Urea/BUN (Blood Urea Nitrogen)

What are urea and BUN?

Urea is the main waste product formed when the liver renders harmless the ammonia released during the breakdown of proteins in the body. Urea produced in the liver is carried in the blood to the kidneys, where it is filtered and excreted from the body in the urine. For this reason the blood urea level is affected both by the kidneys' filtering function and by the liver's urea production and the amount of protein you eat.

BUN, or blood urea nitrogen, refers only to the nitrogen portion of the same molecule; urea and BUN give the same information in different units. Some laboratories report urea, some report BUN, and some report both. The measurement is made on a serum sample using methods involving an enzyme that breaks down urea, and the result is usually evaluated together with creatinine. Because urea is more affected than creatinine by nutrition and fluid status, it is less reliable on its own as an indicator of kidney function.

When is a urea and BUN test ordered?

Urea and BUN are core parts of kidney function testing and are frequently ordered together with creatinine in routine health check-ups. The ratio of the two values can give a clue as to whether an elevation is due to causes outside the kidneys or to the kidney itself. This test also guides doctors in situations such as fluid loss, heart failure or gastrointestinal bleeding. Regular measurements may also be requested for people being treated with medications that can harm the kidneys.

• Regular assessment of kidney function together with creatinine
• Low fluid intake, prolonged vomiting, diarrhea or heavy sweating
• Fatigue, swelling, decreased urine output, nausea or itching
• Suspected heart failure or gastrointestinal bleeding
• Monitoring of chronic kidney disease and dialysis treatment

How do you prepare for a urea and BUN test?

Fasting does not significantly change the result of this test, so you can give your blood at any time of day. Because a very high-protein meal can temporarily raise urea levels, it is recommended that you avoid excessive amounts of meat or products such as protein powder the evening before the test and stick to your usual eating habits. Dehydration can also raise the value, so it is important not to change your daily fluid intake.

Because cortisone-type medications, some antibiotics and diuretics can affect urea levels, you should share your medication list, but do not make any changes to your treatment on your own. For people whose kidneys are monitored regularly, it is helpful to repeat the test under similar conditions so that the results can be compared. You can have your follow-up tests done each time at your home or workplace, with a blood sample taken by a nurse.

What does a urea and BUN result mean?

High urea or BUN may suggest that the kidneys are not removing waste products adequately; acute kidney injury and chronic kidney disease are among the leading causes. However, fluid loss, reduced blood flow to the kidneys due to heart failure, gastrointestinal bleeding, a high-protein diet, illnesses with fever and cortisone use can also raise the value without any kidney disease. Obstructions in the urinary tract can also cause an increase.

Low urea, on the other hand, is usually not significant and can be seen with a very low-protein diet, high fluid intake, pregnancy or severe liver disease in which the liver's urea production is reduced. When interpreting the result, your doctor also looks at the creatinine value and the ratio of the two values. Separate reference ranges are used for urea and BUN, and these ranges may vary by age, sex and laboratory; both ranges of the laboratory that runs your test are shown on this page. The significance of your urea result is assessed by your doctor, taking into account your symptoms and other kidney tests.