Blood Urea Nitrogen (BUN)

Having a small amount of urea nitrogen in your blood is normal. But higher-than-normal BUN levels may be a sign that your kidneys aren't working well. If you have kidney disease, this waste material can build up in your blood and may lead to serious health problems, including high blood pressure, anemia, and heart disease.

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

Normal BUN levels can vary, but generally a high blood urea nitrogen level is a sign that your kidneys are not working well. Levels can also vary with sex and usually rise with age.

High BUN can be caused by kidney problems, but also by dehydration, a high-protein diet, bleeding inside the digestive tract, certain medicines (such as some antibiotics and steroids), heart failure, or anything that lowers blood flow to the kidneys.

Low BUN is less common but can be seen with severe liver disease, eating very little protein, drinking large amounts of fluid, or pregnancy.

Because BUN is affected by so many things outside the kidneys, your provider often looks at it together with your creatinine and the BUN-to-creatinine ratio to better understand your kidney health and hydration.

BUN levels can be higher or lower than normal for a variety of reasons unrelated to kidney function. To learn what your results mean, talk to your provider.