BUN

Definition

BUN stands for blood urea nitrogen. Urea nitrogen is what forms when protein breaks down.

A test can be done to measure the amount of urea nitrogen in the blood.

Alternative Names

Blood urea nitrogen

How the test is performed

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to prepare for the test

Many drugs affect BUN levels. Before having this test, make sure the health care provider knows which medications you are taking.

Drugs that can increase BUN measurements include:

• Allopurinol
• Aminoglycosides
• Amphotericin B
• Aspirin (high doses)
• Bacitracin
• Carbamazepine
• Cephalosporins
• Chloral hydrate
• Cisplatin
• Colistin
• Furosemide
• Gentamicin
• Guanethidine
• Indomethacin
• Methicillin
• Methotrexate
• Methyldopa
• Neomycin
• Penicillamine
• Polymyxin B
• Probenecid
• Propranolol
• Rifampin
• Spironolactone
• Tetracyclines
• Thiazide diuretics
• Triamterene
• Vancomycin

Drugs that can decrease BUN measurements include:

• Chloramphenicol
• Streptomycin

How the test will feel

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

The BUN test is often done to check kidney function.

Normal Values

7 - 20 mg/dL. Note that normal values may vary among different laboratories.

What abnormal results mean

Higher-than-normal levels may be due to:

• Congestive heart failure
• Excessive protein levels in the gastrointestinal tract
• Gastrointestinal bleeding
• Hypovolemia
• Heart attack
• Kidney disease, including glomerulonephritis, pyelonephritis, and acute tubular necrosis
• Kidney failure
• Shock
• Urinary tract obstruction

Lower-than-normal levels may be due to:

• Liver failure
• Low protein diet
• Malnutrition
• Over-hydration

Additional conditions under which the test may be done include:

• Acute nephritic syndrome
• Alport syndrome
• Atheroembolic kidney disease
• Dementia due to metabolic causes
• Diabetic nephropathy/sclerosis
• Digitalis toxicity
• Epilepsy
• Generalized tonic-clonic seizure
• Goodpasture syndrome
• Hemolytic-uremic syndrome (HUS)
• Hepatokidney syndrome
• Interstitial nephritis
• Lupus nephritis
• Malignant hypertension (arteriolar nephrosclerosis)
• Medullary cystic kidney disease
• Membranoproliferative GN I
• Membranoproliferative GN II
• Type 2 diabetes
• Prerenal azotemia
• Primary amyloidosis
• Secondary systemic amyloidosis
• Wilms' tumor

What the risks are

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Special considerations

For people with liver disease, the BUN level may be low even if the kidneys are normal.

References

Molitoris BA. Acute kidney injury. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 121.