Glucose Level

Figure: Blood Glucose Levels Classification (Hypoglycemia, Normal, and Hyperglycemia)

Blood glucose measurement is one of the most frequently requested and clinically vital investigations in routine clinical biochemistry. Maintaining physiological glucose homeostasis is vital for human survival, as glucose serves as the primary metabolic fuel for the central nervous system and erythrocytes.

Diagnostic Classification & Reference Values

According to international diagnostic criteria (American Diabetes Association - ADA and World Health Organization - WHO), blood glucose concentrations in venous plasma are classified into three primary diagnostic categories:

Test Type	Normal Range	Impaired / Prediabetes	Diabetes Mellitus
Fasting Blood Sugar (FBS / FPG)	70 – 99 mg/dL (3.9 – 5.5 mmol/L)	100 – 125 mg/dL (5.6 – 6.9 mmol/L)	≥ 126 mg/dL (7.0 mmol/L)
2-Hour Postprandial (2h PPG / OGTT)	< 140 mg/dL (< 7.8 mmol/L)	140 – 199 mg/dL (7.8 – 11.0 mmol/L)	≥ 200 mg/dL (11.1 mmol/L)
Random Blood Sugar (RBS)	70 – 140 mg/dL (3.9 – 7.8 mmol/L)	—	≥ 200 mg/dL with symptoms

Clinical Interpretation of Abnormal Values

1. Hypoglycemia (< 70 mg/dL or < 3.9 mmol/L)

Hypoglycemia occurs when plasma glucose drops below normal metabolic requirements. It is classified clinically into:

• Alert Value (Level 1): 54 to 70 mg/dL (3.0 to 3.9 mmol/L) – Requires fast-acting carbohydrate administration.
• Clinically Significant (Level 2): < 54 mg/dL (< 3.0 mmol/L) – Sufficient to cause neuroglycopenic symptoms.
• Severe Hypoglycemia (Level 3): Severe cognitive impairment requiring external third-party assistance.

Common Causes: Insulin overdose, sulfonylurea therapy, prolonged starvation, severe sepsis, insulinoma, Addison's disease, and acute hepatic failure.

2. Hyperglycemia (> 126 mg/dL fasting or > 200 mg/dL random)

Sustained elevation in blood glucose occurs due to deficient insulin secretion, insulin resistance, or both.

Common Causes: Diabetes Mellitus (Type 1 and Type 2), Gestational Diabetes, acute physiological stress (sepsis, myocardial infarction, trauma), Cushing syndrome, acromegaly, and corticosteroid therapy.

Laboratory Medicine & Pre-analytical Considerations

Accurate glucose estimation requires strict adherence to pre-analytical protocols:

• Anticoagulant & Glycolysis Inhibition: Unseparated whole blood undergoes in vitro glycolysis at a rate of approximately 5% to 7% per hour (approx. 10 mg/dL/hr) at room temperature. Collect specimens in a Grey-top tube containing Sodium Fluoride (glycolysis inhibitor) and Potassium Oxalate (anticoagulant), or centrifuge plain/gel serum tubes within 30–60 minutes of collection.
• Analytical Methods: Standard clinical laboratory analyzers utilize enzymatic colorimetric methodologies:
  • GOD-POD (Glucose Oxidase - Peroxidase) Method: Highly specific for D-glucose, widely used for routine photometric assays.
  • Hexokinase Method: The designated reference method for high-accuracy reference laboratories and automated biochemistry analyzers.
• Unit Conversion:
  • To convert mg/dL to mmol/L: mg/dL ÷ 18.0182 = mmol/L
  • To convert mmol/L to mg/dL: mmol/L × 18.0182 = mg/dL