Urea Nitrogen, Blood (Blood Urea Nitrogen [BUN], Serum Urea Nitrogen)

Urea Nitrogen, Blood (Blood Urea Nitrogen [BUN], Serum
Urea Nitrogen)

Adult: 10-20 mg/dL or 3.6-7.1 mmol/L (SI units)
Elderly: may be slightly higher than adult
Child: 5-18 mg/dL
Infant: 5-18 mg/dL
Newborn: 3-12 mg/dL
Cord: 21-40 mg/dL

Critical Values

100 mg/dL (indicates serious impairment of renal function)


BUN is an indirect and rough measurement of renal function and glomerular filtration rate (if normal liver function exists). It is also a measurement of liver function. It is performed on patients undergoing routine laboratory testing. It is usually performed as a part of a multiphasic automated testing process.


The BUN measures the amount of urea nitrogen in the blood. Urea is formed in the liver as the end product of protein metabolism and digestion. During ingestion, protein is broken down into amino acids. In the liver these amino acids are catabolized and free ammonia is formed. The ammonia molecules are combined to form urea, which is then deposited in the blood and transported to the kidneys for excretion. Therefore the BUN is directly related to the metabolic function of the liver and the excretory function of the kidney. It serves as an index of the function of these organs. Patients who have elevated BUN levels are said to have azotemia or be azotemic.
Nearly all renal diseases cause an inadequate excretion of urea, which causes the blood concentration to rise above normal. If the disease is unilateral, however, the unaffected kidney can compensate for the diseased kidney and the BUN may not become elevated. The BUN also increases in conditions other than primary renal disease. Prerenal azotemia refers to elevation of the BUN as a result of pathologic conditions that affect urea nitrogen accumulation before it gets to the kidney. Examples of prerenal azotemia include shock, dehydration, congestive heart failure, and excessive protein catabolism.
Another example of prerenal azotemia is gastrointestinal bleeding that causes variable and sometimes significant blood in the intestinal tract. The proteins in the blood and blood cells are digested to urea.
As the marked increase in intestinal urea is absorbed, the BUN can be expected to increase, sometimes
significantly. Postrenal azotemia refers to pathologic conditions that affect urea nitrogen accumulation after it gets to the kidney. Examples of this include ureteral and urethral obstruction.
Finally, the synthesis of urea depends on the liver. Patients with severe primary liver disease will have a decreased BUN. With combined liver and renal disease (as in hepatorenal syndrome), the BUN can be normal because poor hepatic functioning results in decreased formation of urea and is not an indicator that renal excretory function is adequate.
The BUN is interpreted in conjunction with the creatinine test. These tests are referred to as “renal function studies.” The BUN/creatinine ratio is a good measurement of kidney and liver function. The normal adult range is 6 to 25, with 15.5 being the optimal value.


• Changes in protein intake may affect BUN levels. Low-protein diets will decrease BUN if caloric intake is maintained with carbohydrates. High-protein diets or alimentary tube feeding is associated with elevated BUN levels.
• To some degree, muscle mass determines BUN levels. Women and children tend to have lower BUN levels than men.
• Advanced pregnancy may cause increased levels as a result of high protein metabolism.
• Gastrointestinal bleeding can cause increased BUN levels.
• Overhydration and underhydration will affect levels. Overhydrated patients tend to dilute the BUN and have lower levels. Dehydrated patients tend to concentrate BUN and have higher levels.
Drugs that may cause increased BUN levels include allopurinol, aminoglycosides, cephalosporins, chloral hydrate, cisplatin, furosemide, guanethidine, indomethacin, methotrexate, methyldopa, nephrotoxic drugs (e.g., aspirin, amphotericin B, bacitracin, carbamazepine, colistin, gentamicin, methicillin, neomycin, penicillamine, polymyxin B, probenecid, vancomycin), propranolol, rifampin, spironolactone, tetracyclines, thiazide diuretics, and triamterene.

Drugs that may cause decreased levels include chloramphenicol and streptomycin.

Clinical Priorities
• Almost all renal diseases cause an inadequate excretion of urea, which causes the BUN to rise. Since the synthesis of urea depends on the liver, severe liver disease can cause a decreased BUN. Therefore the BUN is directly related to the metabolic function of the liver and the excretory function of the kidney.
• Changes in protein intake can affect BUN levels. Low-protein diets can decrease the BUN and high-protein diets can increase BUN levels.
• Hydration status can also affect levels. Overhydration will dilute the BUN and cause lower levels. Dehydration tends to concentrate the BUN and cause higher levels.

💠Explain the procedure to the patient.
💠Tell the patient that no fasting is required.

• Collect a venous blood sample in a red-top tube.
• Avoid hemolysis.
• Apply pressure or a pressure dressing to the venipuncture site.
• Observe the venipuncture site for bleeding.


⬆️⬆️⬆️ Increased Levels

Prerenal Causes
With reduced blood volume, renal blood flow is diminished. Therefore renal excretion of BUN is decreased and BUN levels rise.
Congestive heart failure,
Myocardial infarction:
With reduced cardiac function, renal blood flow is diminished. Therefore renal excretion of BUN is decreased and BUN levels rise.
GI bleeding,
Excessive protein ingestion (alimentary tube feeding):
Blood or feeding supplements overload the gut with protein. Urea is formed at a higher rate and BUN accumulates.
Excessive protein catabolism,
As protein is broken down to amino acids at an accelerated rate, urea is formed at a higher rate and BUN accumulates.
Sepsis: For a host of reasons, renal blood flow and primary renal function are reduced. BUN levels rise.

Renal Causes
Renal disease (e.g., glomerulonephritis, pyelonephritis, acute tubular necrosis),

Renal failure,
Nephrotoxic drugs:
Primary renal diseases are all associated with reduced excretion of BUN.
Postrenal Azotemia
Ureteral obstruction from stones, tumor, or congenital anomalies,
Bladder outlet obstruction from prostatic hypertrophy or cancer or bladder/urethral congenital anomalies:
Obstruction of the flow of urine causes reduced excretion and BUN levels rise.
⬇️⬇️⬇️ Decreased Levels
Liver failure: BUN is made in the liver from urea. Reduced liver function is associated with reduced BUN levels.

Overhydration because of fluid overload syndrome of inappropriate antidiuretic hormone secretion (SIADH): BUN is diluted by fluid overload.
Negative nitrogen balance (e.g., malnutrition, malabsorption): With protein depletion, urea production is reduced and therefore BUN is reduced.
Pregnancy: Early pregnancy is associated with increased water retention and BUN dilution.
Nephrotic syndrome: This syndrome is associated with protein loss in the urine. With protein depletion,
BUN is reduced.

Creatinine, Blood . This is a more accurate test of renal function that is not dependent on liver function.
Creatinine Clearance . Like creatinine, this is a more accurate test of renal function.

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