Samiksha Jaiswal (Editor)

Blood urea nitrogen

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MeSH  D001806
Blood urea nitrogen
LOINC  6299-2, 59570-2, 12961-9, 12963-5, 12962-7

The liver produces urea in the urea cycle as a waste product of the digestion of protein. Normal human adult blood should contain between 6 and 20 mg of urea nitrogen per 100 ml (6–20 mg/dL) of blood. Individual laboratories will have different reference ranges as the assay used will vary between laboratories.



Blood urea nitrogen (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) and in blood volume (hypovolemia), congestive heart failure, gastrointestinal hemorrhage, fever and increased catabolism. Hypothyroidism can cause both decreased GFR and hypovolemia, however 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.

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.

To convert from mg/dL of blood urea nitrogen to mmol/L of urea, multiply by 0.357 (each molecule of urea having 2 nitrogens, each of molar mass 14g/mol) (BUN is the mass of nitrogen within urea/volume, not the mass of urea)

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

(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):


Blood urea nitrogen Wikipedia

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