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BUN - blood test
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 Results
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
Risks
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)
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.
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; Jatin M. Vyas, MD, PhD, Assistant Professor in Medicine, Harvard Medical School, Assistant in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
