Blood Urea

Introduction

• Urea is the major end product of protein metabolism in humans
  • It’s formed in the liver
• Urea biosynthesis occurs in four stages:
  • Transamination
  • Oxidative deamination of glutamate
  • Ammonia transport
  • Reactions of the urea cycle
• Any defect in any of the enzyme functions of the urea cycle can lead to various metabolic disorders.
• More than 90% of urea is excreted through the kidneys (glomeruli), with losses through GIT and skin in a minor fraction
• Determination of blood urea is important in many kidney diseases and a wide range of conditions that are not primarily renal.

Methods of urea estimation 

Two methods can estimate blood urea

1. Enzymatic method

– Urease method.

– Phenol-hypochlorite method using the Berthelot reaction.

– Nessler’s method.

– Glutamate dehydrogenase method

2. Non-enzymatic method

– Diacetyl monoxime method (DAM)

Principle

Diacetyl monoxime method (DAM)

Under acidic conditions, when urea is heated with compounds containing two adjacent carbonyl groups, such as Diacetyl, in the presence of ferric ions and thiosemicarbazide, pink-coloured diazine is formed at 520 nm.

Enzymatic method

Urease method

urea + H₂O   ———  Urease  —->  2NH₃ + CO₂

Nessler’s method

NH₃ + K₂HgI₄    ———————> Brown compound

            (potassium mercuric iodide)                     (measured at 450nm)

Sample

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.

Reagent’s composition

1. Mixed acid reagent (MAR)

It contains orthophosphoric acid, sulphuric acid, ferric chloride and distilled water.

2. Mixed colour reagent (MCR)

It contains Diacetyl monoxime and thiosemicarbazide.

Std. Conc. is 40mg/dl

Preparation of protein-free filtrates (PFF)

1. DW                                      5 ml
2. Blood                                  1 ml
3. 10% Sod. Tungstate         2 ml
4. 2/3 NH₂SO₄                     2 ml

(1 ml of protein-free filtrate = 0.025 ml of blood)

Procedure

	Test	Standard	Blank
Distilled water, ml	4.0	3.9	3.9
Standard (40 mg/dl), ml	–	0.1	–
PFF, ml	0.1	–	–
Mixed acid reagent, ml	1.5	1.5	1.5
Mixed coloured reagent, ml	1.5	1.5	1.5

Mix thoroughly and place the tubes in a water bath for 30 minutes. Cool and read the pink-coloured solution at 520 nm.

Calculation

Conc. of blood urea = OD of test / OD of std. x Conc. of std. (40mg/dl)

BUN to urea

          Mol. Wt. of urea is 60

2 atoms of N = 14 x 2 = 28

BUN mg = mg urea  x

= mg urea ÷ 2.14

Normal range of Blood Urea Nitrogen and urea

Urea

20 to 40 mg/dl

BUN

1. Blood urea nitrogen = 10 to 20 mg /dl
2. Children (BUN) = 5 to 18 mg/dl
3. Infants = 5 to 18 mg/dl
4. Newborn = 3 to 12 mg/dl

Clinical significance

Azotemia: increased blood urea

Uremia: Azotemia is associated with clinical symptoms indicative of multi-organ failure, also called end-stage renal disease.

Cause of urea plasma elevation: –

1. Pre-renal 
2. Renal
3. Post-renal

Causes of increased urea (BUN)

Impaired Renal Function:

Prerenal causes

1. These are mostly due to decreased blood flow to the kidneys.
2. Congestive heart failure and Myocardial infarction.
3. Salt and water depletion.
4. Stress.
5. Hemorrhage in GI tract.
6. Dehydration.
7. Excessive protein catabolism.
8. Burn.

Chronic Renal Diseases:

Renal causes:

1. Any urinary tract obstruction also increases the BUN/creatinine ratio. In the case of protein catabolism, the serum creatinine is normal.
2. Diabetes mellitus with ketoacidosis

Urinary Tract Obstruction:

Postrenal causes

1. Ureteral obstruction:
   1. By the stone
   2. Cancers
   3. Inflammation
   4. Surgical procedure
   5. Obstruction of the bladder, urethra
   6. Prostatic enlargement
   7. Prostatic cancer
   8. Inflammation
   9. Stones

Causes of decreased Urea/BUN

1. Malnutrition and a low protein diet.
2. Overhydration
3. Pregnancy