BUN (Blood Urea Nitrogen) Test

Definition and Use of the BUN Test

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Blood urea nitrogen (BUN) is a blood test performed as a marker of kidney function. It is part of the basic metabolic panel done for health screening. It is also used to monitor the progression of kidney failure. BUN is produced by the breakdown of protein and usually cleared from the blood by the kidneys. A higher than normal value may show impaired kidney function.

BUN Normal Values

  • The normal range for BUN is 6 to 20 mg/dL, although the range may vary a little from lab to lab. Refer to the range for your lab.
  • A high BUN may be caused by impaired kidney function, congestive heart failure, urine flow obstruction, shock, recent heart attack, dehydration, a high protein diet, or gastrointestinal bleeding.
  • A low BUN is not common but can be found incidentally with malnutrition, severe liver disease or in overhydration.

What Is Blood Urea Nitrogen - BUN?

BUN is a waste product of cell metabolism. You get protein from the food you eat, and it enters the bloodstream from the intestines to be used by cells throughout your body. Your cells break protein down into amino acids to build back up into the proteins they need for various processes. This produces nitrogen-containing ammonia as a byproduct and it is excreted it into the bloodstream. The liver transforms ammonia into urea to make it less toxic and sends the urea out into the bloodstream. Urea is filtered out of the blood by the kidneys.

If all is going well, there is a continuous amount of urea being produced and being excreted by the kidneys into the urine. The BUN level in the blood is, therefore, stable. If the kidneys are damaged and not functioning properly, urea and the nitrogen it contains are not filtered fully from the blood. A BUN of over 20 mg/dl is an indicator of decreased kidney function.

How Is the BUN Test Done?

The BUN test is part of the typical Chem 7 blood chemistry test or basic metabolic panel. This group of tests includes glucose, BUN, creatinine, carbon dioxide, sodium, potassium, and chloride. It is typical that this panel is run during health screenings and in monitoring diabetes management. They are all performed on the same tube of blood, typically analyzed in the lab in an instrument set up to test all of them at the same time.

BUN is also part of the renal panel of tests. This panel includes many other values that assess how your kidneys are functioning.

BUN may also be ordered with the creatinine test to look at the ratio between the two. A high ratio may be due to a condition causing decreased blood flow to the kidneys, such as congestive heart failure or dehydration.

BUN in Diabetes Management

Kidney failure is a complication of diabetes. The BUN (blood urea nitrogen) level in the blood is used to monitor the progression of kidney failure. BUN may also be monitored if you are given drugs that may impair kidney function.

View Article Sources
  • Blood Urea Nitrogen, Lab Tests Online, October 29, 2015. American Association for Clinical Chemistry.
  • Laura J. Martin, MD, BUN - blood test, MedlinePlus, 4/30/2015. U.S. National Library of Medicine.