Blood Urea Nitrogen (BUN)

What Is It, Causes, Treatment, and More

Author:Anna Hernández MD

Editors:Alyssa Haag,Józia McGowan, DO,Kelsey LaFayette, DNP, ARNP, FNP-C

Illustrator:Jessica Reynolds, MS

Copyeditor:Stacy Johnson, LMSW

What is urea nitrogen?

Urea nitrogen is one of the significant end-products of protein metabolism. It is synthesized in the liver from ammonia, a waste product created from the metabolism of amino acids. Amino acids are the building blocks of proteins and are made of a nitrogen group, a carbon skeleton, and a side chain unique to each amino acid. When amino acids are metabolized, nitrogen is formed into ammonia, which is highly toxic to cells. Ammonia is converted into urea through the urea cycle so the kidneys can excrete it. 

Sad kidney with low BUN level due to low protein intake.

What is a blood urea nitrogen (BUN) test?

The blood urea nitrogen (BUN) test measures the amount of urea nitrogen in the blood. Urea nitrogen only reflects the nitrogen component of urea and not the whole urea molecule. In healthy individuals, normal BUN levels range between 5 to 20 mg/dL or 1.8 to 7.1 mmol/L; however, reference ranges may vary between laboratories due to changes in assay methods.

A BUN test is typically obtained with other blood tests to assess kidney function. When the kidneys are not working correctly, blood urea nitrogen increases as less urea gets excreted in the urine. However, urea production can vary independently of kidney function, so a BUN test is less practical than creatinine to assess kidney health. BUN is often combined with creatinine to obtain the ratio of BUN to creatinine, which can help determine renal failure

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What causes a high BUN level?

A high BUN level is generally caused by impaired kidney function; however, it can also be due to a high-protein diet; dehydration; congestive heart failure; or increased protein breakdown caused by gastrointestinal bleeding, trauma, or corticoid therapy.

How are high BUN levels treated?

Treatment of high BUN levels depends on the underlying cause. In otherwise healthy individuals, eating less protein in the diet, stopping protein supplements, and drinking plenty of fluids may help to decrease BUN levels. Alternatively, when high BUN levels are due to kidney disease, treatment may include management of any underlying disorders (e.g., hypertension or diabetes mellitus) and treatment of complications (e.g., electrolyte abnormalities, accumulation of toxins in the body, and bone abnormalities). Dialysis or a kidney transplant might be needed as renal replacement therapy in severe situations.

What causes a low BUN level?

A low BUN level can be caused due to a variety of conditions. For example, a low-protein diet, like a strict vegetarian diet; malnutrition; or starvation, can decrease protein intake, thereby decreasing protein breakdown and, ultimately, urea production. Alternatively, a low BUN level may be caused by overhydration or during pregnancy due to a transient increase in plasma volume. Finally, a low BUN level may be seen in individuals with liver disease due to decreased urea production. 

How are low BUN levels treated?

Low BUN levels are generally not a cause for concern and treatment depends on the underlying cause. In most cases, low BUN levels are related to overhydration, a low-protein diet, or pregnancy and do not require any treatment besides increasing protein intake, if needed. In cases where low BUN levels are caused by underlying liver disease, treatment will depend on the exact underlying cause of the disease. It may include lifestyle modifications like stopping alcohol use or losing weight.

What are the most important facts to know about BUN levels?

Blood urea nitrogen (BUN) is a product of protein metabolism that gets excreted in the urine. BUN levels are often tested as a measure of kidney function and can help to determine the cause of renal failure when used to obtain the BUN to creatinine ratio. High BUN levels may indicate kidney disease but may also be present in individuals with a high-protein diet or dehydration. Conversely, low BUN levels may be due to a low-protein diet, overhydration, pregnancy, or liver damage

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Related links

Nitrogen and urea cycle
Renal system anatomy and physiology
Prerenal azotemia

Resources for research and reference

Assessment of kidney function. (n.d.). Retrieved January 20, 2023, from

Burtis, C. A., & Bruns, D. E. (2015). Tietz fundamentals of clinical chemistry and molecular diagnostics. Saunders.

Salazar, J. H. (2014). Overview of urea and creatinine. Laboratory Medicine, 45(1), e19–e20.

Weiner, I. D., Mitch, W. E., & Sands, J. M. (2015). Urea and ammonia metabolism and the control of renal nitrogen excretion. Clinical Journal of the American Society of Nephrology: CJASN, 10(8), 1444–1458.