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.

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 an unreliable and less specific indicator of the kidney’s function.

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. Remember that muscle mass is higher in males than in females and children.
6. Overhydrated patients will dilute the BUN and give 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 are streptomycin and chloramphenicol.

Definition of Blood urea nitrogen (BUN)

1. It is the nitrogen part of the urea.
2. BUN measures the nitrogen part of the urea.
3. Urea nitrogen is synthesized mainly in the liver. It is mostly the end product of protein metabolism.
4. Urea in the blood is reported as blood urea nitrogen (BUN).

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

1. Blood urea molecule: O = C = ( NH2 )2.
2. Urea production and BUN increase when more amino acids are metabolized in the liver.
3. This occurs with a high-protein diet, tissue breakdown, and or decreased protein synthesis.
4. While urea and BUN production is reduced in case of low protein intake and severe liver disease.
5. The urease enzyme can split urea into ammonia and carbon dioxide.
6. >90% of the urea is excreted through the kidneys, the rest through GIT and skin.
7. Urea is freely filtered from the glomeruli.
   1. 40% to 70% of the urea is highly diffusible and moves passively out of the renal tubule into the interstitium and ultimately enters the blood.

8. 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. Surplus amino acids are catabolized for energy when these are in excess.
2.  The breakdown of the proteins and nucleic acid gives rise to a non-protein nitrogenous compound (NPN) in the blood:
   1. Urea. This is present in high concentrations in the blood. (45% of total NPN).
   2. Amino acids.
   3. Uric acid and Urates.
   4. Ammonia.
   5. Creatinine.
   6. Creatine
   7. Ammonia (NH3)
3. Amino acids are converted into ammonia (NH3), CO₂, H₂O, and energy.
   1. NH3, before it reaches a toxic level, is prevented by the conversion of the NH3 to urea.
      1. Urea is synthesized in the liver by CO2 and ammonia (NH3).
      2. CO2 and ammonia (NH3) are produced from the deamination of the amino acids in the urea cycle.
   2. While ammonia (NH3) forms urea which is excreted into the urine. Conversion of NH3 to urea takes place in the liver.

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

Urea formation from proteins metabolism

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.

3. Urea is cleared by the kidneys.
4. Urea is freely filtered and then partially absorbed by the nephron. 40% to 50% of filtered urea is reabsorbed by the proximal tubules.
5. The BUN is used as the glomerular function index in urea production and excretion.
   1. Urea reabsorption is increased in hypovolemia, so that BUN will underestimate the Glomerular filtration rate (GFR) and more in hypovolemia.

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, which will occur in the liver.
2. Urea production and BUN increase when the liver has increased amino acid metabolism.
   1. It can occur by increased protein intake, tissue breakdown, or decreased protein synthesis.
   2. Urea is decreased in case of severe liver disease and low protein intake.
3. Urea is degraded in the intestine to ammonium ions by the intestinal bacteria.
4. This BUN, or urea, is excreted through the kidney in the urine.
5. 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.
6. In the liver, amino acids are catabolized, producing free ammonia.
   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 liver’s metabolic function and kidneys’ excretory function.

7. BUN is directly related to the liver’s metabolic function and the kidneys’ excretory function.
8. 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 creatinine.

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

9. A high protein diet may increase the BUN, and low protein intake may decrease its level.
10. Blood urea nitrogen and creatinine ratio also explain renal, pre-renal, or post-renal diseases.

Interpretation of the BUN for renal functions:

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 kidney 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 adults.

Source 1

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:

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

B. Chronic renal diseases:

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

C. Urinary tract obstruction:

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

Common causes of uremia:

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

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