High BUN: Causes, Reference Ranges, and Test Results

What BUN Measures

A blood urea nitrogen test measures the quantity of urea nitrogen circulating in your bloodstream. Urea nitrogen forms as a waste product during the breakdown of proteins. The blood carries this waste to your kidneys, which remove it from your body when you urinate.

This laboratory test may also be called a serum BUN, a urea nitrogen test, or part of a BUN to creatinine ratio. A BUN test is often ordered as part of a comprehensive metabolic panel (CMP) or a basic metabolic panel (BMP). Healthcare providers order it during routine checkups or to monitor and diagnose kidney diseases.

A provider may also order the test to monitor kidney disease treatments. In addition, providers use it to screen people at higher risk for kidney problems. High-risk groups include individuals with diabetes, high blood pressure, heart disease, obesity, a family history of kidney problems, people over age 60, and people who use tobacco or smoke.

Causes of High BUN

Higher-than-normal BUN levels may be a sign that the kidneys are not working well. When kidney disease is present, this waste material builds up in the blood instead of being removed properly.

Several other medical and dietary factors can also cause BUN levels to rise above normal. Dehydration can push BUN up, as can burns or consuming a high-protein diet. Furthermore, taking certain medicines or experiencing a recent heart attack can lead to high blood urea nitrogen results.

Reasons for Low BUN Levels

Blood urea nitrogen can also drop below normal levels under certain circumstances, including malnutrition or liver disease. Another reason for low BUN is a lack of protein in your diet.

If your lab report shows lower-than-normal levels, talk to your healthcare provider to learn what your results mean. You can also read our dedicated guide on low blood urea nitrogen to explore these lower patterns.

BUN Reference Ranges by Lab and Age

Normal BUN levels can vary between different testing facilities, sexes, and age groups. Because reference ranges depend on the specific laboratory analyzing your sample, you should always check the reference range printed directly on your lab report.

BUN levels also vary according to your sex. In addition, blood urea nitrogen levels usually increase with age. An abnormal result does not always indicate a medical condition that requires treatment.

Other Kidney Tests Read With BUN

A BUN test is only one measure of overall kidney function. When a provider suspects kidney disease, they usually evaluate BUN alongside other blood tests to obtain an accurate clinical assessment.

A healthcare provider may also measure creatinine, which is another waste product that the kidneys remove from the blood. Providers also assess glomerular filtration rate (GFR), which provides an estimate of how well the kidneys filter blood. You can learn how these markers are interpreted together in our guide to the kidney function test and our review of the eGFR test.

Blood Urea Nitrogen Test Procedure

A blood urea nitrogen test requires a blood sample taken from a vein in your arm. The procedure usually takes under five minutes to complete. The blood draw carries very little risk, though you might feel slight pain or develop mild bruising where the needle entered.

Most patients do not need any special preparation before taking a BUN test. However, if your provider ordered other tests on the same blood sample, you may need to fast for several hours beforehand.

Next Steps for High BUN Results

An elevated BUN result indicates that you should review the reading in context with your healthcare provider. Your provider can help determine whether kidney problems, dehydration, a high-protein diet, medications, or another factor caused the increase. If kidney disease is suspected, a provider may also measure creatinine and GFR.

You can review our guide on blood test results to prepare questions for your appointment. Following up with your provider ensures that you interpret your lab numbers safely and accurately.