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Blood Urea Nitrogen (BUN) Test: What It Is, Why It Is Done, and How to Prepare

Published: July 31, 2026Updated: August 10, 2026

A blood test that measures the amount of urea nitrogen in your blood to evaluate how well your kidneys are working. Urea nitrogen is a waste product your body produces when it breaks down protein, and healthy kidneys filter it out of your blood and remove it through urine. Your doctor may order this test to screen for kidney problems, look into symptoms such as fatigue or swelling, check hydration status, or monitor existing conditions like dehydration, heart failure, or kidney disease over time.

Quick summary

{"BUN test measures waste product levels in your blood","Helps assess how well your kidneys are working","Higher than normal levels may indicate kidney problems or dehydration","Results are often interpreted alongside creatinine levels"}

Medically reviewed by WellAlly Editorial TeamLast reviewed August 10, 2026Editorial policy

Key facts at a glance

LOINC
108689-1
Short code
BUN
Synonyms
BUN Msmt SerPl
Category
chemistry
Sample
serum
Container
SST (Serum Separator Tube) / Red top tube / Lithium heparin plasma tube
Units
not specified
Ref low
7
Ref high
20
Critical range
5 not specified – 100 not specified
Fasting
false
Population
adult
Sex
all
Age range
0–120 years
Method
Enzymatic (Urease) method, spectrophotometric detection
CPT
84520
SNOMED
2739000
Specimen volume
1-3 mL minimum
Stability
Room temperature: 24-72 hours; Refrigerated (2-8°C): 5-7 days; Frozen (-20°C or below): 3-6 months
Hemolysis max
50
Lipemia max
300
Icterus max
20
TAT
1–24 hours
Order
routine

What is this test?

The Blood Urea Nitrogen (BUN) test measures the amount of urea nitrogen in your blood. Urea nitrogen is a waste product that forms when your body breaks down protein. Normally, your kidneys filter urea nitrogen from your blood and excrete it in urine. This test helps doctors evaluate how well your kidneys are functioning.

How is the test performed?

A healthcare professional will draw a small blood sample from a vein in your arm. The sample is then sent to a laboratory for analysis. You may need to fast (not eat) for 8-12 hours before the test. Your doctor may also ask you to avoid certain medications or supplements before the test.

What do the results mean?

Normal BUN levels typically range from 7 to 20 mg/dL (2.5 to 7.1 mmol/L). However, normal ranges may vary slightly between laboratories. Higher than normal BUN levels may indicate kidney problems, dehydration, heart failure, or high protein intake. Lower than normal levels may be seen in liver disease, malnutrition, or overhydration.

Reference range

7-20 mg/dLnot specified

Specimen: serum

No fasting or special preparation required

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

The BUN test is primarily used to evaluate kidney function. It is commonly ordered as part of a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP). The test helps diagnose and monitor conditions such as kidney disease, dehydration, and urinary tract obstruction. BUN levels are often interpreted together with creatinine levels to provide a clearer picture of kidney function (the BUN-to-creatinine ratio).

Frequently asked questions

Do I need to prepare for the BUN test?
You may need to fast for 8-12 hours before the test. Inform your doctor about all medications and supplements you are taking, as some can affect BUN levels. Stay well-hydrated unless otherwise instructed by your doctor.
Is the BUN test painful?
You may feel a brief sting or pinch when the needle is inserted to draw blood. The discomfort is usually minimal and lasts only a few seconds. Some people may experience minor bruising at the injection site.
What can cause high BUN levels?
High BUN levels can be caused by kidney disease, dehydration, heart failure, urinary tract obstruction, high protein diet, certain medications, or bleeding in the digestive tract. Your doctor will interpret your results in the context of your overall health.
Can BUN levels fluctuate?
Yes, BUN levels can change based on factors such as diet (especially protein intake), hydration status, medications, and overall health. This is why multiple tests may be needed to assess kidney function over time.
  1. 1
    MedlinePlus - BUN TestSource
  2. 2
    May Clinic - Blood Urea Nitrogen (BUN) TestSource

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 10, 2026. · Read our editorial policy