Blood Urea Nitrogen (BUN) Test: A Complete Patient Guide to Kidney Health Screening

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

The Blood Urea Nitrogen (BUN) test measures the amount of urea nitrogen in your blood. Urea nitrogen is a waste product that forms when protein breaks down, and healthy kidneys normally filter it out of your blood.

Quick summary

{"BUN test measures waste products (urea) in your blood that your kidneys normally filter out","High BUN levels may indicate kidney problems, dehydration, or high protein intake","Low BUN levels may be caused by liver disease, overhydration, or malnutrition","The test is usually part of a routine blood panel called a comprehensive metabolic panel (CMP)","Results are interpreted together with other kidney function tests like creatinine"}

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

Key facts at a glance

LOINC: 3094-0
Short code: BUN
Synonyms: BUN SerPl-mCnc
Category: chemistry
Sample: serum
Container: SST (Serum Separator Tube) or plain red top tube; Sodium heparin or EDTA plasma acceptable
Units: not specified
Ref low: 7
Ref high: 20
Critical range: 3 not specified – 100 not specified
Fasting: false
Population: adult
Sex: all
Age range: ≥ 0 years
Method: Enzymatic method (urease) or Colorimetric method (DAM/van Slyke)
CPT: 84520
SNOMED: 2730-8
Specimen volume: 1-2 mL minimum
Stability: Room temperature: 7 days; Refrigerated (2-8°C): 7 days; Frozen (-20°C or below): 30 days
Hemolysis max: 50
Lipemia max: 300
Icterus max: 20
TAT: 1–4 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 from food and tissues. Normally, your kidneys filter urea nitrogen from your blood and remove it through urine.

Why is it done?

A BUN test is commonly ordered as part of a routine health checkup or when your doctor suspects kidney problems. It helps evaluate how well your kidneys are working to remove waste from your blood. The test is often performed along with other kidney function tests, such as creatinine, to get a more complete picture of kidney health.

How is it performed?

A healthcare professional will draw a small blood sample from a vein in your arm using a needle. The blood is collected into a tube and sent to a laboratory for analysis. You do not need to fast before a standard BUN test, but your doctor may give you specific instructions.

Understanding your results

Normal BUN ranges typically fall between 7 and 20 mg/dL (milligrams per deciliter). However, reference ranges may vary slightly depending on the laboratory and your individual health factors. Your doctor will interpret your results based on your overall health, diet, and other test results.

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.

Elevated BUN levels (azotemia) can indicate kidney dysfunction, dehydration, heart failure, urinary tract obstruction, or high protein intake. In acute kidney injury, BUN levels often rise before creatinine. Conversely, low BUN levels may suggest liver disease (where urea production is impaired), overhydration, malnutrition, or pregnancy. BUN-to-creatinine ratio is also clinically important—elevated ratios may indicate prerenal azotemia (e.g., dehydration), while normal or low ratios suggest intrinsic kidney problems.

Frequently asked questions

Do I need to fast before a BUN test?

Typically, no fasting is required for a standard BUN test. However, follow any specific instructions your doctor gives you. Some tests may require fasting if they are part of a larger panel that includes glucose or lipid tests.

What can cause high BUN levels besides kidney problems?

High BUN can be caused by dehydration, a high-protein diet, certain medications (like diuretics), heart failure, gastrointestinal bleeding (blood in the digestive tract adds protein), or catabolism (breakdown of body tissue). Your doctor will consider these factors when interpreting your results.

Can BUN levels fluctuate daily?

Yes, BUN levels can vary based on diet, hydration status, time of day, and physical activity. This is why doctors look at trends over time rather than a single result.

Is a BUN test painful?

You may feel a brief sting or pinch when the needle is inserted to draw blood. Some people may experience minor bruising or soreness afterward, but serious complications are rare.

How often should BUN be tested?

The frequency depends on your health condition. For routine health screening, BUN may be checked once a year as part of a comprehensive metabolic panel. If you have kidney disease, diabetes, high blood pressure, or are on certain medications, your doctor may order it more frequently.

Sources

1
Blood Urea Nitrogen (BUN) Test - Mayo ClinicSource
2
BUN Test - MedlinePlusSource

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