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Blood urea nitrogen

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Blood urea nitrogen
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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 18 mg/dL (0.388 to 1 mmol/L) of urea nitrogen.[1] Individual laboratories may have different reference ranges, as they may use different assays.[2][3][4] The test is used to detect kidney problems. It is not considered as reliable as creatinine or BUN-to-creatinine ratio blood studies.[1]


BUN is an indication of kidney health. The normal range is 2.1–7.1 mmol/L or 6–20 mg/dL.[1]

The main causes of an increase in BUN are: high-protein diet, decrease in glomerular filtration rate (GFR) (suggestive of kidney failure), decrease in blood volume (hypovolemia), congestive heart failure, gastrointestinal hemorrhage,[5] fever, rapid cell destruction from infections, athletic activity, excessive muscle breakdown, and increased catabolism.[1]

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 malnutrition (low-protein diet), severe liver disease, anabolic state, and syndrome of inappropriate antidiuretic hormone.[5]

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 is usually reported in mg/dL in some countries (e.g. United States, Mexico, Italy, Austria, and Germany). Elsewhere, the concentration of urea is reported in SI units as mmol/L.

represents the mass of nitrogen within urea/volume, not the mass of whole urea. Each molecule of urea has two nitrogen atoms, each having molar mass 14 g/mol. To convert from mg/dL of blood urea nitrogen to mmol/L of urea:

Note that molar concentrations of urea and urea nitrogen are equal, because both nitrogen gas and urea has two nitrogen atoms.

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

Where 60 represents MW of urea and 14*2 MW of urea nitrogen.

See also[edit]


  1. ^ a b c d Lewis, Sharon Mantik; Dirksen, Shannon Ruff; Heitkemper, Margaret M.; Bucher, Linda; Harding, Mariann (5 December 2013). Medical-surgical nursing : assessment and management of clinical problems (9th ed.). St. Louis, Missouri. ISBN 978-0-323-10089-2. OCLC 228373703.{{cite book}}: CS1 maint: location missing publisher (link)
  2. ^ Tao Le; Vikas Bhushan; Deepak Rao (2007). First Aid for the USMLE Step 1 2008. New York: McGraw-Hill Medical. Last page. ISBN 978-0-07-149868-5.
  3. ^ "Normal Lab Results". Marshal University School of Medicine. Archived from the original on December 16, 2014.
  4. ^ "Blood urea nitrogen (BUN) test - Mayo Clinic". www.mayoclinic.org. Retrieved 2019-05-13.
  5. ^ a b Longo; et al. Harrison's Principles of Internal Medicine (18th ed.). p. 611.[full citation needed]