Blood urea nitrogen (BUN) is a medical test that measures the amount of urea nitrogen found in blood.

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.

It is not considered as reliable as creatinine or BUN-to-creatinine ratio blood studies.

[1] BUN is an indication of kidney health.

[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.

Each molecule of urea has two nitrogen atoms, each having molar mass 14 g/mol.