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Lab Test: Blood Urea Nitrogen, BUN Level

    Lab Test
    • Blood Urea Nitrogen (BUN Serum)
    Description
    • Measurement of serum or plasma blood urea nitrogen (BUN) for the evaluation and management of volume status and renal disorders. It is performed on patients undergoing routine laboratory testing and is usually performed as part of a multiphasic automated testing process.
    Reference Range
    • Adults:  10-20 mg/dL (3.6-7.1 mmol/L)
    • Elderly:  may be slightly higher than adult
    • Children:  5-18 mg/dL (1.8-6.4 mmol/L)
    • 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)
    Indications & Uses
    • Adrenal insufficiency - moderate elevations in BUN levels are consistent with both acute and chronic adrenal insufficiency.  The increased Bun is largely due to dehydration secondary to aldosterone deficiency, which leads to excretion of sodium in excess of intake and results in azotemia.  Patients with secondary adrenal insufficiency are less affected because of intact aldosterone secretion.  Elevation is usually reversible with restoration of normal renal hemodynamics and circulating blood volume.
    • Community-acquired pneumonia - In one study, an elevated BUN, along with increased respiratory rate and decreased diastolic blood pressure, was predictive of mortality in patients with community-acquired pneumonia.
    • Hemolytic uremic syndrome (HUS) - BUN level is consistently increased with the elevation usually occurring very rapidly.  The combination of renal insufficiency, a catabolic state, and reabsorption of blood from the GI tract can cause BUN levels to increase as much as 50 mg/dL/day.  In children with uncomplicated dehydration and diarrhea, the BUN level should fall to one half the admission level within 24 hours; if this does not occur, renal disease should be suspected.
    • Hemorrhagic shock - Acute tubular necrosis (ATN) from prolonged hypotension results in renal failure.  BUN and creatinine levels typically rise over the first few post shock days in a fixed ratio of 10:1.  Increased BUN levels also occur secondary to the breakdown of blood in the gastrointestinal tract, as seen in gastrointestinal tract hemorrhage and as a result of prerenal azotemia. 
    • Initial evaluation and monitoring of hyperosmolar hyperglycemic state - BUN is almost always increased in HHS secondary to dehydration and renal damage, even in the absence of significant diabetic, nephropathy.  The mean level of BUN is 61 mg/dL in patients presenting with HHS.
    • Initial evaluation and monitoring of suspected diabetic ketoacidosis - BUN level is usually mildly to moderately elevated (mean 32 mg/dL) in diabetic ketoacidosis (DKA), attributable to significant volume loss rather than diabetic nephropathy.  Check BUN every 2 to 4 hours until the patient is stable.
    • Metabolic acidosis - Anion gap metabolic acidosis develops when the glomerular filtration rate is less than 20 mL/min (BUN level greater than 40 mg/dL).
    • Sickle cell disease - a chronically elevated BUN level in conjunction with an elevated serum creatinine, even if only mildly increased, indicates renal insufficiency.
    • Suspected cardiogenic shock - elevated BUN may occur secondary to preexisting renal disease or poor renal perfusion; poor renal perfusion may lead to eventual prerenal azotemia or frank acute tubular necrosis (ATN) from renal ischemia.
    • Suspected dehydration in children with acute gastroenteritis - BUN level below 40 mg/dL may be associated with mild to moderate dehydration.  BUN level above 100 mg/dL may be associated with moderate to severe dehydration.
    • Suspected hypertensive crisis - BUN and creatinine levels tend to rise at the onset of therapy but eventually improve as the renal arterioles recover from the effects of the high pressure.  The rise in BUN may be faster in patients with essential hypertension in the malignant phase than in patients with other underlying primary renal disease.
    • Suspected necrotizing soft tissue infection - in one study, a BUN greater than 51 mg/dL was reported in 60% of cases of synergistic necrotizing cellulitis.
    • Suspected or known acute renal failure - BUN level increases progressively at a rate of at least 10 mg/dL /day in the setting of clinically significant renal failure.  In patients with extensive tissue necrosis, it can rise at a rate of 50 to 100 mg/dL/day.  A BUN of 50 to 100 mg/100mL suggest serious renal impairment, and a BUN of 150 to 250 mg/100 mL is virtually diagnostic of severe glomerular dysfunction.  In a steady state, a 50% decrease in the glomerular filtration rate (GFR) will result in a doubling of the BUN.
    • Suspected or known renal failure - in patients with no history of renal disease, acute renal failure may be defined either by a BUN level of at least 40 mg/dL (14.3 mmol/L) or a serum creatinine level of at least 2 mg/dL (177 micromol/L). If the GFR falls to less than 10 mL/min, serum creatinine should increase by 0.5 to 1.5 mg/dL (44 to 133 micromol/L) per day, depending on age, muscle mass, and muscle injury.  The BUN level should increase by 10 to 20 mg/dL (3.6 to 7.1 mmol/L) per day, but the rate of increase can be higher in hypercatabolic states like sepsis, gastrointestinal bleeding, or with corticosteroid use. 
    • Causes of rapid serial increases in BUN levels:
      • Clinically significant renal failure (e.g., GFR < 10 mL/minute):  increases 10 to 20 mg/dL/day.
      • Extensive tissue necrosis:  increases 50 to 100 mg/dL/day.
      • Hypercatabolic states (e.g., sepsis, gastrointestinal hemorrhage, corticosteroid use)
    • Suspected sepsis - BUN levels may be elevated due to prerenal azotemia, gastrointestinal bleeding drug toxicity or ATN.  The Infectious Disease Society of America recommends BUN and serum creatinine be done at baseline and at least every 3 days during the course of intensive antibiotic therapy.
    • Suspected toxic shock syndrome - BUN may be elevated secondary to myoglobinemia or shock.
    • To differentiate between upper and lower acute gastrointestinal bleeding:
      • In patients with acute GI bleeding, a BUN:creatinine ratio of 36 or greater suggests an upper GI bleeding site.
      • In patients with GI bleeding and no renal disease, a BUN level over 40 mg/dL with a normal creatinine level suggests significant GI blood loss.
      • A BUN level greater than 85 mg/dL with a normal creatinine level may indicate a loss of 2 or more units of blood into the GI tract.
      • In patients with normal renal function, a mild BUN elevation (4 to 7 mg/dL above baseline) secondary to hemorrhage should return to normal within 24 hours.  In patients with GI bleeding, persistent BUN elevation greater than 7 mg/dL above baseline values suggest hypovolemia, renal insufficiency, or continued hemorrhage.
      • In patients with acute GI bleeding, the absence of a rise in BUN does not rule out an upper GI source of bleeding.
    • Suspected post-streptococcal glomerulonephritis - BUN is elevated to some extent in 60% to 75% of post-streptococcal glomerulonephritis patients.
    Clinical Application

    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.  The urea is deposited in the blood and transported to the kidneys for excretion.  The BUN serves as an index of the function of the liver and the kidneys.  Patients who have elevated BUN levels are said to have azotemia or be azotemic. 

    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. 

    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. 

    Increased levels:

    Prerenal causes:

    • Hypovolemia, shock, burns, dehydration - with reduced blood volume, renal blood flow is diminished.  Therefore, renal excretion of BUN is decreased and BUN levels rise.
    • Congestive heart failure myocardia infarction - with reduced cardiac function, renal blood flow is diminished 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 and starvation - 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 - Reduced liver function is associated with reduced BUN levels.
    • Overhydration because of fluid overload syndrome of inappropriate antidiuretic hormone secretions (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.
    Related Tests
    • Basic metabolic panel
    • Comprehensive metabolic panel
    • Enteral/parenteral nutritional management panel
    • General health panel
    • Hypertensive panel
    • Prenatal screening panel
    • Renal panel
    • Transplant panel
    • Creatinine, Blood - test of renal function that is not dependent on liver function.
    • Creatinine clearance
    Drug-Lab Interactions
    • Results increased in:  febrile illness, high protein diet, alimentary tube feeding, gastrointestinal bleeding, dehydrated patients
    • Results decreased in:  low protein diet, high carbohydrate diet, overhydrated patients
    • To some extent, 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.
    • 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.
    Test Tube Needed
    • Red top tube
    Procedure
    • Collect a venous blood sample. 
    • Avoid hemolysis.
    • Apply pressure or a pressure dressing to the venipuncture site and observe the site for bleeding.
    Storage and Handling
    • Sample is stable for 24 hours at room temperature, for several days at 4°C to 6°C, and for 2 to 3 months when frozen.
    What To Tell Patient Before & After
    • Explain the procedure to the patient and tell then that no fasting is required.
    References
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
    • Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.

MESH Terms & Keywords

  • Blood, Urea, Nitrogen, BUN