Urea / Blood Urea Nitrogen (BUN)

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Urea is one of the waste products of protein metabolism. It is produced during the urea cycle (also known as the ornithine cycle) when highly toxic ammonia is converted to urea. The urea cycle occurs mostly in the liver. From the liver urea is sent out to the bloodstream and is excreted from the body by the kidneys with urine. This is a continuous process and a small amount of urea is usually present in the blood. Urea contains nitrogen and helps remove excess nitrogen from the body.

Most conditions or diseases that affect the kidneys (if the kidneys aren't functioning properly) usually affect the urea amount in the blood. In this case, urea and the nitrogen it contains are not completely removed from the blood.

Blood urea nitrogen (BUN) test is part of the basic metabolic panel (BMP) or comprehensive metabolic panel (CMP). A metabolic panel is a set of blood tests that helps evaluate your body's chemical balance and metabolism. BMP or CMP is usually ordered during annual physical exams.

To check how well the kidneys and liver are working plasma/serum urea is measured, but the results are expressed in two different ways. The concentration of urea in the blood can be expressed as the amount of urea nitrogen or as the whole molecule of urea (not just the nitrogen part).

Blood urea nitrogen (BUN) is expressed in mg/dL, while the unit of urea concentration is mmol/L (SI units). The conversion between urea nitrogen and urea is that urea is approximately twice (2.14) that of BUN. The molecular weight of urea nitrogen in urea is 28 while molecular weight if urea is 60 (60 / 28 = 2.14). For example, 10 mg/dL of BUN is equivalent to 21.4 mg/dL of urea.

The conversion factor between blood urea nitrogen (BUN) and urea is 0.357.

BUN (mg/dL) * 0.357 = urea (mmol/L)

Urea (mmol/L) / 0.357 = BUN (mg/dL)

Blood urea nitrogen (BUN) test alone is not enough to diagnose any condition. Usually it is combined with a serum creatinine test. Your doctor may also order other tests to make a proper diagnosis.

Normal levels of BUN (urea)

Normal levels of blood urea nitrogen (BUN) are between 7 to 22 mg/dL (blood urea – 2.4-7.8 mmol/L). Children have lower urea levels than adults, also urea levels tend to increase with age.

Normal values can vary slightly from lab to lab. See the ranges your lab provided in your lab report.

Elevated levels of BUN (urea)

If the kidneys do not work properly and are not able to remove urea from the blood, the level of urea in the blood may increase.

Elevated BUN levels may be due to high protein diets, urinary tract obstruction, dehydration, infections, recent heart attack, gastrointestinal bleeding. Certain medications can influence urea levels in the blood.

Diabetes often occurs with kidney failure, which leads to an increased level of urea in the blood.

Low levels of BUN (urea)

Low level of urea in the blood is not common but it may be caused by severe liver disease, overhydration or a diet very low in protein.


Comprehensive Metabolic Panel (CMP) Interpretation