Normal Serum Glucose Values
Normal fasting serum glucose is defined as less than 100 mg/dL (5.6 mmol/L) according to current American Diabetes Association criteria. 1
Fasting Glucose Categories
The diagnostic framework divides glucose metabolism into three distinct categories based on fasting plasma glucose measurements 2, 1:
- Normal glucose: <100 mg/dL (<5.6 mmol/L) 1
- Impaired fasting glucose (prediabetes): 100-125 mg/dL (5.6-6.9 mmol/L) 2, 1
- Diabetes mellitus: ≥126 mg/dL (≥7.0 mmol/L) on two separate occasions 2
Testing Requirements
Fasting is defined as no caloric intake for at least 8 hours before blood draw. 2, 1 This standardization is critical because glucose values vary significantly based on fed versus fasted state.
Important Measurement Considerations
Specimen Type Matters
Plasma is the recommended primary specimen for glucose measurement 2. However, different blood specimens yield different values 2:
- Plasma glucose values are approximately 11% higher than whole blood glucose 2, 1
- Serum glucose is approximately 5% lower than plasma glucose 2
Biological Variability
Even in healthy individuals, fasting glucose demonstrates substantial day-to-day variation with a within-person coefficient of variation of approximately 5-6% 2. This means a true glucose of 100 mg/dL could measure anywhere from 87-113 mg/dL on repeat testing due to biological variation alone 2.
Clinical Pitfalls and Caveats
Values in the "high-normal" range (87-99 mg/dL) carry increased diabetes risk despite being technically normal. Research demonstrates that fasting glucose levels of 87 mg/dL or higher show progressively increased risk of developing type 2 diabetes compared to levels below 81 mg/dL 3. Each 1 mg/dL increase in fasting glucose within the normal range increases diabetes risk by 6% 4.
Very low fasting glucose (<70 mg/dL) is also associated with increased cardiovascular mortality, creating a U-shaped mortality curve 5. The optimal fasting glucose range associated with lowest cardiovascular risk appears to be 85-99 mg/dL 6.
Random (Non-Fasting) Glucose
For random glucose measurements 7:
- Values <140 mg/dL are generally considered normal 7
- Values 140-199 mg/dL warrant further evaluation with formal fasting testing 7
- Values ≥200 mg/dL with classic diabetes symptoms (polyuria, polydipsia, weight loss) are diagnostic for diabetes 2, 7
Random glucose is not recommended for screening or establishing normal ranges—it is primarily useful only when markedly elevated in symptomatic patients 7.
Confirmation Requirements
Any abnormal value suggesting diabetes or prediabetes must be confirmed by repeat testing on a different day before establishing a diagnosis 2, 1. This requirement accounts for the significant biological and analytical variability in glucose measurements 2.