Blood Urea Nitrogen

Blood urea nitrogen, known by the simple initials BUN is the measurement of urea nitrogen levels in the blood. Urea nitrogen is naturally produced by the liver as a waste product of digested protein. After production, urea is transported to the kidneys where it is excreted from the blood stream. The BUN test is therefore conducted to determine how much urea nitrogen is still present in the blood, and this test mainly determines if the kidneys renal functions are working as they should. There are many factors that could bring about high or low levels of BUN. Read on to learn more about blood urea nitrogen test and what abnormal levels mean.

Blood Urea Nitrogen Test

A blood creatinine test could be used to identify the creatinine levels in the blood. This test also determines whether or not the kidneys are fully functional and a high creatinine level could indicate poor kidney function. BUN and creatinine tests are often used together to determine the ratio of blood urea nitrogen to creatinine. The BUN to creatinine ratio ultimately helps the physician to find the cause of the poor kidney function and the causes may vary from dehydration to malnutrition or kidney failure.

Why Is It Done?

There are various reasons as to why a BUN test is conducted and these are:

  • To identify if the kidneys are functioning normally
  • To determine if the patients kidney disease is worsening
  • To verify if kidney disease treatment is working
  • To look for signs of severe dehydration. Dehydration has been known to elevate the levels of BUN to surpass the level of creatinine. Dehydration creates a high ratio of BUN to creatinine. Kidney disease could also elevate the BUN and creatinine levels.

What to Expect

The test involves drawing blood from the patients arm and this blood sample is analyzed to identify the levels of urea nitrogen in the blood. The process is simple and this is not a painful procedure. You can carry on with your normal duties after the blood is drawn.

How to Prepare

Like we mentioned earlier, urea nitrogen is a byproduct of the digestion of protein. You need to stay away from meat or other sources of protein for at least 24 hours before you have the BUN test.


  • Drawing blood from the arm’s vein comes with very little complications. The risks that you could experience following this test include a small bruise at the injection site. However, this could be minimized by placing pressure for a few minutes after the blood has been drawn.
  • Sometimes you could experience continuous bleeding and this is mainly common in persons who have bleeding disorders. Blood thinners and medications such as warfarin and aspirin also thin the blood and make you more susceptible to bleeding. If you have clotting problems or are taking any blood thinners, it is important to let your doctor know before undergoing the test.
  • In very rare cases, the vein could swell. This condition is known as phlebitis and can be easily treated with warm compression.

What Affects the Test

There are several factors that could deter you from taking the tests. Sometimes, the results may not be reliable or even helpful. Some of the reasons are:

  • You are on medication such as cortisteroids, fungizone, and antibiotics such as tetracycline antibiotics as well as Chloromycetin.
  • Diuretics could cause dehydration and taking the test may not produce accurate results.
  • Age is also a consideration since BUN levels tend to rise as one gets older.
  • Gender also could affect the test and generally men have higher BUN levels when compared to children and women.

Normal Values

The normal BUN values may vary amongst different labs and your lab could have a different normal range. The following are the acceptable normal values of BUN.

A normal BUN would range between 6 and 20 mg of urea nitrogen per deciliter. In liters this would be 2.1 to 7.1 mmol/L (millimoles per liter).

The normal BUN to creatinine ratio should be between 10:1 and 20:1.

What Does High Blood Urea Nitrogen Level Mean?



Health Conditions

Various health conditions could bring about high BUN levels. Some of these conditions include heart failure and dehydration which cause low blood supply to the kidneys. Kidney stones and tumors also block the urinary tract while high blood pressure and diabetes does harm to the kidneys directly.


Various types mediations cause high levels of BUN and you need to inform your doctor of any medications you may be taking before undertaking a BUN test.

High-protein Diet

If your diet consists of a large amount of proteins, then you could have a high BUN level.

High BUN-to-creatinine Ratios

This occurs mostly with acute kidney failure, severe dehydration or shock. Urinary tract blockage and bleeding within the respiratory or digestive tract also could bring about a high BUN to creatinine ratio.


Other conditions responsible for a high BUN include: congestive heart failure, severe burns and shock, just to mention a few.

What Does Low Blood Urea Nitrogen Level Mean?



Low Protein Diet

Low blood urea nitrogen level could be caused by a diet that has insufficient amounts of protein.

Over Hydration

Drinking too much water could also contribute to low BUN levels.


Your gender may have a role to play and generally, women and children have much lower BUN levels than men. This is because men have the ability to breakdown protein better.

Low BUN-to-creatinine Ratios

Various health conditions could bring about a low BUN to creatinine ratio and these include cancer and lung disease which sometimes brings about syndrome of inappropriate antidiuretic hormone secretion simply referred to as SIADH. Cirrhosis, rhabdomyolysis which is severe muscle injury and diseases affecting the central nervous system also could lower ones BUN to creatinine ratio. Pregnancy is yet another cause of low BUN levels and this is most common during the pregnancy’s last trimester.

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