UREA

• Kidneys


  • Eliminate excess body water
  • Eliminate waste products of metabolism (urea and creatinine)
  • Eliminate foreign substances like drugs
  • Retain substances necessary for normal body function (proteins & amino acids, glucose)
  • Regulate electrolyte balance and osmotic pressure of the body fluids
  • Endocrine function: production of rennin, prostaglandin and erythropoietin; degradation of insulin, glucagon and aldosterone
• Clinically Significant NPN compounds

  • urea – 45%
  • amino acids – 20%
  • uric acid – 20%
  • creatinine – 5%
  • creatine – 1-2%
  • ammonia – 0.2%
• TOTAL NPN METHODOLOGY

  1. KJELDAHL DIGESTION

  2. MEASUREMENT OF AMMONIA FORMED
• KJELDAHL DIGESTION


  The nitrogen in a pff of the specimen is converted to ammonia using hot conc. H2SO4 with copper sulfate, mercuric sulfate or selenium oxide as the catalysts
• MEASUREMENT OF AMMONIA FORMED

  1. NESSLERIZATION

  2. BERTHELOT METHOD

  3. MONITORING CONSUMPTION OF AMMONIA (Kaplan, Manoukian – Fawaz; Kallet – Cook Reaction)
• NESSLERIZATION


  • Nessler's reagent – double iodide salt of potassium & mercury
  • Gum ghatti – colloidal stabilizer
  • Dimercuric ammonium iodide – yellow to orange brown product
• BERTHELOT METHOD


  • Reagent: phenol and alkaline hypochlorite
  • Catalyst: sodium nitroprusside
  • Product: indophenol blue
• MONITORING CONSUMPTION OF AMMONIA

  • NH3 + α – ketoglutarate + NADH + H → Glutamate + NAD
  • Catalyst: Glutamate dehydrogenase
  • Measure a decrease in the absorbance at 340 nm
• Urea


  • Most abundant NPN compound in plasma
  • Major excretory product of protein metabolism
  • Synthesized in the liver from CO2 and ammonia that arises from the deamination of amino acids in the reaction of the urea cycle
• Urea structure

  NH2- C = O

  NH2

  MW = 60 g/mole
• Urea renal handling

  • 90% excreted through the kidneys
  • 10% excreted through the skin and GIT
  • 40 – 70% reabsorbed in the renal tubules by passive diffusion
• Factors affecting urea concentration

  • Renal function and perfusion
  • Protein content of the diet
  • Amount of protein catabolism
• Methods for urea determination

  • INDIRECT METHOD / ENZYMATIC = measures Blood Urea Nitrogen

  DIRECT METHODS
• INDIRECT METHOD / ENZYMATIC


  Based on the preliminary hydrolysis of urea with urease followed by some process that quantitates the ammonium ion

  Berthelot reaction

  Nessler's reaction

  GLDH-coupled enzymatic method (Dupont ACA Analyzer)

  Conductimetric method: Beckman BUN Analyzer

  Urograph or Urastrat strip

  Indicator dye (uriol): Kodak Ectachem Analyzer
• DIRECT METHODS

  Diacetyl Monoxime (Fearon)

  ortho – phthaldehyde: adapted by automated methods
• Disease correlations

  • Azotemia

  Uremia/ Uremic syndrome

  Decreased Urea
• Azotemia


  A biochemical abnormality pertaining to increase NPN compounds especially creatinine and urea defining GFR defect

  Prerenal – due to reduced renal blood flow

  Renal – decreased renal function

  Postrenal – obstruction of urine flow
• Uremia/ Uremic syndrome


  A clinical syndrome characterized by increased BUN accompanying renal failure seen in metabolic acidosis, hyperkalemia and edema
• Decreased Urea


  Decreased protein intake, severe vomiting and diarrhea
• Specimen Requirements and Interfering Substances:

  Plasma, serum or urine

  Plasma: ammonium ions and high concentrations of sodium citrate and sodium fluoride must be avoided

  Non fasting sample is acceptable

  Nonhemolyzed sample is recommended

  Urine sample guarded against bacterial decomposition of urea
• Reference Interval:

  Adult serum/plasma: 6-20 mg/dL or 2.1-7.1 mmol/L

  Urine, 24hr: 12-20 g/day or 0.43-0.71 mol/day

  Conversion factor: mg/dL ---> mmol/L: 0.357