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• Section Writer: Dr. Om J Lakhani
• Section Editor: Dr. Om J Lakhani

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FAQs on Blood Glucose Testing

Q. What precautions are taken while drawing plasma blood glucose?

• Collect from an arm different from where the IV line is to avoid contamination with IV fluid.
• Centrifuge and ship on ice or use a fluoride bulb.

Q. Which is higher – whole glucose values or plasma glucose values?

• Plasma glucose values are higher than whole glucose values by 12-15%.

Q. What is the difference between venous and arterial blood glucose?

• Fasting: Venous and arterial values are the same.
• Post-meal: Venous values are 10% less than arterial.

Q. What is the SI unit for glucose?

• Mmol/liter
  • To convert to mg/dl: multiply by 18.

Q. Which are the two major ways in which blood glucose is measured?

1. Glucose oxidase methods (GOD/POD).
2. Enzymatic methods.

Q. Which of the above is better?

• Enzymatic methods: More sensitive and specific.
• Glucose oxidase methods: Cheaper.

Q. Which is the reference enzymatic method?

• Hexokinase/G6PDH method.

Q. Which substances in the blood produce errors with GOD/POD?

• Reducing substances: Ascorbic acid, tetracycline, glutathione, bilirubin.

Q. Describe the enzymatic reaction in the GOD-POD method:

1. Reaction:
   Glucose + O₂ → (Via Glucose oxidase enzyme) → Glucuronic acid + H₂O₂
2. Measurement:
   H₂O₂ + Reduced dye → H₂O + Oxidized dye (measured electrochemically).

Q. What type of glucose does Glucose oxidase act upon?

• Acts on beta D-glucose, not alpha D-glucose (specific step in GOD-POD).

Q. Give the chemical reaction for Hexokinase/G6PDH method:

1. Glucose → (Hexokinase enzyme) → Glucose-6-phosphate + ADP.
2. Glucose-6-phosphate → (G6PDH enzyme) → 6-phosphogluconate + NADPH + H.
   • NADPH: Measured using fluorescence.

Q. What method is used in most labs?

• Glucose oxidase method on Beckman Coulter Unicel DXC-800 analyzer.

Q. What blood glucose values are reported by glucometers compared to laboratories?

• Glucometers: Whole blood glucose values.
• Laboratories: Plasma glucose values.
  • Modern glucometers: Adjust to plasma glucose values.

Q. What are the ISO standards for glucometers?

• Blood glucose <70 mg/dl: 95% of readings should be within 15 mg/dl.
• Blood glucose >70 mg/dl: Should be within 20%.

Q. What is the purpose of fluoride bulbs, and what does it inhibit?

• Fluoride inhibits the enolase enzyme, preventing glycolysis during transport.

Q. How long is glucose stable:

• With fluoride bulb: 72 hours at room temperature.
• Without fluoride: 8 hours at 25°C or 72 hours at 4°C.

Q. If heparin is used to separate plasma, what is the difference in values?

• Glucose values are 5% lower.

Q. Is finger prick glucose arterial or venous?

• It is arterial.

Q. Where is the finger prick done, and why?

• Lateral aspect of the fingertip (fewer nerve endings, less pain).

Q. What is the typical range for blood glucose strips?

• 10–600 mg/dl.

Q. If glucometer strips are cut in half, does it reduce accuracy?

• No.

Q. How do glucometers measure glucose?

1. Reflectance photometry: Measures light reflected by chromogen.
2. Amperometric method: Measures current generated by glucose oxidase.
   • Reflectance photometry is better.

Q. How does hematocrit affect glucose values?

• BG >300 mg/dl:
  • Hematocrit >55%: Lowers BG by 15%.
  • Hematocrit <35%: Increases BG by 10%.

Q. Which common glucometers use the GDH-PQQ method?

• ACCU-CHEK (Roche), FreeStyle (Abbott Diabetes Care).

Q. Does the ACCU-CHEK in India use GDH-PQQ?

• No, it uses the Mut Q-GDH method.

Interfering Substances with Glucometers (ADA Guidelines 2021):

Glucose oxidase monitors:

• Interfering Substances: Uric acid, galactose, xylose, acetaminophen, L-DOPA, ascorbic acid.

Glucose dehydrogenase monitors:

• Interfering Substances: Icodextrin (used in peritoneal dialysis).

FDA Standards for Blood Glucose Monitors:

Home Use:

• 95% readings: Within 15% for usable BG range.
• 99% readings: Within 20% for usable BG range.

Hospital Use:

• 95% readings:
  • Within 12% for BG ≥75 mg/dl.
  • Within 12 mg/dl for BG <75 mg/dl.
• 98% readings:
  • Within 15% for BG ≥75 mg/dl.
  • Within 15 mg/dl for BG <75 mg/dl.

Impact of Oxygen Levels:

• Glucose oxidase glucometers: Affected by oxygen levels.
• Glucose dehydrogenase glucometers: Not affected.
  • Higher oxygen: False low values.
  • Lower oxygen: False high values (e.g., high altitude, hypoxia).