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Osmolality of Blood (Plasma or Serum)

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

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

Quick summary

["Osmolality helps doctors check if your body is properly balanced with fluids and electrolytes."]

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

Key facts at a glance

LOINC
89213-3
Short code
OB H&P note
Synonyms
OB H&P 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?

Your healthcare provider may order this test if you have symptoms like excessive thirst, confusion, swelling, or unexplained weight changes. It’s also used to monitor conditions like diabetes, kidney disease, or dehydration.

What Does the Result Mean?

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

How Is the Test Performed?

A small blood sample is taken from a vein in your arm. The sample is sent to a lab where it’s analyzed to measure the concentration of solutes in the plasma or serum.

What Are Common Causes of Abnormal Results?

Changes in osmolarity can occur due to dehydration, kidney problems, diabetes (especially uncontrolled), excessive alcohol use, or certain medications.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Blood Osmolality

Frequently asked questions

What if my osmolarity is too high?
High osmolarity often means your body is losing too much water (dehydration). Your doctor may recommend drinking more fluids, adjusting medications, or monitoring symptoms closely.
What if my osmolarity is too low?
Low osmolarity may signal overhydration or kidney/liver problems. Your doctor could check for other conditions like heart failure or fluid retention and may suggest lifestyle changes or further tests.
Does osmolarity change with diet?
Dietary factors like salt intake can indirectly affect osmolarity, but the test itself measures solutes in blood, not what you eat. Hydration status and kidney function play a bigger role.
  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