Introduction to Blood Urea Nitrogen (BUN)
BUN is an abbreviation for Blood Urea Nitrogen. It is a vital parameter when checking whether the kidneys are functioning normally or not. It is also an indicator of a person’s overall health status. Urea nitrogen is a waste product that the kidneys remove. 
If the kidneys are not functioning well then there will be an accumulation of this waste product in the blood causing a variety of complications of which cardiac conditions and hypertension is the most common.  This article provides more insight as to what is Blood Urea Nitrogen (BUN), how is it measured, what is the significance of normal, high, and low BUN levels and what needs to be done to keep the BUN levels normal.
What is BUN (Blood Urea Nitrogen)?
As stated above, BUN stands for blood urea nitrogen. One of the main waste products formed when we eat food is urea nitrogen. This is formed when the liver breaks down the proteins present in the food and releases urea and nitrogen in the blood.
The Blood Urea Nitrogen (BUN) then passes to the kidneys where most of it is eliminated in the form of urine. Thus, checking the levels of BUN in the blood is a vital indicator to check the health of the kidneys. If the kidneys are not functioning normally, the levels of Blood Urea Nitrogen (BUN) will be high, which makes BUN levels important. 
Physicians tend to order Blood Urea Nitrogen (BUN) levels during a routine checkup or if they suspect that something is wrong with the liver or the kidneys. To check the levels of BUN a sample of blood will be taken from the veins of the patient and sent to laboratory for analysis. 
It should be noted that there is some difference between BUN levels and the levels of urea in the blood. The BUN test will measure amount of nitrogen present in the urea whereas a urea test will measure the amount of urea present n the blood. In terms of numbers, the relation between Blood Urea Nitrogen (BUN) and Urea can be understood as the BUN level being approximately one half of the level of blood urea. 
Levels of Bun and Its Significance
Normal BUN levels are different for infants, children, and adults. In infants and children, it ranges between 5 to 18 mg/dL and in adults it is between 7 and 20 mg/dL. 
The exact levels tend to vary from individual to individual depending on the age, gender, and diet. There are also certain other factors which influence the BUN levels in a person, which are illustrated in detail below. 
High BUN Levels: This is an indicator that something is not right with either the kidneys or the liver, and the BUN is getting accumulated in the blood. Elevated BUN levels is medically termed as azotemia and there are various reasons for this. 
It can signify a medical condition that leads to rapid breakdown of proteins. It can be a cardiac condition like CHF or something common like plain dehydration due to exposure to excessive heat. Elevated BUN can also be caused by certain medications, especially NSAIDs that are frequently used to treat chronic pain and inflammation and certain classes of antibiotics. [3, 5]
This is because these drugs tend to interfere with the blood flow to the kidneys, thereby interfering with their ability to clear BUN from the body, causing the elevation of BUN levels. Additionally, diuretics which are frequently given to people with cardiac condition or have those who have problems with fluid buildup in the body increases urea in the blood thereby increasing the BUN levels. [3, 5]
Elevated BUN levels can also be caused due to gastrointestinal bleeding or urinary tract obstruction. In all of these cases once the underlying cause is identified and treated the levels of BUN come back to normal pretty quickly. Dietary intake also has a role when it comes to elevated BUN levels. Foods high on protein cause the liver to produce excess urea which in turn elevates the level of BUN in the body. 
Some female who are pregnant tend to have fluctuations in their BUN levels. This is because of the changes in protein metabolism that take place in the body of a pregnant female which in turn causes fluctuations in the BUN levels. [3, 4]
Low BUN Levels: This is usually a sign that something is wrong with the liver as it is not producing enough urea. It can also be a sign that the person is overhydrated or is malnourished. A malnourished person will have reduced protein breakdown due to poor oral intake of food which in turn lowers the Blood Urea Nitrogen (BUN) levels. Once a cause for the depleted BUN levels is identified and treated the levels of BUN tend to normalize.
How to Test Blood Urea Nitrogen (BUN) Levels?
BUN levels are measured by doing a blood test. There are no special instructions for the patient to follow before blood test for BUN levels. Some people may be asked to limit fluid intake a few hours before the test and if the healthcare provider needs more information to be collected from the blood test aside from BUN like LFTs or lipids then the patient will be asked to give blood sample early in the morning on an empty stomach. 
For BUN levels the blood is drawn using a needle from the vein. This usually takes less than 5 minutes and the sample is then sent to the laboratory for investigation. It generally takes a few days before the results of the Blood Urea Nitrogen (BUN) test are known. 
Sometimes the physician may also ask for BUN levels in the urine. This will be done by conducting a urine test in which the patient will be asked to submit a sample of the urine to be sent for analysis. 
BUN or Blood Urea Nitrogen is an important clinical parameter which gives valuable insight into the functioning of the kidneys and liver. A normal BUN level is also an indicator of good health of a person. [1, 2]
The food that a person eats contains proteins which are then broken down in the liver to produce urea and nitrogen. This is then absorbed in the blood and reaches kidneys from where most of it is eliminated by the body. If by any chance the liver or the kidneys are not functioning well then the urea and nitrogen starts accumulating causing a variety of problems. [1, 2]
For a normal functioning kidneys, the normal levels of BUN in children is between 5 and 18 mg/dL and in adults it is between 7 and 20 mg/dL. Elevated BUN levels may point towards a kidney dysfunction, cardiovascular problem, or dehydration. 
There are also certain medications especially NASAIDs, diuretics, and antibiotics which also have a tendency to increase the BUN levels as they facilitate increased production of urea by the liver. 
Just as is the case with elevated BUN, depleted BUN levels also indicate an underlying medical condition involving the liver. This is generally caused when there is reduced breakdown of protein by the liver resulting in less production of urea. This usually happens in cases of overhydration or malnutrition. 
Blood Urea Nitrogen (BUN) levels are tested by taking a blood sample from the vein. In case there is an abnormality on the Blood Urea Nitrogen (BUN) levels the physician will look for an underlying cause by further investigations and if any other symptoms are evident. In majority of the cases when the underlying cause is identified and treated there is rapid normalization of the BUN levels.