Blood urea nitrogen (BUN)

check icon 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.

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FAQs:

Is Blood urea nitrogen (BUN) the same as urea?

Blood Urea Nitrogen (BUN) and urea are related but not exactly the same. Here’s the distinction?

Urea:

- Urea is a chemical compound with the formula (NH2)2CO.

- It is a waste product formed in the liver through the urea cycle, which converts ammonia, a byproduct of protein metabolism, into urea.

- Urea is then transported to the kidneys via the bloodstream, where it is excreted in urine.

Blood Urea Nitrogen (BUN):

- BUN is a measurement of the nitrogen portion of urea in the blood.

- The BUN test is used to evaluate kidney function. Higher levels of BUN indicate that the kidneys may not be filtering urea properly.

- The term 'nitrogen' in BUN refers specifically to the nitrogen atom that is part of the urea molecule.

In summary, BUN is a specific measure of the nitrogen content in urea found in the blood, while urea itself is the actual compound that contains nitrogen.

References:

Testing.com: OneCare Media; c2022. Blood Urea Nitrogen - https://www.testing.com/tests/blood-urea-nitrogen-bun/

Lyman JL. Blood urea nitrogen and creatinine. Emerg Med Clin North Am - https://www.ncbi.nlm.nih.gov/pubmed/3516645

Mayo Clinic. Mayo Foundation for Medical Education and Research; c1998-2022. Blood Urea Nitrogen (BUN) Test - http://www.mayoclinic.org/tests-procedures/blood-urea-nitrogen/home/ovc-20211239

Mayo Clinic, Mayo Foundation for Medical Education and Research; c1998-2022. Chronic Kidney Disease - https://www.mayoclinic.org/diseases-conditions/chronic-kidney-disease/symptoms-causes/syc-20354521

National Heart, Lung, and Blood Institute - U.S. Department of Health and Human Services; Blood Tests - https://www.nhlbi.nih.gov/health-topics/blood-tests

National Institute of Diabetes and Digestive and Kidney Diseases: U.S. Department of Health and Human Services; Kidney Disease - https://www.niddk.nih.gov/health-information/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 - https://www.niddk.nih.gov/-/media/Files/Health-Information/Health-Professionals/Kidney-Disease/Your_Kidney_Test_Results_EN.pdf

National Kidney Foundation. New York: National Kidney Foundation Inc., c2022. About Chronic Kidney Disease - https://www.kidney.org/kidneydisease/aboutckd

National Kidney Foundation. New York: National Kidney Foundation Inc., c2022. Understanding Lab Values - https://www.kidney.org/kidneydisease/understandinglabvalues

University of Rochester Medical Center: University of Rochester Medical Center; c2022. Health Encyclopedia: Blood Urea Nitrogen - https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=urea_nitrogen_serum

A.D.A.M. Medical Encyclopedia. Basic Metabolic Panel - https://medlineplus.gov/ency/article/003462.htm

A.D.A.M. Medical Encyclopedia. BUN – Blood Test - https://medlineplus.gov/ency/article/003474.htm

A.D.A.M. Medical Encyclopedia. Comprehensive Metabolic Panel - https://medlineplus.gov/ency/article/003468.htm

American Board of Internal Medicine. ABIM Laboratory Test Reference Ranges - https://www.abim.org/Media/bfijryql/laboratory-reference-ranges.pdf

Gounden V, Bhatt H, Jialal I. Renal Function Tests. In: StatPearls - https://www.ncbi.nlm.nih.gov/books/NBK507821/

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 - https://www.ncbi.nlm.nih.gov/books/NBK305/

Inker LA, Perrone RD. Assessment of Kidney Function. In: Sterns RH, ed. UpToDate - https://www.uptodate.com/contents/assessment-of-kidney-function

MedlinePlus: National Library of Medicine. BUN (Blood Urea Nitrogen) - https://medlineplus.gov/lab-tests/bun-blood-urea-nitrogen/

MedlinePlus: National Library of Medicine. Comprehensive Metabolic Panel (CMP) - https://medlineplus.gov/lab-tests/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?

A high blood urea nitrogen (BUN) level means your kidneys may not be clearing urea - a waste product from protein metabolism - as efficiently as they should, or that your body is producing excess urea due to increased protein breakdown. BUN is a useful signal, but it is not specific on its own. Doctors always interpret it alongside other markers to understand the underlying cause.

Why BUN Can Be Elevated

BUN can rise for many reasons, including:

  • Dehydration (one of the most common causes)

  • High-protein diet or increased protein breakdown

  • Gastrointestinal bleeding

  • Severe infection, burns, trauma, or surgery

  • Medications such as corticosteroids, tetracyclines, or diuretics

  • Heart failure or conditions that reduce blood flow to the kidneys

  • Kidney disease, both acute and chronic

Symptoms of Significantly High BUN

If BUN rises to very high levels, you may notice:

  • Swelling in the legs, hands, or face

  • Fatigue or weakness

  • Itching or dry skin

  • Muscle cramps

  • Nausea or poor appetite

  • Changes in urine frequency or volume
    These symptoms usually appear later in the course of kidney dysfunction.


How BUN Correlates With Other Markers

1. Creatinine

Creatinine is the most important companion marker.

  • Both BUN and creatinine elevated: Suggests kidney dysfunction (acute or chronic), reduced kidney perfusion, or severe dehydration.

  • High BUN with normal creatinine: More likely due to dehydration, high protein intake, GI bleeding, steroid use, or increased catabolism.

2. BUN/Creatinine Ratio

This ratio helps identify why BUN is high.

  • >20:1 (high ratio): Often dehydration, GI bleed, heart failure, shock, or high-protein intake.

  • Normal ratio (10–20:1) with both elevated: Likely intrinsic kidney disease.

  • Low ratio: Possible liver disease, malnutrition, or overhydration.

3. eGFR (Estimated Glomerular Filtration Rate)

  • Low eGFR + high BUN strongly suggests reduced kidney filtration (CKD or AKI).

  • Normal eGFR + high BUN typically points to non-kidney causes such as dehydration, protein load, GI bleeding, or medications.

4. Electrolytes (Sodium, Potassium, Chloride)

Kidney dysfunction often affects electrolytes:

  • High potassium (hyperkalemia) + high BUN: Possible advanced CKD or reduced kidney perfusion.

  • Low sodium (hyponatremia) may occur in heart failure or overhydration.

  • Metabolic acidosis (low CO2/bicarbonate): Common in chronic or acute kidney injury.

5. Liver Markers (ALT, AST, Albumin)

Because urea is produced in the liver:

  • High BUN + abnormal liver markers may indicate combined liver–kidney involvement.

  • Low BUN + poor liver function suggests reduced urea production due to liver dysfunction.

6. Urine Tests (Urinalysis, Albumin, Protein, Microalbumin/Creatinine Ratio)

These can help confirm kidney disease:

  • Proteinuria or albuminuria + high BUN: Damage to kidney filtering units (glomeruli).

  • Hematuria: Possible glomerulonephritis or infection.

7. CBC (Complete Blood Count)

  • Anemia + high BUN: Suggests chronic kidney disease, as the kidneys also help regulate red blood cell production.

  • High BUN + low hemoglobin may also indicate GI bleeding.


Treatment Options for High BUN

Treatment depends entirely on the cause—not just the number. Below are common approaches:

1. Dehydration

  • Increase water intake

  • IV fluids if severe

  • Avoid excess caffeine or diuretics unless medically needed

Expected effect: BUN often normalizes quickly.

2. High-Protein Diet or Excess Protein Breakdown

  • Reduce excessive protein intake

  • Address causes of catabolism (infection, fever, trauma, hyperthyroidism)

  • Review supplements (e.g., protein shakes, creatine)

3. Gastrointestinal Bleeding

  • Identify and treat the bleeding source

  • Medications to reduce stomach acid

  • Endoscopic treatment if needed

Note: GI bleeding can dramatically raise BUN even when kidneys are normal.

4. Kidney Disease (Acute or Chronic)

  • Control blood pressure (ACE inhibitors/ARBs are often first-line)

  • Tight blood sugar control if diabetic

  • Manage electrolytes

  • Reduce sodium intake

  • Treat underlying causes such as obstruction, autoimmune disease, or infection

  • Avoid nephrotoxic medications (NSAIDs, contrast dyes where possible)

5. Heart Failure or Reduced Kidney Blood Flow

  • Optimize heart failure treatment (diuretics, ACE inhibitors, beta blockers)

  • Reduce fluid overload

  • Manage blood pressure and vascular health

6. Medication-Related BUN Elevation

If steroids, tetracyclines, diuretics, or other drugs are contributing:

  • Adjust dosage or switch medications if appropriate

  • Discuss with your doctor before making any changes

7. Severe Kidney Failure

If kidney function declines significantly:

  • Dialysis may be necessary

  • Address anemia, bone health, and electrolyte imbalances caused by CKD


Why Tracking BUN Matters

Monitoring BUN over time - alongside creatinine, eGFR, electrolytes, and urinalysis - can:

  • Detect early kidney stress

  • Distinguish hydration issues from true kidney disease

  • Guide dietary changes and medication adjustments

  • Help evaluate the severity of existing kidney conditions

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

What Does a Low BUN (Blood Urea Nitrogen) Level Mean?

Lower-than-normal BUN levels can be caused by several factors, including:

  • Overhydration (excess fluid intake)

  • Low-protein diet, malnutrition, or prolonged starvation

  • Liver dysfunction or liver failure

Depending on your specific test results, your healthcare provider may recommend additional testing to confirm a diagnosis or determine the best course of treatment. Other values within a Comprehensive Metabolic Panel (CMP) can provide valuable context when interpreting a low BUN level.

It's important to understand that BUN levels are just one piece of a larger puzzle. They should always be interpreted in relation to your overall health, symptoms, and other lab results. While the typical reference range for BUN is about 7 to 20 mg/dL, this can vary slightly between laboratories based on testing methods.

BUN alone is not a reliable indicator of kidney function. It is typically assessed alongside other markers—especially creatinine—to get a clearer picture of how well the kidneys are working. Because many factors can influence BUN, further evaluation is often necessary to determine the root cause of an abnormal result.

For the most accurate interpretation and personalized advice, discuss your BUN results with your doctor.

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