Blood Urea Nitrogen (BUN)

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)

Increased BUN

An increase in the BUN level is known as azotemia. An elevated BUN may be caused by:

Diseased or damaged kidneys cause an elevated BUN because the kidneys are less able to clear urea from the bloodstream. In conditions in which renal perfusion is decreased, such as hypovolemic shock or congestive heart failure, BUN levels rise. A patient who is severely dehydrated may also have a high BUN due to the lack of fluid volume to excrete waste products. Because urea is an end product of protein metabolism, a diet high in protein, such as high-protein tube feeding, may also cause the BUN to increase. Extensive bleeding into the gastrointestinal (GI) tract will also cause an elevated BUN because digested blood is a source of urea. For example, a hemorrhage of one liter of blood into the GI tract may elevate the BUN up to 40mg/ml.

Decreased BUN

A decreased BUN may be seen in:

Because urea is synthesized by the liver, severe liver failure causes a reduction of urea in the blood. Just as dehydration may cause an elevated BUN, overhydration causes a decreased BUN. When a person has "syndrome of inappropriate anti-diuretic secretion" (SIADH), the anti-diuretic hormone responsible for stimulating the kidney to conserve water causes excess water to be retained in the bloodstream rather than being excreted into the urine. SIADH can cause the BUN level, along with other important substances, to decrease because the fluid volume of the bloodstream may significantly increase.

Pathophysiology Hyponatremia and sometimes hypervolemia resulting from elevated plasma ADH/vasopressin. ADH causes the distal tubule to reabsorb more water but not sodium.
  • Cranial injury- ADH released from posterior pituitary
  • Cancer - Small cell cancers, esp. lung, produce ADH
  • Infections- intracranial, lung infection
  • Drugs- including SSRIs
Treatment Fluid restriction or isotonic saline IV

An assessment of the BUN is used as a gross index of glomerular function. Because the BUN is affected by the patient's hydration status, it is a less sensitive indicator of declining renal function than a creatinine clearance test. A BUN of over 100 mg/dl is a panic value.

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Patients on high-protein tube feedings may have increased BUN values