Blood Urea Nitrogen

What is Blood Urea Nitrogen

Blood urea nitrogen is the nitrogen present in the waste product urea found in the blood. Urea is the waste product of protein breakdown carried out in the liver; urea is excreted to the bloodstream, after which it is sent to the kidneys to be eliminated through the urine. The levels of urea nitrogen can be assessed when it is in the bloodstream by obtaining blood samples. These levels are indicative of kidney (or renal) health but they can also help identify other diseases and damage to body systems such as the respiratory, the digestive and the circulatory systems.

Elimination of Protein

The elimination of protein from the body goes through a few steps. First, proteins reach the liver, where they are broken down by enzymes. Aminoacids—the “pieces” that form proteins—are used by our cells for endless purposes. In the process of breaking down proteins, urea is created as a waste product; urea is then secreted by the liver to the bloodstream and eventually reaches the kidneys, whose function is to separate urea from the blood in order to excrete urea into the urine. If the kidneys are malfunctioning, the urea will not be separated from the blood and the blood urea nitrogen will hence be higher.

Blood Urea Nitrogen Test

A blood urea nitrogen test can be done for several reasons: to determine whether a kidney is healthy, to see whether a malfunctioning kidney is worsening or improving, to find out whether treatment of a kidney disease is working, and to analyze other factors such as severe dehydration, the latter of which is usually carried out together with another test called creatinine test. The results of a blood urea nitrogen test can also provide information about the health state of the respiratory, the digestive and the circulatory systems.

Reference Range of Blood Urea Nitrogen

As with multiple clinical assessments, the levels of urea nitrogen in the blood have been well studied and a reference range has been established to determine whether the kidney is healthy or not, i.e. whether the blood urea nitrogen levels fall within normal values. In adults, normal values are considered between 10 and 20 mg/dL, or between 3.6 and 7.1 mmol/L. In children, the healthy reference range is from 5 to 18 mg/dL or between 1.8 and 6.4 mmol/L. When the blood urea nitrogen values fall out of the reference range, it is very likely that there is some kind of abnormality in the body, most probably related to the kidney.

High Levels of Blood Urea Nitrogen

Several factors can give rise to high levels of blood urea nitrogen, which are levels above the reference value. The most probable factor is kidney damage. Kidney disease or kidney injury can in turn originate from multiple sources such as from high blood pressure and from diabetes. Other factors that cause high blood urea nitrogen may involve the respiratory, circulatory and gastrointestinal tracts, for instance internal bleeding or tissue damage. Additional factors include dehydration and a high-protein diet (more protein leads to more protein breakdown and consequently to more urea in the blood). Some medicines can also instigate high levels of blood urea nitrogen.

Low Levels of Blood Urea Nitrogen

Low levels of blood urea nitrogen, i.e. levels below the reference values, are not as indicative of kidney issues as high levels. Low levels are instead usually associated with liver damage, malnutrition, overhydration or a low-protein diet. Some medicines may also induce low levels of blood urea nitrogen.


1. What is blood urea nitrogen?
A. A type of nitrogen only found in blood urea.
B. A test to assess the levels of nitrogen in urea.
C. The nitrogen from the kidneys that can be measured in blood urea.
D. The levels of nitrogen from protein breakdown found in the blood.

Answer to Question #1
D is correct. After proteins are broken down in the liver, the waste product urea travels in the bloodstream to the kidneys. The nitrogen in urea found in the bloodstream is blood urea nitrogen.

2. What can high levels of blood urea nitrogen indicate?
A. High-protein diet.
B. Kidney disease.
C. Internal bleeding.
D. Tissue damage, for instance in the respiratory tract.
E. All of the above.

Answer to Question #2
E is correct. High levels of blood urea nitrogen can originate from many sources, the most common of which is kidney disease. High-protein diets, internal bleeding, tissue damage and dehydration are other factors that may cause high levels of blood urea nitrogen.

3. What can low levels of blood urea nitrogen indicate?
A. Too much exercise
B. Low-protein diet
C. Dehydration
D. All of the above

Answer to Question #3
B is correct. Low levels of blood urea nitrogen can originate from low-protein diets but also from overhydration, malnutrition and liver damage.


  • Lindenfeld, JA., Schrier, R.W. (2011). Blood Urea Nitrogen: A Marker for Adverse Effects of Loop Diuretics? Journal of the American College of Cardiology, 58, 4.
  • WebMD (2015, November 20). Blood urea nitrogen. Retrieved from:
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