Blood Urea Nitrogen (BUN)

The BUN (blood urea nitrogen) test is conducted to check the level of urea nitrogen in the blood, so as to evaluate kidney function. This article provides some information about the test.
Urea is a waste product formed in the liver, when proteins get digested. It is the function of the kidneys to remove urea from the blood, and pass it out of the body through urine. An increase in the level of blood nitrogen urea may indicate an impaired kidney functioning. So, a blood urea nitrogen or BUN test is conducted to check the level of urea nitrogen in the blood. A creatinine test may also be done to evaluate the functioning of the kidneys. A BUN test is also called the urea nitrogen test or serum urea nitrogen test.

The Test: The person who has to undergo the BUN test, must abstain from protein-rich food for 24 hours prior to the test, as it may increase the BUN level. During the test, a blood sample is taken from the vein in the arm. There are very little chances of bleeding or other related problems. First, an elastic band is wrapped around the upper arm to stop the blood flow through veins. The skin is rubbed with alcohol before inserting the needle into the vein. A tube is attached to the needle. After collecting the blood, the band is removed.

Normal Range

Generally, a BUN value that ranges between 5 to 20 mg/dL is considered normal. In case of adults, the normal value ranges between 10 to 20 mg/dL; whereas in kids, the value is found to be between 5 to 18 mg/dL.

Apart from an impaired kidney function, there are certain other factors that may cause variations in the normal BUN level. For example, medicines such as tetracycline antibiotics, nafcillin, aminoglycosides, amphotericin B (such as Fungizone), aureomycin (Kantrex), tobramycin (Nebcin), corticosteroids, diuretics, and chloramphenicol (Chloromycetin) may affect the BUN level. A woman may have an increased or decreased BUN value during pregnancy. A person may have a normal BUN level, even if one of his kidneys fail to function properly, provided the other kidney is fully functional. A person may have abnormally high BUN levels, if he is on a high-protein diet. The BUN level may become abnormally low, for those who are on a low-protein diet. Women and kids have a slightly lower BUN level, when compared to men.

High BUN values

A high BUN level may indicate:
  • Kidney diseases caused by high blood pressure or diabetes
  • Low blood flow to the kidneys due to heart failure or dehydration
  • Blockage of urine flow because of kidney stone or tumor
  • Excessive intake of protein-rich food
  • Bleeding in the gastrointestinal tract
  • Tissue damage due to burns
  • Addison's disease, stress, mental shock
Low BUN Values

A low BUN level could be due to:
  • A low-protein diet, malnutrition
  • Severe liver damage
  • Impaired nutrient absorption
  • Use of anabolic steroids
  • Syndrome of inappropriate anti-diuretic secretion (SIADH)
  • Overhydration caused by excess intake of liquids
  • Rhabdomyolysis
What is BUN:Creatinine Ratio?

The BUN test is often done along with the creatinine test, if an underlying kidney problem is suspected. The normal BUN:creatinine ratio should be between 10:1 and 20:1. The ratio should be less than 30:1 in babies younger than 12 months. Elderly people have a slightly higher BUN:creatinine ratio, when compared to healthy, young adults. The value can be slightly higher in men than in women.

An increased ratio indicates dehydration, acute kidney failure, mental shock, decreased blood flow to the kidneys or congestive heart failure. On the other hand, a decreased BUN:creatinine ratio indicates malnutrition, syndrome of inappropriate antidiuretic hormone secretion (SIADH), which is sometimes found with lung diseases, cancer, diseases of the central nervous system, muscle injuries like rhabdomyolysis, bleeding in the digestive or respiratory tract and liver diseases.

Patients who undergo kidney dialysis need to check BUN and creatinine levels regularly. It is a reliable method to evaluate kidney function. Keep in mind that the normal ranges may vary with various factors such as the age, gender, medical condition, etc. The reference range used by different labs may also vary. So, your healthcare provider is the best person to interpret and evaluate the test results.

Disclaimer: This article is for informational purposes only, and should not be used as a replacement for expert medical advice.