Blood urea nitrogen

Blood urea nitrogen

The blood urea nitrogen (BUN) test is a measure of the amount of nitrogen in the blood in the form of urea, and a measurement of renal function. Urea is a substance secreted by the liver, and removed from the blood by the kidneys.

Physiology

The liver produces urea in the urea cycle as a waste product of the digestion of protein. Normal human adult blood should contain between 7 and 18 to 21 mg of urea nitrogen per 100 ml (7-21 mg/dL) of blood. Individual laboratories may have different reference ranges, and this is because the procedure may vary. Last page of cite book |author=Deepak A. Rao; Le, Tao; Bhushan, Vikas |title=First Aid for the USMLE Step 1 2008 (First Aid for the Usmle Step 1) |publisher=McGraw-Hill Medical |location= |year=2007 |pages= |isbn=0-07-149868-0 |oclc= |doi= |accessdate= ] [http://pathcuric1.swmed.edu/PathDemo/nrrt.htm Normal Reference Range Table] from The University of Texas Southwestern Medical Center at Dallas. Used in Interactive Case Study Companion to PATHOLOGIC BASIS of DISEASE. ]

Interpretation

The most common cause of an elevated BUN, azotemia, is poor kidney function, although a serum creatinine level is a somewhat more specific measure of renal function (see also renal function).

A greatly elevated BUN (>60 mg/dL) generally indicates a moderate-to-severe degree of renal failure. Impaired renal excretion of urea may be due to temporary conditions such as dehydration or shock, or may be due to either acute or chronic disease of the kidneys themselves.

An elevated BUN in the setting of a relatively normal creatinine may reflect a physiological response to a relative decrease of blood flow to the kidney (as seen in heart failure or dehydration) without indicating any true injury to the kidney. However, an isolated elevation of BUN may also reflect excessive formation of urea without any compromise to the kidneys.

Increased production of urea is seen in cases of moderate or heavy bleeding in the upper gastrointestinal tract (e.g. from ulcers). The nitrogenous compounds from the blood are resorbed as they pass through the rest of the GI tract and then broken down to urea by the liver. Enhanced metabolism of proteins will also increase urea production, as may be seen with high protein diets, steroid use, burns, or fevers.

A low BUN usually has little significance, but its causes include liver problems, malnutrition (insufficient dietary protein), or excessive alcohol consumption. Overhydration from intravenous fluids can result in a low BUN. Normal changes in renal bloodflow during pregnancy will also lower BUN.

Urea itself is not toxic. This was demonstrated by Johnson "et al." by adding large amounts of urea to the dialysate of hemodialysis patients for several months and finding no ill effects.ref|Johnson. However, BUN is a marker for other nitrogenous waste. Thus, when renal failure leads to a buildup of urea "and other nitrogenous wastes" (uremia), an individual may suffer neurological disturbances such as altered cognitive function (encephalopathy), impaired taste (dysgeusia) or loss of appetite (anorexia). The individual may also suffer from nausea and vomiting, or bleeding from dysfunctional platelets. Prolonged periods of severe uremia may result in the skin taking on a grey discolouration or even forming frank urea crystals ("uremic frost") on the skin.

Because multiple variables can interfere with the interpretation of a BUN value, GFR and creatinine clearance are more accurate markers of kidney function. Age, sex, and weight will alter the "normal" range for each individual, including race. In renal failure or chronic kidney disease (CKD), BUN will only be elevated outside "normal" when more than 60% of kidney cells are no longer functioning. Hence, more accurate measures of renal function are generally preferred to assess the clearance for purposes of medication dosing.

Units

BUN is reported as mg/dL in the United States. Elsewhere, the concentration of urea is reported as mmol/L. To convert from mg/dL of blood urea nitrogen to mmol/L of urea, divide by 2.8 (each molecule of urea having 2 nitrogens, each of molar mass 14g/mol) ::Urea (in mmol/L) = BUN (in mg/dL of nitrogen) / 2.8

Methodology

:"See also: Wikibooks: "The test as originally carried out was by flame photometry; now chemical colorimetric tests are more widely used. Three methods are common: Diacetyl Monoxime, Urograph and Modified Berthelot Enzymatic methods:

ee also

*BUN-to-creatinine ratio
*Kt/V
*Urea reduction ratio (URR)
*Standardized Kt/V

References

#Johnson WJ, Hagge WW, Wagoner RD, Dinapoli RP, Rosevear JW. Effects of urea loading in patients with far-advanced renal failure. Mayo Clin Proc. 1972 Jan;47(1):21-9. PMID 5008253


Wikimedia Foundation. 2010.

Игры ⚽ Нужно решить контрольную?

Look at other dictionaries:

  • Blood urea nitrogen — (dt.: „Blut Harnstoff Stickstoff“), meistens in mit BUN abgekürzt, ist ein Maß für die Menge des Stickstoffs im Blut, der durch den Harnstoff hervorgerufen wird. Die Leber produziert Harnstoff im Harnstoffzyklus als Abfallprodukt der… …   Deutsch Wikipedia

  • blood urea nitrogen — BUN BUN n. [acronym] (Med.) same as {blood urea nitrogen}; the concentration of nitrogen in blood present in the form of urea; used as a measure of kidney function. Note: Blood usually contains 10 to 15 mg of nitrogen per 100 ml in the form of… …   The Collaborative International Dictionary of English

  • blood urea nitrogen — see urea nitrogen, under urea …   Medical dictionary

  • Blood urea nitrogen (BUN) — A measure primarily of the urea level in blood. Urea is cleared by the kidney. Diseases that compromise the function of the kidney frequently lead to increased blood levels …   Medical dictionary

  • blood urea nitrogen — (Medicine) waste product created during the metabolic process and excreted into the blood (can be used as an indication of renal function), BUN …   English contemporary dictionary

  • urea nitrogen appearance — the amount of urea in grams produced by a person s body over a specific period of time, closely related to the amount of nitrogen that has not been absorbed by the body. It is calculated as the sum of the urea excreted in the urine plus that… …   Medical dictionary

  • urea nitrogen — the urea concentration of blood or serum stated in terms of nitrogen content; converted to urea concentration by multiplying by 60/28 or 2.14. The serum or plasma urea nitrogen is traditionally referred to as blood urea nitrogen (BUN) …   Medical dictionary

  • urea nitrogen — A chemical in the blood produced by the breakdown of protein. Urea nitrogen is removed from the blood by the kidneys. Blood urea nitrogen (BUN) tests are sometimes done to see how well the kidneys are working …   English dictionary of cancer terms

  • Urea — Urea …   Wikipedia

  • Nitrogen balance — is the measure of nitrogen output subtracted from nitrogen input.[1] Blood urea nitrogen can be used in estimating nitrogen balance, as can the urea concentration in urine. A positive value is often found during periods of growth, tissue repair… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”