Blood urea nitrogen

Blood urea nitrogen (BUN) is a medical test that measures the amount of urea nitrogen found in blood. The liver produces urea in the urea cycle as a waste product of the digestion of protein. Normal human adult blood should contain 7 to 20 mg/dL (2.5 to 7.1 mmol/L) of urea nitrogen. Individual laboratories will have different reference ranges as the assay used can vary between laboratories.[1][2][3]

Blood urea nitrogen
Medical diagnostics
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BUN is an indication of renal (kidney) health. The normal range is 1.8–7.1 mmol/L or 6–20 mg/dL.

The main causes of an increase in BUN are: high protein diet, decrease in glomerular filtration rate (GFR) (suggestive of renal failure), decrease in blood volume (hypovolemia), congestive heart failure, gastrointestinal hemorrhage,[4] fever, and increased catabolism.

Hypothyroidism can cause both decreased GFR and hypovolemia, but BUN-to-creatinine ratio has been found to be lowered in hypothyroidism and raised in hyperthyroidism.

The main causes of a decrease in BUN are severe liver disease, anabolic state, and syndrome of inappropriate antidiuretic hormone.[4]

Reference ranges for blood tests, comparing urea (yellow at right) to other blood constituents

Another rare cause of a decreased BUN is ornithine transcarbamylase deficiency, which is a genetic disorder inherited in an X-linked recessive pattern. OTC deficiency is also accompanied by hyperammonemia and high orotic acid levels.


BUN (urea-N) is mg/dL in the United States, Mexico, Italy, Austria, and Germany. Elsewhere, the concentration of urea is reported as mmol/L, generally depending on the lab.

BUN (urea-N) SI unit is mmol/L

BUN [mmol/L] = BUN [mg/dL] * 0.3571

To convert from mg/dL of blood urea nitrogen to mmol/L of urea, multiply by 0.357 (each molecule of urea having two nitrogens, each of molar mass 14g/mol) (BUN is the mass of nitrogen within urea/volume, not the mass of urea)

Urea [mmol/L] = BUN [mg/dL of nitrogen] x 10 [dL/L] / 14x2 [mg N/mmol urea] (the mass of nitrogen within urea is used)

convert BUN to urea in mg/dL by using following formula:

Urea [mg/dL]= BUN [mg/dL] * 2.14

(conversion factor derived by: MW of urea = 60, MW of urea nitrogen = 14x2 => 60/28 = 2.14)

factor = 1 for conversions in mmol (1 mole N2 = 2 moles N per mole of urea):

BUN [mmol/L]= urea [mmol/L]

See also


  1. Last page of Deepak A. Rao; Le, Tao; Bhushan, Vikas (2007). First Aid for the USMLE Step 1 2008 (First Aid for the Usmle Step 1). McGraw-Hill Medical. ISBN 0-07-149868-0.
  2. Normal Lab Results Archived December 16, 2014, at the Wayback Machine from Marshal University School of Medicine
  3. "Blood urea nitrogen (BUN) test - Mayo Clinic". Retrieved 2019-05-13.
  4. Longo et al., Harrison's Principles of Internal Medicine, 18th ed., p.611
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