Blood urea nitrogen
Encyclopedia
The blood urea nitrogen (BUN, pronounced "B-U-N") 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 by- product from metabolism of proteins by the liver
and is removed from the blood by the kidneys.
as a waste product of the digestion
of protein
. Normal human adult blood should contain between 7 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.
, 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, patients on total parenteral nutrition, steroid
use, burns, or fevers.
When the ratio of BUN to creatinine (BUN:Cr) is greater than 20, the patient is suspected of having prerenal azotemia. This means that the pathologic process is unlikely to be due to intrinsic kidney damage.
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. 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 sallow, yellowish discoloration 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.
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)
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 = 2 for conversions in mmol (2 moles N per mole of urea):
Renal function
Renal function, in nephrology, is an indication of the state of the kidney and its role in renal physiology. Glomerular filtration rate describes the flow rate of filtered fluid through the kidney...
. Urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
is a by- product from metabolism of proteins by the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
and is removed from the blood by the kidneys.
Physiology
The liver produces urea in the urea cycleUrea cycle
The urea cycle is a cycle of biochemical reactions occurring in many animals that produces urea from ammonia . This cycle was the first metabolic cycle discovered , five years before the discovery of the TCA cycle...
as a waste product of the digestion
Digestion
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....
of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
. Normal human adult blood should contain between 7 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.
Interpretation
The most common cause of an elevated BUN, azotemiaAzotemia
Azotemia is a medical condition characterized by abnormally high levels of nitrogen-containing compounds, such as urea, creatinine, various body waste compounds, and other nitrogen-rich compounds in the blood...
, is poor kidney function, although a serum creatinine
Creatinine
Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body...
level is a somewhat more specific measure of renal
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
function (see also renal function
Renal function
Renal function, in nephrology, is an indication of the state of the kidney and its role in renal physiology. Glomerular filtration rate describes the flow rate of filtered fluid through the kidney...
).
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
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...
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
Creatinine
Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body...
may reflect a physiological response to a relative decrease of blood flow to the kidney (as seen in heart failure or dehydration
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...
) 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
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
. Enhanced metabolism of proteins will also increase urea production, as may be seen with high protein diets, patients on total parenteral nutrition, steroid
Steroid
A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...
use, burns, or fevers.
When the ratio of BUN to creatinine (BUN:Cr) is greater than 20, the patient is suspected of having prerenal azotemia. This means that the pathologic process is unlikely to be due to intrinsic kidney damage.
A low BUN usually has little significance, but its causes include liver problems, malnutrition
Malnutrition
Malnutrition is the condition that results from taking an unbalanced diet in which certain nutrients are lacking, in excess , or in the wrong proportions....
(insufficient dietary protein), or excessive alcohol consumption. Overhydration from intravenous fluids can result in a low BUN. Normal changes in renal bloodflow during pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
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
Hemodialysis
In medicine, hemodialysis is a method for removing waste products such as creatinine and urea, as well as free water from the blood when the kidneys are in renal failure. Hemodialysis is one of three renal replacement therapies .Hemodialysis can be an outpatient or inpatient therapy...
patients for several months and finding no ill effects. However, BUN is a marker for other nitrogenous waste. Thus, when renal failure leads to a buildup of urea and other nitrogenous wastes (uremia
Uremia
Uremia or uraemia is a term used to loosely describe the illness accompanying kidney failure , in particular the nitrogenous waste products associated with the failure of this organ....
), an individual may suffer neurological disturbances such as altered cognitive function (encephalopathy
Encephalopathy
Encephalopathy means disorder or disease of the brain. In modern usage, encephalopathy does not refer to a single disease, but rather to a syndrome of global brain dysfunction; this syndrome can be caused by many different illnesses.-Terminology:...
), impaired taste (dysgeusia
Dysgeusia
Dysgeusia is the distortion of the sense of taste. Dysgeusia is also often associated with ageusia, which is the complete lack of taste, and hypogeusia, which is the decrease in taste sensitivity. An alteration in taste or smell may be a secondary process in various disease states, or it may be...
) or loss of appetite (anorexia
Anorexia (symptom)
Anorexia is the decreased sensation of appetite...
). 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 sallow, yellowish discoloration 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 (urea-N) is reported as mg/dL in the United States, Italy, Austria, 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, divide by 2.8 (each molecule of urea having 2 nitrogens, each of molar mass 14g/mol)
-
- Urea [mmol/L] = BUN [mg/dL of nitrogenNitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
] x 10 [dL/L] / 14x2 [mg N/mmol urea]
- Urea [mmol/L] = BUN [mg/dL of nitrogen
convert BUN to urea in mg/dL by using following formula:
-
- Urea [mg/dL]= BUN [mg/dL] * 2.14
(conversion factor derived by: MW of urea = 60, MW of urea nitrogen = 14x2 => 60/28 = 2.14)
factor = 2 for conversions in mmol (2 moles N per mole of urea):
-
- BUN [mmol/L]= 2x urea [mmol/L]
See also
- BUN-to-creatinine ratioBUN-to-creatinine ratioIn medicine, the BUN-to-creatinine ratio is the ratio of two serum laboratory values, the blood urea nitrogen and serum creatinine . Outside the United States, particularly in Canada and Europe, the truncated term urea is used and the units are different...
- Kt/VKt/VIn medicine, Kt/V is a number used to quantify hemodialysis and peritoneal dialysis treatment adequacy.*K - dialyzer clearance of urea*t - dialysis time*V - volume of distribution of urea, approximately equal to patient's total body water...
- Urea reduction ratio (URR)
- Standardized Kt/VStandardized Kt/VStandardized Kt/V, also std Kt/V, is a way of measuring dialysis adequacy. It was developed by Frank Gotch and is used in the USA to measure dialysis. Despite the name, it is quite different from Kt/V...