HealthFlex
×
  • Home
  • Immunology Book
  • Lab Tests
    • Hematology
    • Fluid analysis
    • CSF
    • Urine Analysis
    • Chemical pathology
    • Blood banking
    • Fungi
    • General pathology
    • Immune system
    • Microbiology
    • Parasitology
    • Pathology
    • Tumor marker
    • Virology
    • Cytology
  • Lectures
    • Bacteriology
    • Liver
    • Lymph node
    • Mycology
    • Virology
  • Blog
    • Economics and technical
    • Fitness health
    • Mental health
    • Nutrition
    • Travel
    • Preventive health
    • Nature and photos
    • General topic
  • Medical Dictionary
  • About Us
  • Contact

Blood Urea Nitrogen (BUN), or Urea Nitrogen, and Interpretations

Blood Urea Nitrogen (BUN), or Urea Nitrogen, and Interpretations
April 27, 2022Chemical pathologyLab Tests

Blood Urea Nitrogen (BUN)

Sample for Blood Urea Nitrogen (BUN)

  1. It is done on the serum of the patient.
  2. No special preparation is needed.
  3. This test can be done on a random sample.

Precautions for Blood Urea Nitrogen (BUN)

  1. If there is Fluoride, that will inhibit the Urease reaction.
  2. Avoid hemolysis.
  3. Protein intake will affect BUN. A low protein diet will give low BUN.
  4. A high protein diet or nasogastric tubing will increase BUN.
  5. Keep in mind muscle mass is more in males than females and children.
  6. Overhydrated patients will dilute the BUN and gives a lower value.
  7. The dehydrated patient will concentrate BUN and gives high value.
  8. GI bleeding can cause an increase in BUN levels.
  9. Advanced pregnancy may increase the BUN level.
  10. Drugs increasing the BUN level are cephalosporin, indomethacin, gentamicin, polymyxin B, rifampicin, bacitracin, neomycin, and tetracycline, thiazide diuretics, and aspirin.
  11. Some drugs that decrease the BUN level is streptomycin and chloramphenicol.

Indications for blood urea nitrogen (BUN)

  1. To assess renal function.
  2. As a routine test in the patient with dialysis.
  3. To assess liver function.
  4. This may be part of the routine test.
  5. In patients:
    1. Has nonspecific symptoms.
    2. During the hospital stay.
    3. Before some drug therapy.
    4. Acutely ill patients are admitted in an emergency.
  6. BUN is a less specific indicator of the kidney’s function and is not reliable.

Definition of Blood urea nitrogen (BUN)

  1. It is the nitrogen part of the urea.
Blood urea nitrogen (BUN) formula

Blood urea nitrogen (BUN) formula

  1. For to calculate BUN from the total blood urea = 60/28 = 2.1
  2.  Now the value of blood urea nitrogen (BUN) will be = suppose Blood urea = 100 mg/dL =  100/2.1 = 47.6 mg/dL

Pathophysiology of blood urea nitrogen (BUN) and blood urea:

  1. Blood urea molecule: O = C = ( NH2 )2 .
  2. The urease enzyme can split urea into ammonia and carbon dioxide.
Blood urea and action of urease enzyme

Blood urea and action of urease enzyme

  1. The molecular weight of urea = 60 grams
    1. Each molecule contains 2 nitrogen = 28 grams. It is called blood urea nitrogen (BUN).
    2. While in the SI unit, this is meaningless because it is reported as mmol/L.
    3. The serum concentration of 28 mg/dL of urea-nitrogen (BUN) is equivalent to 60 mg/dL of blood urea or 10 mmol/L of urea or urea-nitrogen in SI units.

Protein metabolism and urea formation:

  1. Proteins cannot be stored in the body. When these are in excess, then surplus amino acids are catabolized for energy.
  2.  Breakdown of the proteins and nucleic acid gives rise to a non-protein nitrogenous compound in the blood:
    1. Urea.
    2. Amino acids.
    3. Urates.
    4. Ammonia.
    5. Creatinine.
  3. Amino acids are converted into ammonia, CO2, H2O, and energy.
    1. While ammonia forms urea which is excreted into the urine.
BUN, urea catabolism and clearance from the body

BUN, urea catabolism, and clearance from the body

    1. Urea is water-soluble and is a waste product excreted in the urine.
    2. Urea concentration in the glomerular filtrate is the same as in the plasma.
    3. Under normal conditions, 40% of the urea filtered is reabsorbed in the tubules.
Urea formation from proteins

Urea formation from proteins

Clearance of the urea:

  1. Blood urea nitrogen is the main waste product of protein metabolism.
  2. Urea forms in the liver with CO2 and is the final product of protein metabolism.
Urea cycle and role of liver and kidney

Urea cycle and role of liver and kidney

Blood urea cycle and role of liver

Blood urea cycle and role of liver

  1. Urea is freely filtered and then partially absorbed by the nephron.
  2. The BUN is used as the index of glomerular function in the production and excretion of urea.
    1. Urea reabsorption is increased in hypovolemia so that BUN will underestimate the Glomerular filtration rate (GFR) and more in hypovolemia.
  3. BUN measures the nitrogen part of the urea.

Blood urea/Blood urea nitrogen (BUN) and role of kidneys and liver:

  1. The toxic level of NH3 is prevented by converting the NH3 into urea, this will take place in the liver.
  2. Urea production and BUN are increased when there is increased amino acids metabolism in the liver.
    1. It can take place by increased intake of the proteins, tissue breakdown, or decreased protein synthesis.
    2. Urea is decreased in case of liver severe liver disease, and low protein intake.
    3. Urea is degraded in the intestine to ammonium ions by the intestinal bacteria.
Urea formation and role of liver

Urea formation and role of liver

  1. This BUN or urea is excreted through the kidney in the urine.
  2. The measurement of urea nitrogen gives an idea of the ratio between excretion and production of urea.
    1. Urea is filtered at the glomerulus, and approximately 40%  to 50% is reabsorbed in the proximal tubules by passive back-diffusion.
    2. In normal conditions, urea clearance values parallel the glomerular filtration rate (GFR) at about 60% of it.
    3. At a low level when urine output is <2 mL/minute, the values are very inaccurate, even with the application of correction formulas.
  3. In the liver, amino acids are catabolized, and free ammonia is produced.
    1. Ammonia molecules combine to form urea.
    2. The urea through blood goes to the kidney and is excreted in the urine.
    3. So BUN depends upon the metabolic function of the liver and excretory function of the kidneys.
Blood urea excretion and role of kidneys and liver

Blood urea excretion and role of kidneys and liver

  1. The BUN is directly related to the metabolic function of the liver and the excretory function of the kidneys.
  2. In chronic renal diseases, the BUN level correlates better than creatinine with the sign and symptoms of the patient.
    1. As the synthesis of BUN depends upon life, patients with severe primary liver disease will have decreased BUN.
    2. In combined liver and renal disease, as in hepatorenal syndrome, the BUN may be normal because of poor liver function resulting in decreased formation of urea.
    3. Overall the BUN is less accurate than creatinine for renal diseases.
    4. In chronic renal diseases, BUN correlates better with the symptoms than the creatinine.

Blood urea nitrogen (BUN) level and degree of azotemia:

BUN level Clinical implication
10 to 20 mg/dL Normal kidney function
<20 mg/dL  (7 mmol/L) Normal and no azotemia
20 to 25 mg/dL   (7 to 18 mmol/L) Mild azotemia
>50 mg/dL (>18 mmol/L) Moderate to severe azotemia
50 to 150 mg/dL Severe kidney disease
Low BUN of 6 to 8 mg/dL
  1. Overhydration
  2. Liver diseases
  1. A high protein diet may increase the BUN, and low protein intake may decrease its level.
  2. Blood urea nitrogen and creatinine ratio also give the idea about the renal, pre-renal, or post-renal diseases.
Clinical condition BUN
Primary liver disease decreased
Combined liver and kidney disease (Hepatorenal syndrome) normal
Dehydration increased
Overhydration decreased

Interpretation of the BUN for the renal functions:

BUN level Clinical interpretations
  • 6 to 8 mg/dL
  • It is associated with overhydration
  • 10 to 20 mg/dL
  • It indicates the normal glomerular function
  • 50 to 150 mg/dL
  • It indicates severe renal function impairment

Clinical presentation of renal dysfunction (S/S of renal disease):

  1. The patient may have edema around the eyes, legs, abdomen, and wrist.
  2. There is a history of fatigue, poor appetite, lack of concentration, and disturbed sleep.
  3. There may be flank pain in the kidneys area.
  4. There may be burning urination, abnormal discharge, and increased frequency.
  5. There is a decrease in the amount of urine.
  6. The urine is bloody or coffee-colored and foamy.
  7. There may be hypertension.

NORMAL BUN and Urea

Source 2

  • Urea
    •  20 to 40 mg/dl
  • BUN
    • Blood urea nitrogen (BUN) = 10 to 20 mg /dl
    • Children (BUN) = 5 to 18 mg/dl
    • Infants = 5 to 18 mg/dL
    • Newborn = 3 to 12 mg/dL
    • Cord blood = 21 to 40 mg/dL
    • Older people may have a higher level than adult.

Source 1

Age Urea nitrogen mg/dL
Cor blood 21 to 40
Premature one week 3 to 25
<1 year 4 to 19
Infant/child 5 to 18
18 to 60 year 6 to 20
60 to 90 8 to 23
>90 years 10 to 31

Source 3 Blood urea nitrogen (BUN)

  • Adult = 10 to 20 mg/dL
  • Older people have a higher value
  • Cord blood = 21 to 40 mg/dL
    • Newborn = 3 to 12 mg/dL
    • Infants = 5 to 18 mg/dL
    • Child = 5 to 18 mg/dL

A level above 100 mg/dL is the critical value indicating severe renal dysfunction.

Increased Urea (BUN) Azotemia seen in:

A. Impaired renal function:

  • Prerenal causes: These are mostly due to decreased blood flow to the kidneys.
  1. Congestive heart failure and Myocardial infarction (CHF).
  2. Salt and water depletion.
  3. Shock.
  4. Stress.
  5. Acute MI.
  6. Hemorrhage in GI tract.
  7. Dehydration.
  8. Excessive protein catabolism.
  9. Burn.

B. Chronic renal diseases:

  • Renal causes: Any obstruction of the urinary tract, also increases the BUN/creatinine ratio. In the case of protein catabolism, the serum creatinine is normal.
  1. Glomerulonephritis (GN).
  2. Pyelonephritis (PN).
  3. Acute tubular necrosis.
  4. Renal failure.
  5. Diabetes mellitus with ketoacidosis.
  6. Anabolic steroids use.
  7. Nephrotoxic drugs.

C. Urinary tract obstruction:

  • Postrenal causes
  1. Ureteral obstruction from stones, tumors, or congenital abnormality.
  2. Bladder outlet obstruction from prostatic hypertrophy, cancer.
  3. Bladder/urethral congenital abnormality.
Uremia causes

Uremia causes

Decreased Urea/BUN seen in:

  1. Severe liver diseases (liver failure).
  2. Malnutrition and a low protein diet.
  3. Impaired absorption of Celiac disease.
  4. Syndrome of inappropriate antidiuretic hormone secretion.
  5. Increased utilization of protein for synthesis:
    1. Late pregnancy.
    2. Acromegaly.
    3. Infants.
    4. Anabolic hormones.
    5. Malnutrition.
  6. Overhydration.
  7. Nephrotic syndrome.

Effect of drugs and other conditions on a BUN:

  1. Some of the drugs that may cause a decrease in BUN like Dextrose infusion, Phenothiazine, and Thymol.
  2. Increased BUN levels may be seen in late pregnancy and infancy because of increased use of proteins.

Questions and answers:

Question 1: What is the difference between blood urea and blood urea nitrogen (BUN)?
Show answer
Blood urea molecular weight is 60, while BUN is only two molecules of nitrogen with a molecular weight of only 28.

Possible References Used
Go Back to Chemical pathology

Add Comment Cancel


  • Lab Tests
    • Blood banking
    • Chemical pathology
    • CSF
    • Cytology
    • Fluid analysis
    • Fungi
    • General pathology
    • Hematology
    • Immune system
    • Microbiology
    • Parasitology
    • Pathology
    • Tumor marker
    • Urine Analysis
    • Virology

About Us

Labpedia.net is non-profit health information resource. All informations are useful for doctors, lab technicians, nurses, and paramedical staff. All the tests include details about the sampling, normal values, precautions, pathophysiology, and interpretation.

[email protected]

Quick Links

  • Blog
  • About Us
  • Contact
  • Disclaimer

Our Team

Professor Dr. Riaz Ahmad Bhutta

Dr. Naheed Afroz Syed

Dr. Asad Ahmad, M.D.

Dr. Shehpar Khan, M.D.

Copyright © 2014 - 2022. All Rights Reserved.
Web development by Farhan Ahmad.