Normal Fasting Blood Glucose Range
A normal fasting blood glucose 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
Defining Normal Fasting Glucose
The American Diabetes Association (ADA) establishes the upper limit of normal fasting plasma glucose at <100 mg/dL (5.6 mmol/L), which represents a threshold below which diabetes risk remains low 2. The specific normal range is:
- Normal fasting glucose: 70-99 mg/dL (3.9-5.5 mmol/L) 1
- Upper limit of normal: <100 mg/dL (5.6 mmol/L) 3, 2
Diagnostic Categories Beyond Normal
Understanding the full spectrum of glucose metabolism helps contextualize what "normal" means:
- Impaired fasting glucose (prediabetes): 100-125 mg/dL (5.6-6.9 mmol/L) 3, 2
- Diabetes: ≥126 mg/dL (7.0 mmol/L) on two separate occasions 3
Testing Requirements for Accuracy
Fasting is defined as no caloric intake for at least 8 hours before blood draw 3, 2. This standardization is critical because:
- Day-to-day biological variation in fasting glucose ranges from 4.8% to 6.1% within the same individual 1
- For a true fasting glucose of 100 mg/dL, normal biological variation means values could range from 87-113 mg/dL on different days 1
- Laboratory testing is preferred over point-of-care meters for diagnostic purposes due to greater accuracy 1
Important Clinical Caveats
Very low fasting glucose levels (<70 mg/dL or 3.89 mmol/L) may paradoxically indicate increased cardiovascular risk, not just optimal glucose control 4, 5. Research demonstrates a U-shaped mortality curve, with the lowest cardiovascular risk occurring in the glucose range of 85-99 mg/dL 5.
Any abnormal result should be confirmed with repeat testing on a separate day before establishing a diagnosis 1, 2. This prevents misdiagnosis due to normal biological variation or pre-analytical factors such as stress, illness, or medications (particularly glucocorticoids) 1.
Historical Context
The ADA lowered the threshold for normal fasting glucose from <110 mg/dL to <100 mg/dL in 2003, though this change remains contentious and has not been universally adopted by all organizations including the WHO 3. The rationale was that individuals with fasting glucose between 100-109 mg/dL demonstrate increased risk for progression to type 2 diabetes 3.