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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
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.
urea cycle
The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions that produces urea (NH2)2CO from ammonia (NH3). Animals that use this cycle, mainly amphibians and mammals, are called ureotelic.
The urea cycle converts hi ...
as a waste product of the digestion
Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...
of protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...
. Normal human adult blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in th ...
should contain 6 to 20 mg/dL (2.1 to 7.1 mmol/L) of urea nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...
. Individual laboratories will have different reference range
In medicine and health-related fields, a reference range or reference interval is the range or the interval of values that is deemed normal for a physiological measurement in healthy persons (for example, the amount of creatinine in the blood ...
s as the assay used can vary between laboratories. The test is used to detect renal problems. It is not considered as reliable as creatinine or BUN/creatinine ratio blood studies.
Interpretation
BUN is an indication ofrenal
The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; bloo ...
(kidney) health. The normal range is 2.1–7.1 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 kidney failure
Kidney failure, also known as end-stage kidney disease, is a medical condition in which the kidneys can no longer adequately filter waste products from the blood, functioning at less than 15% of normal levels. Kidney failure is classified as eit ...
), decrease in blood volume (hypovolemia
Hypovolemia, also known as volume depletion or volume contraction, is a state of abnormally low extracellular fluid in the body. This may be due to either a loss of both salt and water or a decrease in blood volume. Hypovolemia refers to the lo ...
), congestive heart failure
Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, ...
, gastrointestinal hemorrhage, fever
Fever, also referred to as pyrexia, is defined as having a temperature above the normal range due to an increase in the body's temperature set point. There is not a single agreed-upon upper limit for normal temperature with sources using val ...
, rapid cell destruction from infections, athletic activity, excessive muscle breakdown, and increased catabolism
Catabolism () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipids ...
.
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
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is characterized by excessive unsuppressible release of antidiuretic hormone (ADH) either from the posterior pituitary gland, or an abnormal non-pituitary source. Unsuppressed ADH ...
.
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.
Units
BUN (urea-N) is mg/dL in the United States, Mexico, Italy, Austria, and Germany. Elsewhere, the concentration of urea is reported in SI unit as mmol/L, which is generally depending on the lab. represents the mass of nitrogen within urea/volume, not the mass of whole urea. Each molecule of urea has two nitrogen atoms, each of molar mass 14g/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
* Kt/V * Standardized Kt/V * Urea reduction ratio * Urine urea nitrogenReferences
{{DEFAULTSORT:Blood Urea Nitrogen Chemical pathology Nitrogen cycle Nephrology