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

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]

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.

${\displaystyle BUN_{mg/dL))$ 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:

${\displaystyle Urea_{mmol/L}=BUN_{mmol/L}=BUN_{mg/dL}*{\frac {10_{dL/L)){14*2))=BUN_{mg/dL}*0.3571}$

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:

${\displaystyle Urea_{mg/dL}=BUN_{mg/dL}*{\frac {60}{14*2))=BUN_{mg/dL}*2.14}$

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

• Kt/V
• Standardized Kt/V
• Urea reduction ratio
• Urine urea nitrogen