Blood urea nitrogen (BUN)

Optimal Result: 6 - 24 mg/dL, or 2.14 - 8.57 mmol/L.

What Is Blood Urea Nitrogen (BUN)?

Urea nitrogen is a waste product. It develops when your body breaks down the protein in the foods you eat. It forms in your liver and travels through your blood to your kidneys, which then filter it out of your blood. It leaves your body through your urine. When your kidneys are healthy, they remove the BUN, usually leaving a small amount of it in the blood. But for the most part, your kidneys get rid of it by flushing it out of your body through urine. When your kidneys are not working well, your BUN level goes up. Over time, this may lead to serious health problems, including high blood pressure, anemia, and heart disease.

Your blood urea nitrogen (BUN) level is based on a blood test that measures the amount of urea nitrogen in your blood.

- BUN is a waste product of cell metabolism. You get protein from the food you eat, and it enters the bloodstream from the intestines to be used by cells throughout your body.

- Your cells break protein down into amino acids to build back up into the proteins they need for various processes. This produces nitrogen-containing ammonia as a byproduct, which is excreted into the bloodstream.

- The liver transforms ammonia into urea to make it less toxic and sends the urea out into the bloodstream. Urea is filtered out of the blood by the kidneys.

- If all is going well, there is a continuous amount of urea being produced and being excreted by the kidneys into the urine. The BUN level in the blood is, therefore, stable. If the kidneys are damaged and not functioning properly, urea and the nitrogen it contains are not filtered fully from the blood.

The blood urea nitrogen (BUN) test measures how much urea nitrogen is in your blood. It helps a healthcare provider determine if your kidneys are working as they should.

People with early kidney disease may not have any symptoms. A BUN test can help uncover kidney problems at an early stage when treatment can be more effective.

What are normal BUN levels?

BUN levels vary according to your age and sex. Abnormal levels may indicate a health condition, including kidney damage.

Normal Ranges in mg/dL by ages:

0-2 years:      4-15  
3-16 years:    9-18  
17-64 years:  8-22
>64 years:    10-28 

What is the purpose of the BUN test?

Urea nitrogen levels in your blood are one marker that allows healthcare providers to understand how well your kidneys are working. A small amount of urea nitrogen in your blood is normal. If you have too much urea nitrogen in your blood, your kidneys aren’t filtering it properly. You may have a condition that’s affecting your kidneys’ health.

The biomarker Blood urea nitrogen (BUN) is part of a standardized blood test panel called the Comprehensive Metabolic panel (CMP). This panel is usually performed at your biannual health check up. 

Your health care professional might also perform this test if you: 

a) have symptoms of kidney disease, such as urinary changes, swelling in your arms or legs, muscle cramps, or frequent episodes of fatigue, or 

b) if you have a higher risk of kidney disease because of diabetes, cardiovascular problems, or a family history of kidney disease.

c) need to  monitor for side effects when you begin a new medication that can affect the kidneys.

Your doctor may also test how well your kidneys are removing waste from the blood. To do this, you may have a blood sample taken to calculate your estimated glomerular filtration rate (GFR). The GFR estimates the percentage of kidney function you have.

You may need a BUN test for other reasons. Depending on your overall health, it may be part of a routine health check to find out how your kidneys are working. If you need dialysis or medicine for kidney function, you may have the test to check the health of your kidneys before the procedure, after the procedure, or both. BUN tests are also routine during hospital stays for certain conditions. 

You may be more likely to develop kidney disease if you have:

- Family of kidney problems

- Diabetes

- High blood pressure

- Heart disease

What are symptoms of later stage kidney disease?

Your provider may check your BUN levels if you are having symptoms of later stage kidney disease, such as:

- Needing to urinate more often or less often than usual

- Itching

- Fatigue

- Swelling in your legs, feet, or ankles

- Muscle cramps

- Trouble sleeping

Is there anything else I need to know about a BUN test?

A BUN test is only one type of measurement of kidney function. If your provider thinks you may have kidney disease, you may need other tests. 

These may include tests to measure:

- Creatinine, which is another waste product that your kidneys remove from your body

- GFR (Glomerular Filtration Rate), which estimates how well your kidneys are filtering blood

What might affect my test results?

Your BUN levels might rise even if your kidneys are working as they should for many reasons. These include:

- A high-protein diet

- Steroid use

- Dehydration

- Burn injuries

- Aging

This is why the ratio of BUN levels to creatinine levels is a more reliable measure of kidney health.

References: OneCare Media; c2022. Blood Urea Nitrogen -

Lyman JL. Blood urea nitrogen and creatinine. Emerg Med Clin North Am -

Mayo Clinic. Mayo Foundation for Medical Education and Research; c1998-2022. Blood Urea Nitrogen (BUN) Test -

Mayo Clinic, Mayo Foundation for Medical Education and Research; c1998-2022. Chronic Kidney Disease -

National Heart, Lung, and Blood Institute - U.S. Department of Health and Human Services; Blood Tests -

National Institute of Diabetes and Digestive and Kidney Diseases: U.S. Department of Health and Human Services; Kidney Disease -

National Institute of Diabetes and Digestive and Kidney Diseases: U.S. Department of Health and Human Services; National Kidney Disease Education Program: Your Kidney Test Results -

National Kidney Foundation. New York: National Kidney Foundation Inc., c2022. About Chronic Kidney Disease -

National Kidney Foundation. New York: National Kidney Foundation Inc., c2022. Understanding Lab Values -

University of Rochester Medical Center: University of Rochester Medical Center; c2022. Health Encyclopedia: Blood Urea Nitrogen -

A.D.A.M. Medical Encyclopedia. Basic Metabolic Panel -

A.D.A.M. Medical Encyclopedia. BUN – Blood Test -

A.D.A.M. Medical Encyclopedia. Comprehensive Metabolic Panel -

American Board of Internal Medicine. ABIM Laboratory Test Reference Ranges -

Gounden V, Bhatt H, Jialal I. Renal Function Tests. In: StatPearls -

Hosten AO. Chapter 193: BUN and Creatinine. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. Third Edition. 1990 -

Inker LA, Perrone RD. Assessment of Kidney Function. In: Sterns RH, ed. UpToDate -

MedlinePlus: National Library of Medicine. BUN (Blood Urea Nitrogen) -

MedlinePlus: National Library of Medicine. Comprehensive Metabolic Panel (CMP) -

Seki M, Nakayama M, Sakoh T, et al. Blood urea nitrogen is independently associated with renal outcomes in Japanese patients with stage 3-5 chronic kidney disease: a prospective observational study. BMC Nephrol. 2019;20(1):115. doi:10.1186/s12882-019-1306-1

Tomizawa M, Shinozaki F, Hasegawa R, et al. Patient characteristics with high or low blood urea nitrogen in upper gastrointestinal bleeding. World J Gastroenterol. 2015;21(24):7500-5. doi:10.3748/wjg.v21.i24.7500

Jujo K, Minami Y, Haruki S, et al. Persistent high blood urea nitrogen level is associated with increased risk of cardiovascular events in patients with acute heart failure. ESC Heart Fail. 2017;4(4):545-553. doi:10.1002/ehf2.12188

American Association for Clinical Chemistry. Blood Urea Nitrogen, Lab Tests Online.

Laura J. Martin, MD, BUN - blood test, MedlinePlus. U.S. National Library of Medicine.

National Heart, Lung and Blood Institute. Types of Blood Tests.

What does it mean if your Blood urea nitrogen (BUN) result is too high?

Increased BUN levels suggest kidney impairment

Impairment can be acute, chronic, damage, or failure.

It may also be due to a condition that has decreased blood flow to the kidneys, such as:

- congestive heart failure,

- recent heart attack,

- shock,

- stress,

- severe burns,

- or dehydration.

Excessive protein breakdown will also elevate BUN levels, which can result from a high protein diet or gastrointestinal bleeding

Your doctor can discuss your BUN levels and how they relate to your overall health, symptoms, and other test measurements. This is important because BUN levels alone are not a consistent predictor of kidney function. Elevated BUN can occur with kidney problems, but it can also happen from eating lots of protein, taking certain medications, or other issues like dehydration or burns. BUN levels often rise with aging as well.

Independently, blood urea nitrogen may not reflect kidney function. For this reason, it is often interpreted in the context of other measurements, such as creatinine, a breakdown product of the muscle filtered by the kidneys. In some cases, the doctor may look at the ratio of BUN to creatinine to help determine the underlying cause of the altered kidney function.

Working with your doctor is the best way to understand the significance of your BUN test. The American Board of Internal Medicine lists a typical reference range for BUN as 8 to 20 mg/dL. However, this range is not universal. Labs can use different methods to measure BUN or report BUN in different units, and what constitutes a normal result can vary from lab to lab.

What does it mean if your Blood urea nitrogen (BUN) result is too low?

Lower BUN levels can indicate:

- Overhydration

- Low-protein diet, malnutrition, or starvation

- Liver failure

Depending on your test results, your doctor may also run other tests to confirm a diagnosis or recommend treatments. Other markers included in the CMP panel may provide helpful information for understanding the significance of low BUN.

Working with your doctor is the best way to understand the significance of your BUN test. The American Board of Internal Medicine lists a typical reference range for BUN as 8 to 20 mg/dL. However, this range is not universal. Labs can use different methods to measure BUN or report BUN in different units, and what constitutes a normal result can vary from lab to lab.

Your doctor can discuss your BUN levels and how they relate to your overall health, symptoms, and other test measurements. This is important because BUN levels alone are not a consistent predictor of kidney function.

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