Blood Urea Nitrogen (BUN) · What Is It, Causes, Treatment, and More

Published: Aug 21, 2025
Author: Anna Hernández, MD•
Editor: Alyssa Haag, MD•
Editor: Józia McGowan, DO•
Editor: Kelsey LaFayette, DNP, ARNP, FNP-C
Illustrator: Jessica Reynolds, MS•
Copyeditor: Stacy Johnson, LMSW
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What is urea nitrogen?

Urea nitrogen is one of the most significant end-products of protein metabolism. It’s 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 converted into ammonia, which is highly toxic to cells. Ammonia is then converted into urea through the urea cycle to allow the kidneys to excrete it in the urine.  

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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 aren’t working correctly, BUN 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 the cause of renal failure.  

How is the BUN to creatinine ratio interpreted?

The BUN to creatinine (BUN/creatinine) ratio is commonly used to help determine the cause of renal failure. Specifically, it helps distinguish whether the problem is before the kidneys (prerenal), in the kidneys (intrarenal), or after the kidneys (postrenal). 

The normal ratio of BUN to creatinine is around 20 to 1, meaning the blood carries around 20 molecules of urea for every molecule of creatinine. The kidneys handle urea and creatinine a bit differently. Almost all of the urea in the blood is freely filtered by the glomerulus into the kidney tubules, and then around half of it is reabsorbed back into the blood. In states of dehydration or low blood volume, urea reabsorption increases to help the body preserve water. Creatinine, on the other hand, is freely filtered by the kidneys and is not significantly reabsorbed, even in states of dehydration.  

With prerenal kidney injury, there is a decrease in the amount of blood that gets filtered by the kidneys, which decreases the amount of urea and creatinine that gets into the urine. Additionally, part of the little urea that has been filtered gets reabsorbed back in the tubules, so even more urea gets into the blood, resulting in a BUN to creatinine ratio greater than 20:1. 

With intrarenal kidney failure, on the other hand, there is damage to the kidney structure, causing the kidneys to lose their ability to filter both BUN and creatinine proportionally. As a result, the BUN to creatinine ratio stays within the normal range. 

Finally, in postrenal kidney failure, the problem is due to a blockage in urine flow after the kidneys (e.g., kidney stones, enlarged prostate, tumors). This obstruction causes back pressure on the kidneys, which damages their ability to filter and reabsorb substances properly. Damaged tubules lose their ability to reabsorb urea, leading to a disproportionate drop in BUN compared to creatinine. As kidney function declines, creatinine levels accumulate in the blood, but since BUN levels stay relatively lower, the BUN to creatinine ratio drops below 10:1.  

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.  

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.  

Key Takeaways

Definition 

BUN, synthesized in the liver from ammonia, is a waste product created from the metabolism of amino acids. 

BUN Test 

- Tests measuring the amount of BUN in the blood 

- Normal range: 5 to 20 mg/dL or 1.8 to 7.1 mmol/L 

- Aim: assess kidney function  

- Increases when kidney function is impaired  

- Not specific for kidney failure → often combined with creatinine (BUN/creatinine ratio)  

BUN/Creatinine 

- Normal BUN/creatinine → 20:1  

- Prerenal kidney injury → greater than 20:1 

- Intrarenal kidney failure → within range 

- Postrenal kidney failure → below 10:1  

High BUN 

- Causes:  

     - Impaired kidney function 

     - High protein diet  

     - Dehydration  

     - Congestive heart failure  

     - Increased protein breakdown 

- Treatment:  

     - If otherwise healthy → decrease protein intake, stop protein supplements, hydration  

     - Kidney disease → manage underlying disorders or complications 

Low BUN  

- Causes:  

     - Low-protein diet 

     - Malnutrition  

     - Starvation  

     - Strict vegetarian diet 

     - Overhydration  

     - Pregnancy (transient increase in plasma volume)  

     - Liver disease  

- Treatment:  

     - Increase protein intake, if needed  

     - Liver disease → address underlying cause  

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References

Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier - Health Sciences Division; 2020.  


Koeppen BM, Stanton BA. Renal Physiology: Mosby Physiology Monograph Series (with Student Consult Online Access). 5th ed. Mosby; 2012. 


Salazar JH. Overview of urea and creatinine. Lab Med. 2014;45(1):e19-e20. doi:10.1309/lm920sbnzpjrjgut 


Weiner ID, Mitch WE, Sands JM. Urea and ammonia metabolism and the control of renal nitrogen excretion. Clin J Am Soc Nephrol. 2015;10(8):1444-1458. doi:10.2215/CJN.10311013