Normal Fasting Blood Sugar (FBS)
A normal fasting blood sugar level is less than 100 mg/dL (5.6 mmol/L), with the optimal range being 70-99 mg/dL (3.9-5.5 mmol/L) for adults without diabetes. 1, 2
Diagnostic Categories Based on Fasting Plasma Glucose
The classification of fasting glucose levels follows a clear hierarchy that determines metabolic status:
- Normal: FPG <100 mg/dL (5.6 mmol/L), with the ideal range being 70-99 mg/dL (3.9-5.5 mmol/L) 1, 2
- Impaired Fasting Glucose (Prediabetes): FPG 100-125 mg/dL (5.6-6.9 mmol/L) 3, 2, 4
- Diabetes: FPG ≥126 mg/dL (7.0 mmol/L) on two separate occasions, with fasting defined as no caloric intake for at least 8 hours 3, 2
Important Nuances About "Normal" Glucose Levels
While values below 100 mg/dL are classified as normal, emerging evidence suggests cardiovascular risk stratification within this range:
- Optimal cardiovascular health: FPG <80 mg/dL appears to confer the lowest cardiovascular risk 5
- High-normal range (95-99 mg/dL): Associated with 1.53-fold increased cardiovascular disease risk compared to levels <80 mg/dL, even after adjusting for traditional risk factors 5
- Diabetes risk increases progressively: Even within the "normal" range, subjects with FPG 100-104 mg/dL have 3.83-fold increased risk of developing diabetes, and those with 105-109 mg/dL have 7.87-fold increased risk compared to FPG <85 mg/dL 6
Critical Caveats About Very Low Glucose
A common pitfall is assuming all values within the "normal" range are equally healthy. Paradoxically, very low fasting glucose levels may also indicate increased risk:
- FPG <70 mg/dL (<3.89 mmol/L) is associated with 3.3-fold increased cardiovascular mortality risk 7
- FPG 70-79 mg/dL (3.89-4.43 mmol/L) carries 2.4-fold increased cardiovascular mortality risk 7
- This U-shaped relationship suggests that both extremes of the "normal" range warrant clinical attention 7
Factors Affecting Measurement Accuracy
To ensure accurate interpretation of fasting glucose results, clinicians must account for:
- Biological variation: Normal day-to-day fluctuation ranges from 4.8-6.1%, meaning a true FPG of 100 mg/dL could measure anywhere from 87-113 mg/dL on different days 1
- Proper fasting: At least 8 hours without caloric intake is required for valid results 3
- Confirmation of abnormal results: Any abnormal value should be confirmed with repeat testing on a different day before making a diagnosis 3, 1
- Point-of-care limitations: Capillary glucose meters are less accurate than laboratory plasma glucose analyzers and should not be used for diagnostic purposes 1
- Confounding factors: Stress, illness, medications (especially glucocorticoids), and time of day can all affect results 1
Practical Clinical Approach
When interpreting a fasting glucose result:
- Verify proper fasting status (minimum 8 hours) before drawing conclusions 3
- Use laboratory plasma glucose rather than point-of-care meters for diagnostic decisions 1
- Confirm any abnormal result (≥100 mg/dL or <70 mg/dL) with repeat testing on a separate day 3, 1
- Consider cardiovascular risk even in the high-normal range (95-99 mg/dL), as these patients may benefit from lifestyle interventions 5
- Investigate very low values (<70 mg/dL) for underlying causes, as these carry increased mortality risk 7