The blood urea nitrogen or BUN test is primarily used, along with the creatinine test, to evaluate kidney function in a wide range of circumstances, to help diagnose kidney disease, and to monitor people with acute or chronic kidney dysfunction or failure. It also may be used to evaluate a person's general health status when ordered as part of a renal panel, basic metabolic panel (BMP) or comprehensive metabolic panel (CMP).

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.

The kidneys are a pair of bean-shaped organs that are located at the bottom of the ribcage in the right and left sides of the back. Within them are about a million tiny blood filtering units called nephrons. In each nephron, blood is continually filtered through a microscopic cluster of looping blood vessels, called a glomerulus. The glomerulus allows the passage of water and small molecules but retains blood cells and larger molecules. Attached to each glomerulus is a tiny tube (tubule) that collects the fluid and molecules that pass through the glomerulus and then reabsorbs what can be used by the body. The remaining waste forms urine.

If the creatinine and BUN tests are found to be abnormal or if someone has an underlying disease that is known to affect the kidneys, such as diabetes or high blood pressure, then creatinine and BUN tests may be used to monitor for kidney dysfunction and the effectiveness of treatment. Blood creatinine and BUN tests may also be ordered to evaluate kidney function prior to some procedures, such as a CT (computed tomography) scan, that may require the use of drugs that can damage the kidneys.

BUN is part of both the BMP and CMP, groups of tests that are widely used:

• As part of a routine health checkup
• To check how the kidneys are functioning before starting to take certain drug therapies
• When an acutely ill person comes to the emergency room and/or is admitted to the hospital
• During a hospital stay

BUN is often ordered with creatinine or renal panel when kidney problems are suspected. Some signs and symptoms of kidney dysfunction include:

• Fatigue, lack of concentration, poor appetite, or trouble sleeping
• Swelling or puffiness (edema), particularly around the eyes or in the face, wrists, abdomen, thighs, or ankles
• Urine that is foamy, bloody, or coffee-colored
• A decrease in the amount of urine
• Problems urinating, such as a burning feeling or abnormal discharge during urination, or a change in the frequency of urination, especially at night
• Mid-back pain (flank), below the ribs, near where the kidneys are located
• High blood pressure

BUN also may be ordered:

• At regular intervals to monitor kidney function in those with chronic diseases or conditions such as diabetes, congestive heart failure, and myocardial infarction (heart attack)
• At regular intervals to monitor kidney function and treatment in people with known kidney disease
• Prior to and during certain drug treatments to monitor kidney function
• Along with a creatinine when a CT scan is planned
• At regular intervals to monitor the effectiveness of dialysis

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.

BUN concentrations may be elevated when there is excessive protein breakdown (catabolism), significantly increased protein in the diet, or gastrointestinal bleeding (because of the proteins present in the blood).

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.

If one kidney is fully functional, BUN concentrations may be normal even when significant dysfunction is present in the other kidney.

BUN levels can increase with the amount of protein in the diet. High-protein diets may cause abnormally high BUN levels while very low-protein diets can cause an abnormally low BUN.

A wide variety of drugs can cause an increase in BUN. Drugs that can decrease BUN include chloramphenicol and streptomycin. Inform your healthcare provider of any medications you are taking.

Both decreased and increased BUN concentrations may be seen during a normal pregnancy.