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Blood Urea

TestBlood Urea Nitrogen
Known asBUN, Urea Nitrogen, BUN to Creatinine Ratio
Sample Required2 ml Blood Clotted Sample
Sample PreparationsNo special preparation is needed.
Purpose1. To evaluate the health of your kidneys
2. To help diagnose kidney disease
3. To monitor the effectiveness of dialysis and other treatments related to kidney disease or damage
Reference ValuesUrea = 20 to 40 mg/dl
BUN= 10 to 20 mg /dl
Critical Value Upto 100 mg/dl

A blood urea test is used to determine how well your kidneys are working. It does this by measuring the amount of urea nitrogen in the blood. Urea is a waste product that’s created in the liver when the body breaks down proteins. Normally, the kidneys filter out this waste, and urinating removes it from the body.

Blood Urea

Urea levels tend to increase when the kidneys or liver are damaged. Having too much urea nitrogen in the blood can be a sign of kidney or liver problems.

Also Known as: Blood Urea, Urea, Serum Urea


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Panel Tests: Urea, Blood Urea Nitrogen, Creatinine, Calcium, Phosphorus, Albumin, Magnesium, Total Protein, Electrolytes

Why Get Tested:

  1. To assess the 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. Prior to some drug therapy.
    4. Acutely ill patient admitted in an emergency.

When to get Tested:

  • As part of a routine comprehensive or basic metabolic panel (CMP or BMP) during a health exam.
  • When you have signs and symptoms that may be due to kidney disease or you have a condition that may cause or be worsened by kidney dysfunction.
  • At regular intervals when you are being treated for kidney disease or damage.

Sample Required:

  • This is done on the serum of the patient.
  • No special preparation is needed.

Precautions for Sample:

  • If there is Fluoride that will inhibit Urease reaction.
  • Avoid hemolysis.
  • Protein intake will affect BUN. Low protein diet will give low BUN.
  • High protein diet or nasogastric tubing will increase BUN.
  • Keep in mind muscle mass which is more in males than females and children.
  • Overhydrated patient will dilute the BUN and gives a lower value.
  • The dehydrated patient will concentrate BUN and gives high value.
  • GI bleeding can cause an increase in BUN level.
  • Advanced pregnancy may increase the BUN level.
  • Drugs increasing the BUN level are cephalosporin, indomethacin, gentamicin, polymyxin B, rifampicin, bacitracin, neomycin, tetracycline, thiazide diuretics, and aspirin.
  • There are drugs which decrease the BUN level are streptomycin and chloramphenicol.

Normal Ranges:

  • Urea = 20 to 40 mg/dl
  • Blood urea nitrogen (BUN) = 10 to 20 mg /dl

Increased Urea (BUN) Azotemia seen in:

A. Impaired renal function:

Prerenal causes:
  1. congestive heart failure and Myocardial infarction (CHF).
  2. Salt and water depletion
  3. Shock
  4. Stress
  5. Acute MI
  6. Hemorrhage into GI tract
  7. Dehydration.
  8. excessive protein catabolism.
  9. Burn.

B. Chronic renal diseases:

Renal causes
  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.

Decreased Urea (BUN) seen in:

  1. Liver failure.
  2. Malnutrition, and low protein diet.
  3. Impaired absorption of Celiac disease.
  4. Syndrome of inappropriate antidiuretic hormone secretion.
  5. Pregnancy.
  6. Overhydration.
  7. Nephrotic syndrome.

Effect of drugs and other condition on a BUN:

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

BUN/Creatinine ratio:

  1. The normal BUN/creatinine ratio is around 10:1.
  2. BUN/creatinine ratio in the normal range, in the case of raised BUN and creatinine.  This will suggest intrarenal disease:
    1. Glomerulonephritis.
    2. Tubulointerstitial nephritis.
  3. When the BUN / Creatinine ratio is raised will suggest:
    1. Prerenal azotemia.
    2. Postrenal azotemia.
      1. These conditions may be seen in hypovolemia or hypotension.
  4. BUN/creatinine ratio decreased is very rare and this may be seen in:
    1. Protein deficiency in the diet.
    2. In severe liver disease

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