CMP Updated Apr 17, 2026

Blood Urea Nitrogen (BUN)

Blood Urea Nitrogen (BUN) is a lab value that measures the amount of urea nitrogen in the blood, a waste product made from protein breakdown. It is reported as BUN on a blood test or Blood Urea Nitrogen on a lab report and helps describe how blood chemistry is changing over time.

What Is Blood Urea Nitrogen (BUN)?

Blood Urea Nitrogen (BUN) is a blood value that measures the amount of nitrogen in urea, a waste product that circulates in the bloodstream. BUN on a blood test reflects how much of this waste is present in the blood at the time of sampling. It is one part of the overall blood chemistry picture and is often viewed as a marker of how concentrated the blood is and how much protein waste is being handled.

Why Is Blood Urea Nitrogen (BUN) Tested?

BUN is included in a Metabolic Panel, especially a Comprehensive Metabolic Panel (CMP) and a Basic Metabolic Panel (BMP). A BUN test is also commonly reviewed alongside other chemistry values in routine lab work, pre-procedure screening, or general health checks. On a Blood Urea Nitrogen on a lab report, it helps add context to the broader blood chemistry pattern.

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Blood Urea Nitrogen (BUN) Normal Range

Group Range Unit
Adult Male 7–20 mg/dL
Adult Female 7–20 mg/dL

Reference ranges may vary by laboratory and individual factors.

What Does High BUN Mean?

A high BUN test result generally means that more urea nitrogen is present in the blood than expected. In many labs, values above about 20 mg/dL are considered high BUN, though the exact Blood Urea Nitrogen reference range can vary by laboratory. High BUN often reflects a more concentrated blood sample or a shift in how protein waste is being processed and cleared.

Associated factors

Dehydration — lower plasma volume can concentrate the measured BUN.
High protein intake — more protein breakdown can increase urea production.
Recent heavy exercise — increased protein turnover can raise BUN.
Fasting or low calorie intake — less energy intake can shift protein use.
Certain medicines — some drugs can change urea handling or blood volume.
Older age — BUN may run somewhat higher with age-related changes in body water.
Altitude — lower oxygen environments can change fluid balance and nitrogen handling.
Smoking — nicotine-related effects on circulation and hydration can influence BUN.
Recent bleeding into the digestive tract — absorbed blood protein can raise BUN.
Catabolic stress — rapid tissue breakdown can increase urea formation.

What Does Low BUN Mean?

A low BUN test result means that less urea nitrogen is present in the blood than expected. In many labs, values below about 7 mg/dL are considered low BUN, but the normal Blood Urea Nitrogen range depends on the lab method. Low BUN often points to a blood sample that is less concentrated or to reduced urea production.

Associated factors

Low protein intake — less dietary protein can reduce urea formation.
Overhydration — extra body water can dilute the measured BUN.
Pregnancy — increased blood volume can lower the measured concentration.
Severe calorie restriction — reduced protein breakdown can lower urea production.
Certain medicines — some drugs can affect liver handling of nitrogen.
Low muscle mass — less overall protein turnover may reduce BUN.
Liver-related processing changes — reduced urea formation can lower BUN.
Recent intravenous fluids — added fluid can dilute the blood sample.
Celiac-like dietary restriction patterns — very low protein intake may lower BUN.
Laboratory variation — method differences can shift a BUN test result slightly.

How Blood Urea Nitrogen (BUN) Relates to Other Values

Blood Urea Nitrogen is often read with creatinine, because both appear on a Metabolic Panel and together describe different parts of blood chemistry. It is also commonly considered alongside sodium, potassium, and carbon dioxide (CO2), which help show the overall balance of salts and acid-base status. When BUN is reviewed with albumin and total protein, the pattern can suggest whether the blood is more concentrated or more diluted. In some reports, BUN on a blood test is easier to interpret when compared with creatinine ratio and with markers such as hematocrit (Hct), RBC, and MCV on a complete blood count.

What Factors Affect Blood Urea Nitrogen (BUN) Levels?

Blood Urea Nitrogen can vary with hydration, protein intake, recent exercise, and the time of day. BUN may also shift with age, sex, pregnancy, and altitude because body water and nitrogen handling change naturally. Different labs may use different measurement methods, so the Blood Urea Nitrogen normal range can vary slightly from one lab report to another. BUN on a blood test can also move after recent fluid intake or fasting. Ethnicity is not usually a major direct factor, but diet patterns and body composition can influence results indirectly.

How It Is Tested

BUN is measured from a blood draw, usually from a vein in the arm. The lab analyzes the amount of urea nitrogen in the sample and reports it as BUN, most often in mg/dL or mmol/L depending on the lab. A BUN test is a standard chemistry measurement on many lab reports.

How to Prepare

No special preparation is usually needed for a routine BUN test as part of a Metabolic Panel. If the panel includes other measurements, those may have different preparation rules.

Frequently Asked Questions

What is a normal BUN level?
A normal Blood Urea Nitrogen normal range is commonly about 7 to 20 mg/dL in adults, although ranges can vary by laboratory. BUN on a lab report should always be read using the lab’s own reference interval.
What does BUN stand for?
BUN stands for Blood Urea Nitrogen. It is the common abbreviation used on a BUN test and on many lab reports.
What does a high Blood Urea Nitrogen mean on a lab report?
A high BUN result usually means the blood contains more urea nitrogen than expected. In many labs, a value above about 20 mg/dL is considered high BUN, but the exact Blood Urea Nitrogen reference range depends on the lab.
What does a low Blood Urea Nitrogen mean on a lab report?
A low BUN result means the measured urea nitrogen is below the lab’s usual range. In many labs, values below about 7 mg/dL are considered low BUN.
Can hydration affect Blood Urea Nitrogen?
Yes. Hydration can change BUN because added body water can dilute the blood and low fluid intake can make the blood more concentrated. This is one reason BUN on a blood test is often read with other chemistry values.
What is the difference between Blood Urea Nitrogen and creatinine?
BUN measures urea nitrogen, while creatinine measures a different waste product from muscle metabolism. Both are common on a Metabolic Panel, and the pattern between them gives more context than either value alone.
What unit is Blood Urea Nitrogen measured in?
Blood Urea Nitrogen is usually measured in mg/dL in the United States. Some labs may report BUN in mmol/L, so the unit on the lab report matters.
How much can Blood Urea Nitrogen change between tests?
BUN can change from test to test because hydration, diet, and recent activity can shift the value. Small changes may be within normal day-to-day variation, while larger changes may reflect a stronger shift in blood concentration.
Is Blood Urea Nitrogen different for men and women?
BUN is often reported with the same adult reference range for men and women, such as 7 to 20 mg/dL. Some labs may still show small differences based on method or local reference data.
Why is Blood Urea Nitrogen tested in a Metabolic Panel?
BUN is included in a Metabolic Panel because it adds information about blood chemistry and waste handling. Along with creatinine, sodium, potassium, and carbon dioxide, it helps describe the overall pattern on the lab report.
What does BUN on a blood test mean?
BUN on a blood test is the measured amount of blood urea nitrogen in the sample. It is a basic chemistry value that helps describe blood composition and fluid balance.

Disclaimer

This content is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment recommendation. Reference ranges may vary by laboratory. Always discuss your results with a qualified healthcare professional.

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