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Osmolality of urine (measured as mass concentration)

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

This test measures how concentrated or diluted your urine is, which can help assess kidney function and hydration status.

Quick summary

["Urine osmolality helps doctors check if your kidneys are properly balancing fluids in your body."]

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

Key facts at a glance

LOINC
37866-1
Short code
XR tomo Sinuses
Synonyms
XR tomo Sinuses
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 It’s Done

Your doctor may order urine osmolality testing if you have symptoms like excessive thirst, frequent urination, or signs of dehydration. It can also be part of evaluating kidney function, especially in conditions like diabetes or kidney disease.

What to Expect

A small urine sample is collected, usually during a routine urine test. The lab then measures its concentration using specialized equipment. No special preparation is needed, but drinking extra water beforehand may affect results.

How It Works

Osmolality measures the total number of dissolved particles (like salts and waste products) in your urine. Higher values suggest concentrated urine (less water), while lower values indicate diluted urine (more water).

What the Results Mean

Results alone don’t diagnose a condition but can support other findings. For example, very high values might suggest dehydration or kidney issues, while very low values could indicate overhydration or kidney problems.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Urine Osmolality

Frequently asked questions

Could my urine osmolality test be affected by what I eat or drink?
Yes, drinking more water before the test can dilute your urine and lower osmolality. Avoid excessive fluids or certain diuretics (like caffeine) 24 hours before testing for more accurate results.
What if my results are outside the usual range?
Results outside typical ranges may signal potential kidney problems, dehydration, or other health concerns. Your doctor will discuss these findings with you and may recommend further testing or adjustments to your care plan.
Is urine osmolality testing painful or uncomfortable?
No, this test involves a simple urine sample collection—usually just peeing into a cup. There’s no discomfort, and it doesn’t require needles or special equipment at home.
  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