Laboratory Glucose Testing Methods and Diabetes Insipidus

Laboratory Glucose Testing Methods

• Glucose testing is distinct from analyzing hemoglobin A1C.
• Two primary instrumental methods for glucose testing in the laboratory are glucose oxidase and hexokinase.

Glucose Oxidase Method

• This method involves the oxidation of glucose.
• Reaction Mechanism:
  • Glucose is oxidized to gluconolactone, producing hydrogen peroxide ($H<em>2O</em>2$) as a byproduct.
  • Hydrogen peroxide is named such because there is one hydrogen atom per one oxygen atom per oxide, with the formula being $H<em>2O</em>2$ (two hydrogen atoms, two oxygen atoms).
  • This initial reaction is a peroxidase reaction.
  • The $H<em>2O</em>2$ produced further oxidizes a chromogen using a peroxidase enzyme, leading to the formation of a colored product.
• Measurement: Anytime a colored product is formed, it can be measured spectrophotometrically.
  • The deeper the color of the product, the more glucose was present in the original sample.
  • The oxidized chromogen increases proportionately with increased glucose levels.
  • Therefore, more glucose equals more colored product.
• Applications:
  • Commonly used in in-home blood sugar monitoring using finger stick cassettes.
  • Also utilized in urine dipstick testing for glucose; the colored pads on dipsticks change color based on glucose concentration in urine.
• Specificity and Interferences:
  • Considered the second most specific measurement method.
  • However, it has interferences that compete with the chromogen formation.
  • Elevated uric acid and ascorbic acid (vitamin C) can cause falsely decreased glucose values.
  • Due to these interferences, many laboratories have shifted towards the hexokinase method.

Hexokinase Method

• The hexokinase method is most often used in laboratories and is considered a reference method due to its higher specificity and fewer false results from interferences.
• Reaction Mechanism (Cascading Enzymatic Reaction):
  • Step 1: Glucose reacts with ATP (adenosine triphosphate) in the presence of the enzyme hexokinase, converting ATP to ADP (adenosine diphosphate) and forming glucose-6-phosphate ($G6P$).
    • Equation: Glucose + ATP $\xrightarrow{\text{hexokinase}}$ ADP + Glucose-6-phosphate ($G6P$)
  • Step 2: The $G6P$ then reacts with NADP (nicotinamide adenine dinucleotide phosphate) in the presence of the enzyme glucose-6-phosphate dehydrogenase ($G6PD$) to form NADPH (the reduced form of NADP).
    • Equation: $G6P$ + NADP $\xrightarrow{\text{G6PD}}$ NADPH
• Measurement: NADPH is formed in direct proportion to the amount of glucose in the system.
  • The greater the initial glucose, the greater the change from NADP to NADPH.
  • NADPH formation can be detected spectrophotometrically due to its unique absorbance characteristics.
• Specificity and Interferences:
  • This is considered the most specific method because it employs glucose-specific enzymes.
  • Unlike glucose oxidase, it does not interact with uric acid or ascorbic acid, thus having far fewer interferences.
  • Potential interferences can still occur with severe hemolysis and extreme bilirubin levels, as these can alter the NADP substrate going into the reaction, leading to false interactions and potentially falsely decreased glucose values.

Diabetes Insipidus

• Diabetes insipidus (DI) is an honorarium slide included to distinguish it from diabetes mellitus, as there are more differences than similarities.
• Nomenclature and Etiology:
  • Informally called