Blood Urea Nitrogen

What is it?

The blood urea nitrogen or BUN test is used to evaluate kidney function. Urea is a waste product formed in the liver when protein is metabolized. Urea is released by the liver into the blood and is carried to the kidneys, where it is filtered out of the blood and released into the urine. Since this is an ongoing process, there is usually a small but stable amount of urea nitrogen in the blood. However, when the kidneys cannot filter wastes out of the blood due to disease or damage, then the level of urea in the blood will rise. A Normal BUN is roughly between 10 – 20 mg/dl.

Abnormal BUN

Increased BUN levels suggest impaired kidney function. This may be due to acute or chronic kidney disease, damage, or failure. It may also be due to a condition that results in decreased blood flow to the kidneys, such as congestive heart failure, shock, stress, recent heart attack, or severe burns, to conditions that cause obstruction of urine flow, or to dehydration.

Low BUN levels are not common and are not usually a cause for concern. They may be seen in severe liver disease, malnutrition, and sometimes when a person is overhydrated (too much fluid volume), but the BUN test is not usually used to diagnose or monitor these conditions.

Causes For Elevated BUN
  • Urinary tract obstruction
  • Congestive heart failure or recent heart attack
  • Gastrointestinal bleeding
  • Dehydration
  • Shock
  • Severe burns
  • Certain medications, such as corticosteroids and some antibiotics
  • A high protein diet

Although the blood urea nitrogen (BUN) also varies inversely with the GFR, it is generally less useful than the serum creatinine because the BUN can change independently of the GFR. The rate of urea production is not constant, increasing with a high-protein diet and with enhanced tissue breakdown due to hemorrhage, trauma, or glucocorticoid therapy. By comparison, a low-protein diet or liver disease can lower the BUN without change in GFR. Thus, liver disease may be associated with near-normal values for both the BUN (due to decreased urea production) and the serum creatinine (due to muscle wasting) despite a relatively large reduction in GFR