Blood urea nitrogen (BUN) measures the amount of urea nitrogen, a waste product of protein metabolism, in the blood. Urea is formed by the liver and carried by the blood to the kidneys for excretion. Because urea is cleared from the bloodstream by the kidneys, a test measuring how much urea nitrogen remains in the blood can be used as a test of renal function. However, there are many factors besides renal disease that can cause BUN alterations, including protein breakdown, hydration status, and liver failure.

Reference values for BUN: (values may differ slightly from laboratory to laboratory for all tests; the nurse should consult the laboratory manual in each agency for reference values)

Adult: 7-20 mg/100 ml; men may have slightly higher values than women

Pregnancy: values decrease about 25%

Newborn: values slightly lower than adult ranges

Elderly: values may be slightly increased due to lack of renal concentration.

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