• Urea is the chief nitrogenous waste formed during protein metabolism in man.
• It is devised principally from the amino groups of amino acids.
• Liver is the main organ where urea is synthesized by a process called ornithine cycle.
• After urea is formed in the liver, it passes into the blood and then excreted in the urine.
• The concentration of urea in blood depends upon the relationship between urea production and urea excretion.

Normal value of blood urea

• The normal blood (serum) urea = 10-50 mg/100ml of blood.
• This value varies directly with the protein intake of the individual.
• The urea molecule contains two nitrogen atoms.
• So, the concentration of urea is expressed as blood urea nitrogen (BUN).
• The conversion of blood urea nitrogen (BUN) value to the blood urea is done by the following formula.
• Value of blood urea = BUN X 2.14
• Hence the normal BUN = 6-22mg/100ml of serum.

Clinical significance blood urea:

• Increase in BUN = The value increase in kidney disease, shock, dehydration, diabetes, acute myocardial infarction.
• Decrease in BUN = The value decreases in liver failure, impaired absorption and overhydration (increased urinary outflow).

Method for estimation of blood urea

• Blood urea can be estimated by the following method.

• Berthelot method:
  • In this method to estimate blood urea, blood plasma or serum is used.
  • It is based on the principle that urea is hydrolysed into carbonic acid and ammonia by enzyme urease.
  • The ammonia produced is measured photometrically after its reaction with phenol in the presence of hypochlorite (Berthelot reaction).
  • This blue color reaction product is determined photometrically.
  • This method is widely used because of absolute specificity of enzyme urease.

Materials required:

• Buffered enzyme:
  • It is prepared by missing 1ml of enzyme concentrate urease with 100ml of reagent 1 consisting of phosphate buffer.
    1ml enzyme concentrate = 100ml reagent 1 =buffered enzyme.
• Reagent 2:
  • It is a ready to use buffered hypochlorite solution.
• Standard:
  • This is also a ready to use solution. It consists of BUN equivalent of 37.28 mg/dl.
• The reagents are stable for 6 weeks at 2-8^oC after opening.

Procedure for estimation blood urea:

• Sample to be tested (blood plasma/ serum separated out)
• Take 3 test-tubes and label them as blank, test (or sample) and standard. Add the contents in each as given.
  • Blank tube- 1ml of buffered enzyme + 1ml of reagent 2.
  • Test or sample tube – 1ml of buffered enzyme + 1ml of reagent 2 + 10 l of sample
  • Standard tube – 1 ml of buffered enzyme + 1ml of reagent 2 + 10 l of standard solution.
• Mix the contents in each tube and incubate all three test-tubes for 10 minutes at 20-25^oC.
• Measure the absorbance of the test or sample tube and the standard against the reagent blank at 600nm.

Blood Urea: normal value, clinical significance and methods of estimation

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