depression of the blood sample. This helps determine how concentrated your blood is.

What Are Possible Results?

Results are reported in milliosmoles per kilogram (mOsm/kg). Your doctor will compare your result to your usual range or to a reference range provided by the lab.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Osmolality can reveal imbalances in fluid and electrolyte levels. Low osmolality (hypo-osmolality) may suggest overhydration or liver/kidney problems, while high osmolality (hyper-osmolality) could indicate dehydration, diabetes insipidus, or diabetes mellitus.

Short name

Serum/plasma osmolality

Frequently asked questions

What causes abnormal osmolality?
Abnormal osmolality can result from conditions like dehydration (low osmolality), severe diabetes (high osmolality), kidney disease, liver failure, or certain medications that affect fluid balance.
How often is this test repeated?
Your doctor will decide how often to repeat this test based on your symptoms, medical history, and treatment plan. Some patients may need it weekly or monthly depending on their condition.
Can I eat or drink before this test?
No special diet or restrictions are needed before this test. However, if you’re being monitored for fluid balance, your doctor may advise you to avoid certain fluids or foods for a short time.
  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