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Latest Edit: Iva Lloyd, ND 2021-08-24 (EDT)

See Also Lab Tests

BUN, or Blood Urea Nitrogen, reflects the ratio between the production and clearance of urea. Urea, which is formed in the liver, is the final product in protein catabolism, along with CO2. The amount of urea exreted as BUN varies with the amount of dietary protein intake. If BUN is increased, this may be due to either an increased production of urea by the liver or decreased excretion by the kidney.[1],[2]


  • The BUN is a test that is commonly used to measure kidney function.
  • BUN is useful as a first indicator of renal insufficiency, particularly if all the other renal indicators are normal.
  • BUN is also used to monitor the effectiveness of dialysis and other treatments related to kidney disease.

Patient Preparation

  • No fasting required.
  • Factors which can cause increased levels
  • late pregnancy, dehydration, high-protein diets, alimentary tube feeding
  • Drugs: allopurinol, aminoglycosides, cephalosporins, chloral hydrate, cisplatin, furosemide, guanethidine, indomethacin, methotrexate, methyldopa, nephrotoxic drugs (e.g., aspirin, amphotericin B, bacitracin, carbamazepine, colistin, gentamicin, methicillin, neomycin, penicillamine, polymyxin B, probenecid, vancomycin), propanolol, rifampin, spironolactone, tetracyclines, thiazide diuretics, and triamterene.
  • Factors which can cause decreased levels
  • small muscle mass, overhydration
  • Drugs: chloramphenicol and streptomycin.

Clinical Implications

Ranges: The following are the reference ranges for this lab. However, lab ranges can vary by laboratory and country. [2]

Standard U.S. Units Standard International Units
Conventional Laboratory Range 5-25 mg/dL 1.79-8.93 mmol/L
Optimal Range 10-16 mg/dL 3.57-5.71 mmol/L
Alarm Ranges <5 mg/dL or >50 mg/dL <1.79 mmol/L or >17.85 mmol/L

High levels indicate:

Low levels indicate:

  • Low protein diet
  • Malabsorption
  • Pancreatic insufficiency
  • Liver dysfunction or failure
  • Posterior pituitary dysfunction
  • Drug causes: anabolic steroids, some antibiotics
  • Other conditions: dysbiosis, pregnancy

Associated Tests


  1. Pagana Kathleen D, Pagana Timothy J (1998) Mosby's Manual of Diagnostic and Laboratory Tests, Mosby, Inc
  2. 2.0 2.1 Weatherby Dicken, Ferguson Scott (2002) Blood Chemistry and CBC Analysis: Clinical Laboratory Testing from a Functional Perspective, Bear Mountain