Blood Urea Nitrogen Test

Blood Urea Nitrogen Test 

Definition

The blood urea nitrogen (BUN) test measures the level of urea nitrogen in a sample of the patient's blood. Urea is a substance that is formed in the liver when the body breaks down protein. Urea then circulates in the blood in the form of urea nitrogen. In healthy people, most urea nitrogen is filtered out by the kidneys and leaves the body in the urine. If the patient's kidneys are not functioning properly or if the body is using large amounts of protein, the BUN level will rise. If the patient has severe liver disease, the BUN will drop.

Purpose

The BUN level may be checked in order to assess or monitor:

• the presence or progression of kidney or liver disease.
• blockage of urine flow.
• mental confusion. Patients with kidney failure are sometimes disoriented and confused.
• abnormal loss of water from the body (dehydration).
• recovery from severe burns. The body uses larger than normal amounts of protein following serious burns.

Description

The BUN test is performed on a sample of the patient's blood, withdrawn from a vein into a vacuum tube. The procedure, which is called a venipuncture, takes about five minutes.

Preparation

The doctor should check to make sure that the patient is not taking any medications that can affect BUN results. These drugs include the antibiotics chloramphenicol, streptomycin, amphotericin B, methicillin, gentamicin, tobramycin, and kanamycin, as well as diuretics and corticosteroids.

The patient should be advised not to eat large amounts of meat the day before the test.

Aftercare

Aftercare consists of routine care of the area around the venipuncture.

Risks

The primary risk is the possibility of a bruise or swelling in the area of the venipuncture. The patient can apply moist warm compresses.

Normal results

Normal BUN levels are 5-18 mg/dL for children; 7-18 mg/dL for adults; and 8-20 mg/dL in the elderly.

Abnormal results

BUN levels can be too low as well as too high.

Abnormally low bun

Low levels of BUN may indicate overhydration, malnutrition, celiac disease [a disease characterized by the inability ot tolerate foods containing wheat protein (gluten)], liver damage or disease, or use of corticosteroids. Low BUN may also occur in early pregnancy.

Abnormally high bun

High levels of BUN may indicate kidney disease or failure; blockage of the urinary tract by a kidney stone or tumor; a heart attack or congestive heart failure; dehydration; fever; shock; or bleeding in the digestive tract. High BUN levels can sometimes occur during late pregnancy or result from eating large amounts of protein-rich foods. A BUN level higher than 100 mg/dL points to severe kidney damage.

Resources

Books

Pagana, Kathleen Deska. Mosby's Manual of Diagnostic and Laboratory Tests. St. Louis: Mosby, Inc., 1998.

Key terms

Urea — A compound containing nitrogen that occurs in the urine and other body fluids as a result of protein metabolism.