Carbohydrates

• Composed of carbon, hydrogen and oxygen
• Are water soluble
• Are important source of energy for the body’s mechanisms
• Classifications:
  
  • Monosaccharides: Glucose, fructose and galactose
  • Disaccharides: maltose (glucose + glucose), lactose (galactose + glucose) and sucrose (fructose + glucose; most common non reducing sugar)
  • Polysaccharides: starch and glycogen

GLUCOSE

• Primary sugar found circulating in the body
• Carbohydrate metabolism:
  
  • Glycolysis: glucose à lactate or pyruvate à energy (↑ glucose)
  • Glycogenolysis: breakdown of glycogen to glucose (↑ glucose)
  • Glycogenesis: formation of glycogen from sugars for storage (↓glucose)
  • Gluconeogenesis: formation of glucose from non-carbohydrate sources (↓ glucose)
• Hormones for glucose regulation
  
  • Hypoglycemic
    
    • Insulin – released by β cells of islet of Langerhans
      
      • Entry of glucose in the cell
      • Falsely low measurement of serum insulin is seen in the presence of
  • Hyperglycemic
    
    • Glucagon – released by α cells of islet of Langerhans
      
      • Primary hormone that increases glucose concentration.
      • NV in fasting plasma: 25-50pg/mL
    • Somatostatin – released by delta cells of islet of Langerhans
      
      • Inhibits the action on inulin, GH and glucagon.
    • Cortisol
    • Epinephrine
    • Growth hormone
    • Thyroxine
    • ACTH
• MUST KNOW FOR SPECIMEN FOR GLUCOSE DETERMINATION
  
  • FBS should be obtained from an 8-10 hours fasting sample
  • In terms of glucose levels: capillary > venous but < arterial
  • Glucose is metabolized at:
  • Room temperature: 7 mg/dL/hr
  • 4°C: 2 mg/dL/hr

GLUCOSE DETERMINATION

I. Chemical Method

1. Chemical Method
   1. Alkaline Copper Reduction Method
      • Folin Wu – Modification: Benedict Test
        • Principle: Copper Reduction
        • Reagents
          • Alk. Copper reagent
          • Phosphomolybdic Acid
        • End Color / Color Reaction: Molybdenum – BLUE
      • Nelson- Somogyi
        • Principle: Copper Reduction
        • Reagents
          • Alk. Copper reagent
          • Arsenomolybdic acid
          • End Color / Color Reaction: Molybdenum – BLUE
      • Neocuproine
        • Principle: Copper Reduction
        • Reagents
          • Cuprous ions
          • Neocuproine
        • End Color / Color Reaction: Cuprous-Neocuproine Complex – YELLOW/ YELLOW ORANGE
   2. Alkaline Ferric Reduction Method
      • Autoanalyzer (Hagedorn-Jensen)
        • Principle: Ferricyanide reduction (Inverse Colorimetry)
        • Reagents: K3Fe(CN)6
        • End Color / Color Reaction: K₃Fe(CN)₆^-4
2. Condensation Method
   • • Ortho-Toluidine
       • Principle: Dubowski reaction; Condensation Method
       • Reagent: 
         • O-toluidine
         • Glacial Acetic Acid
       • End Color / Color Reaction: Glycosylamine – BLUE GREEN

II. Enzymatic Method

• • • Glucose Oxidase
      • Saifer Gernstenfield 
      • Clarke electrode
      • Principle: 
        • Enzymatic 
          • Colorimetric 
          • Polarographic
      • Reagents: 
        • Glucose Oxidase
        • Peroxidase
        • O-dianisidine
      • End Color / Color Reaction: Oxidized o-dianisidine – ORANGE BROWN
    • Hexokinase (REFERENCE METHOD)
      • Principle: Enzymatic 
      • Reagents:
        • Hexokinase
        • G6PD
      • End Color / Color Reaction: NADPH+

LABORATORY TESTS

• Screening Tests
  
  • Fasting Blood Sugar – 8-10 hours fasting
    
    • Normal: <100 mg/dL
    • Impaired fasting glucose: 100-125 mg/dL
    • Diabetic: >126 mg/dL
  • 2-hours post-prandial – a fasting blood samples is extracted, after which, patient is given glucose load (75g). After 2 hours, blood glucose is measured.
    
    • Normal: <140 mg/dL
    • Impaired: 140-199 mg/dL
    • Diabetic: > 200 mg/dL

• Confirmatory Tests
  
  • Oral Glucose Tolerance Test – series of glucose testing
    
    • Patient is instructed to consume a normal to high CHO diet per day for 3 days prior to procedure
    • Patient should be ambulatory
    • The patient should be finished within 5 minutes
    • Glucose loads: adult (75g), pregnant (100g) and children (1.75g/kg)
    • Normal: <140 mg/dL
    • Impaired: 140-199 mg/dL
    • Diabetic: > 200 mg/dL
• Monitoring Test
  
  • HbA1c – long term monitoring (2-3 months)
    
    • Dependent upon the patients’ RBCs lifespan
    • Sample: EDTA whole blood, non-fasting
    • For every 1% increase in HbA1c = 35mg/dL change in plasma glucose!
  • Fructosamine – short term monitoring (2-3 weeks)
    
    • Levels of albumin affects results

CLINICAL SIGNIFICANCE