Blood Urea Nitrogen

BUN (Blood Urea Nitrogen), BUN:Creatinine Ratio, Urea Nitrogen, Blood

Results

A blood urea nitrogen (BUN) test measures the amount of nitrogen in your blood that comes from the waste product urea.

Normal

Normal values may vary from lab to lab.

Blood urea nitrogen (BUN)

Normal:	3.6–7.1 millimoles per litre (mmol/L) or 10–20 milligrams per decilitre (mg/dL)

 

BUN-to-creatinine ratio

Over 12 months of age:	10:1–20:1
Babies less than 12 months of age:	Up to 30:1

 

High values

• A high BUN value can mean kidney injury or disease is present. Kidney damage can be caused by diabetes or high blood pressure that directly affect the kidneys. High BUN levels can also be caused by blockage of the urinary tract (by a kidney stone or tumour) or low blood flow to the kidneys caused by dehydration or heart failure.
• Many medicines may cause a high BUN. Be sure to tell your doctor about all the non-prescription and prescription medicines you take.
• A high BUN value may be caused by a high-protein diet, Addison's disease, tissue damage (such as from severe burns), or from bleeding in the gastrointestinal tract.
• High BUN-to-creatinine ratios occur with sudden (acute) kidney failure, which may be caused by shock or severe dehydration. A blockage in the urinary tract (such as a kidney stone) can cause a high BUN-to-creatinine ratio. A very high BUN-to-creatinine ratio may be caused by bleeding in the digestive tract or respiratory tract.

Low values

• A low BUN value may be caused by a diet very low in protein, malnutrition, or severe liver damage.
• Drinking excessive amounts of liquid may cause overhydration and cause a low BUN value.
• Women and children may have lower BUN levels than men because of how their bodies break down protein.
• A low BUN-to-creatinine ratio may be caused by a diet low in protein, a severe muscle injury called rhabdomyolysis, pregnancy, cirrhosis, or syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH sometimes occurs with lung disease, cancer, diseases of the central nervous system, and the use of certain medicines.

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Author:	Jan Nissl, RN, BS Sydney Youngerman-Cole, RN, BSN, RNC	Last Updated: November 21, 2006
Medical Review:	Patrice Burgess, MD - Family Medicine Andrew Swan, MD, CCFP, FCFP - Family Medicine
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