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

A blood sample is needed. For information on how this is done, see: Venipuncture

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
  • Aminoglycoside antibiotics
  • Amphotericin B
  • Aspirin (high doses)
  • Bacitracin
  • Carbamazepine
  • Cephalosporins
  • Chloral hydrate
  • Cisplatin
  • Colistin
  • Furosemide
  • 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

The normal result is generally 6 - 20 mg/dL.

Note: Normal values may vary among different labs. Talk to your doctor about your specific test results.

The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

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
  • Diabetes
  • Digitalis toxicity
  • Epilepsy
  • Generalized tonic-clonic seizure
  • Goodpasture syndrome
  • Hemolytic-uremic syndrome (HUS)
  • Hepatorenal syndrome
  • Interstitial nephritis
  • Lupus nephritis
  • Malignant hypertension (arteriolar nephrosclerosis)
  • Medullary cystic kidney disease
  • Membranoproliferative GN I
  • Membranoproliferative GN II
  • 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

Clarkson MR, Friedewald JJ, Eustace JA, Rabb H. Acute kidney injury. In: Brenner BM, eds. Brenner and Rector’s The Kidney. 8th ed. Philadelphia, Pa: Saunders Elsevier; 2008:chap 29.

Review Date: 5/30/2011

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