Blood Urea Nitrogen
A blood urea nitrogen (BUN) test measures the amount of nitrogen in your blood that comes from the waste product urea. Urea is formed when protein is broken down in your body. It is produced in the liver and eliminated from your body in urine.
A BUN test is done to estimate how well your kidneys are functioning. If your kidneys are not able to remove urea from the blood normally, your BUN level increases. Heart failure, dehydration, or a diet high in protein can also increase your BUN level. Liver disease or damage can decrease your BUN level, because urea is made in the liver. A decreased BUN level can occur normally in the second or third trimester of pregnancy.
Increased BUN
An increase in the BUN level is known as azotemia. An elevated BUN may be caused by:
Impaired renal function
Congestive heart failure as a result of poor renal perfusion
Dehydration
Shock
Hemorrhage into the gastrointestinal tract
Acute myocardial infarction
Stress
Excessive protein intake or protein catabolism
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:
Liver failure
Malnutrition
Anabolic steroid use
Overhydration, Which can result from prolonged intravenous fluids
Pregnancy (due to increased plasma volume)
Impaired nutrient absorption
Syndrome of inappropriate anti-diuretic secretion (SIADH)
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.