Synonym

• BUN
• Urea nitrogen (Blood)

Tubes

• Red top, tiger top, marbled top, or green top tube
• 5 mL of venous blood

Additional information on sample collection

• Reject if there is hemolysis

Info

The test measures the nitrogen fraction of urea. Urea is a nonprotein nitrogen compound formed as an end product of protein metabolism in the liver and is excreted in the urine. The blood urea nitrogen (BUN) represents the balance between urea production and excretion.

Clinical

The BUN level is used to:

• Evaluate renal function
• Evaluate liver function
• Estimate hydration (BUN/Creatinine ratio)
• Monitor the effects of drugs:
• Nephrotoxic drugs
• Hepatotoxic drugs
• Chemotherapy
• Monitor effects of hemodialysis therapy

Additional information:

• BUN is part of the basic metabolic panel (BMP) or the comprehensive metabolic panel (CMP)
• BUN is a less reliable indicator of uremia than the serum creatinine level.
• The serum creatinine levels should be evaluated along with BUN.
• Normal BUN/Creatinine ratio is <20:1 ( 20:1 is termed pre-renal dehydration).
• BUN is slightly higher in men than in women.
• In Upper GI bleeding; BUN will often be substantially elevated due to digestion of blood and absorption of nitrogenous blood compounds
• Signs and symptoms of grossly elevated BUN include:
• Acidemia
• Agitation
• Confusion
• Fatigue
• Nausea
• Vomiting
• Coma

Nl Result

Consult your laboratory for their normal ranges as these may vary somewhat from the ones listed below

^a Not dehydrated or with no history of any renal disease

^b With history of renal diseaseWith history of renal disease

^c Increase in 24 hr indicates acute renal failure

During pregnancy, BUN may decrease by 25%.

High Result

Causes of elevated serum urea include:

• Prerenal azotemia
• Decreased glomerular perfusion
• Congestive heart failure
• Dehydration
• Diabetic coma or ketoacidosis
• Diarrhea
• Fever
• Hemolytic uremic syndrome
• Renal artery stenosis
• Shock
• Thromboembolic renal vascular disease
• Thrombotic thrombocytopenic purpura
• Increased formation of urea
• Addison's disease
• Drugs as listed below
• Hemorrhage (GI or respiratory)
• High protein diet
• Hyperalimentation
• Muscle wasting from starvation
• Myocardial infarction (Recent)
• Severe burns (third degree)
• Sickle cell disease (Nephropathy)
• Tissue necrosis
• Renal disease
• Acute tubular necrosis
• Congenital kidney anomalies
• Glomerulonephritis (Chronic)
• Pyelonephritis
• Renal failure (Chronic or acute)
• Postrenal disease
• Renal vein thrombosis
• Urinary tract obstruction
• Obstruction from within urinary tract
• Benign prostatic hypertrophy
• Blood clots or sloughed cells
• Congenital anomalies
• Infection
• Neurogenic bladder
• Prostatic carcinoma
• Tumors
• Ureteral stricture
• Ureterolithiasis
• Obstruction originating outside the urinary tract
• Cancer of the cervix
• Pregnancy
• Retroperitoneal disorders
• Tumor or mass adjacent to the urinary tract
• Uterine prolapse

Drugs, substances and vitamins that increase the serum urea level include:

• Acetaminophen
• Alanine
• Aldatense
• Alkaline antacids
• Allopurinol
• Aminoglycosides
• Amphotericin B
• Antimony compounds
• Arsenicals
• Aspirin (high dose)
• Bacitracin
• Bismuth subsalicylate
• Capreomycin
• Carbamazepine
• Carbenoxolone
• Carbutamide
• Cephalosporins
• Chloral hydrate
• Chloramphenicol
• Chlorthalidone
• Cisplatin
• Colistimethate
• Colistin
• Corticosteroids
• Cotrimoxazole
• Dexamethasone
• Dextran
• Diclofenac
• Doxycycline
• Ethylene glycol
• Furosemide
• Guanethidine
• Guanoxan
• Ibuprofen
• Ifosfamide
• Indomethacin
• Ipodate
• Mephenesin
• Methicillin
• Methotrexate
• Methyldopa
• Metolazone
• Mitomycin
• Neomycin
• Penicillamine
• Phosphorus
• Plicamycin
• Polymyxin B
• Probenecid
• Propranolol
• Rifampin
• Spironolactone
• Tertatolol
• Tetracycline
• Thiazides
• Triamterene
• Triethylenemelamine
• Vancomycin
• Viomycin
• Vitamin D

Low Result

Causes of decrease in serum urea levels include:

• Low protein/High carbohydrate diet
• Malabsorption syndromes
• Malnutrition
• Nephrotic syndrome
• Normal Pregnancy
• Overhydration
• Severe hepatic damage
• Syndrome of inappropriate anti-diuretic secretion (SIADH)

Drugs that may decrease the serum urea levels include:

• Anabolic steroids
• Chloramphenicol
• Fluorides
• Para-methasone
• Phenothiazines
• Streptomycin

References

• A-Z Health Guide from WebMD: Medical Tests®. Blood Urea Nitrogen. [Homepage on the Internet]. Last updated on September 20, 2004. Last accessed on August 17, 2006. Available at URL: http://www.webmd.com/hw/kidney_failure/aa36271.asp#aa36274
• Iglesias J et al. Predictors of worsening renal function in adult patients with congestive heart failure receiving recombinant human B-type brain natriuretic peptide (nesiritide). Nephrol Dial Transplant. 2006 Jul 28; [Epub ahead of print]
• Lab Tests online®. BUN The Test. [Homepage on the Internet]. Last reviewed on June 22, 2005. Last accessed on August 17, 2006. Available at URL: http://www.labtestsonline.org/understanding/analytes/bun/test.html
• Laboratory Corporation of America®Urea Nitrogen, Serum. [Homepage on the Internet] © 2003. Last accessed on August 17, 2006. Available at URL: http://www.labcorp.com/datasets/labcorp/html/chapter/mono/pr008900.htm
• Mathur NB et al. Acute renal failure in neonatal sepsis. Indian J Pediatr. 2006 Jun;73(6):499-502.
• MedlinePlus Medical Encyclopedia®. BUN. Last updated on June 13, 2005. Last accessed on August 17, 2006. Available at URL: http://www.nlm.nih.gov/medlineplus/ency/article/003474.htm
• Toyoguchi T et al. [Nephrotoxicity and drug interaction of vancomycin (2)] [Article in Japanese] Nippon Yakurigaku Zasshi. 1996 May;107(5):225-35.