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Blood Urea Nitrogen (BUN)

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

The BUN test measures urea in the blood and provides information about kidney function, hydration, and protein breakdown.

Quick summary

["BUN is a blood marker that helps assess kidney function, but it is most informative when interpreted with other tests."]

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

Key facts at a glance

LOINC
57810-4
Short code
HEDIS 2010-11 MPM-A BUN
Synonyms
HEDIS 2010-11 MPM-A BUN
Category
kidney
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
7
Ref high
20
Critical range
— – 100 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
84520
SNOMED
2739004
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

What is this test?

Blood urea nitrogen, commonly called BUN, measures the amount of urea in the blood. Urea is a waste product formed in the liver when the body breaks down protein.

Why might I need this test?

This test may be ordered to evaluate kidney function or monitor kidney disease. It can also help assess hydration and changes in protein breakdown, particularly when interpreted with creatinine and estimated glomerular filtration rate.

How is the test performed?

A blood sample is drawn, usually by a venous blood collection. The laboratory measures urea in serum or plasma, and the results are reported as a blood urea nitrogen concentration.

How do I prepare for the test?

Fasting is generally not required for an isolated BUN test. Follow your clinician's instructions about fluids, supplements, and medications, especially if the test is being used to monitor kidney disease.

Reference range

Adult: 7-20 mg/dLN/A

Specimen: serum

No special preparation required. Stay well hydrated.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

BUN test

Frequently asked questions

Is BUN the same as creatinine?
No. BUN measures urea from protein breakdown, while creatinine reflects muscle breakdown and is filtered by the kidneys. They are often evaluated together to help assess kidney function.
Does drinking water affect the BUN result?
Yes. Dehydration can raise BUN, while high fluid intake may lower it. Your clinician may give specific fluid instructions if the test is part of a monitoring plan.
Do I need to fast before a BUN test?
Fasting is usually unnecessary unless your clinician specifically requests it. Tell the healthcare team about any supplements or medicines that may affect the result.
  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