Urea Nitrogen (BUN)
A blood urea nitrogen (BUN) test measures the amount of urea nitrogen in the blood. Urea nitrogen is a waste product that develops when the body breaks down protein. It forms in the liver and then travels through the blood to the kidneys, which filter it out so that it leaves the body in urine. A small amount of urea nitrogen in the blood is normal.
Why it matters: A BUN test is often done during a regular checkup, and it is usually part of a comprehensive metabolic panel or basic metabolic panel. It may also be used for someone at risk of chronic kidney disease, whose early stages usually have no symptoms, and as part of routine testing in late-stage chronic kidney disease. High BUN levels may suggest the kidneys are not working as well as expected, though levels can be elevated even when the kidneys are working properly. Providers do not use BUN on its own to define kidney failure: if BUN is higher than expected and creatinine is also high, kidney failure is likely. Creatinine is a waste product from muscle tissue breakdown, and BUN can rise while creatinine stays low when someone does not drink enough water. Low BUN levels are not common. Abnormal results do not necessarily mean chronic kidney disease, and if chronic kidney disease is suspected further tests may follow, including imaging such as a kidney ultrasound.
Source · Cleveland Clinic — what it is, why it matters, what moves it, related conditions
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What is Urea Nitrogen (BUN)?
A blood urea nitrogen (BUN) test measures the amount of urea nitrogen in the blood. Urea nitrogen is a waste product that develops when the body breaks down protein. It forms in the liver and then travels through the blood to the kidneys, which filter it out so that it leaves the body in urine. A small amount of urea nitrogen in the blood is normal.
What might a high or low Urea Nitrogen (BUN) mean?
A BUN test is often done during a regular checkup, and it is usually part of a comprehensive metabolic panel or basic metabolic panel. It may also be used for someone at risk of chronic kidney disease, whose early stages usually have no symptoms, and as part of routine testing in late-stage chronic kidney disease. High BUN levels may suggest the kidneys are not working as well as expected, though levels can be elevated even when the kidneys are working properly. Providers do not use BUN on its own to define kidney failure: if BUN is higher than expected and creatinine is also high, kidney failure is likely. Creatinine is a waste product from muscle tissue breakdown, and BUN can rise while creatinine stays low when someone does not drink enough water. Low BUN levels are not common. Abnormal results do not necessarily mean chronic kidney disease, and if chronic kidney disease is suspected further tests may follow, including imaging such as a kidney ultrasound.
What is the typical reference range for Urea Nitrogen (BUN)?
The general-population reference range shown here is 2.1 – 7.1 mmol/L. Reference ranges describe the general population and are not a personal target — discuss your results with your physician.
What can affect Urea Nitrogen (BUN)?
It may be higher with: Eating a lot of protein, Dehydration, Aging, since infants and children have lower levels, Certain medicines, including carbamazepine, methotrexate and tetracycline, Burns, A blockage in the urinary system that prevents urination, Stress, Heart attack, Gastrointestinal bleeding. It may be lower with: Not eating enough protein, Eating more low-protein foods, which can bring down high levels, Having a small body type, Having too much water in the body, known as overhydration, Liver disease.
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