Normal Fasting Glucose Range
For adults without diabetes, a normal fasting plasma glucose is less than 100 mg/dL (5.6 mmol/L), with optimal values typically between 70-99 mg/dL (3.9-5.5 mmol/L). 1, 2
Diagnostic Categories
The American Diabetes Association defines three key categories based on fasting plasma glucose after at least 8 hours of no caloric intake 3, 2:
- Normal: <100 mg/dL (<5.6 mmol/L) 1, 2
- 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, 1
Important Clinical Context
Biological Variation
Normal day-to-day variation in fasting glucose is substantial and must be considered when interpreting results 3:
- Within-person biological variation ranges from 4.8% to 6.1% 3
- 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
- This biological variability is substantially greater than analytical laboratory error 3
Cardiovascular Risk Considerations
Even within the "normal" range, higher fasting glucose values carry increased cardiovascular risk 4:
- Fasting glucose levels of 95-99 mg/dL show 1.53-fold increased cardiovascular disease risk compared to levels <80 mg/dL 4
- This relationship is independent of traditional cardiovascular risk factors 4
- Conversely, very low fasting glucose (<70 mg/dL) may also be associated with increased mortality risk in some populations 5, though this finding is less consistent 6
Special Population: Pregnancy
Pregnant women have different fasting glucose targets due to altered physiology 3:
- Normal pregnancy: Fasting glucose 70-95 mg/dL (3.9-5.3 mmol/L) 3
- Gestational diabetes diagnosis: Fasting glucose ≥95 mg/dL (≥5.3 mmol/L) 3
- Lower fasting values in pregnancy reflect increased insulin sensitivity and red blood cell turnover 3
Critical Pitfalls to Avoid
Confirmation testing is essential - A single abnormal fasting glucose should be confirmed with repeat testing on a different day before establishing a diagnosis of diabetes or prediabetes 3, 1. The exception is when a patient presents with classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) plus random glucose ≥200 mg/dL 3.
Proper fasting is required - The patient must have no caloric intake for at least 8 hours before testing 3, 2. Failure to fast properly invalidates the result 1.
Laboratory vs. point-of-care testing - Diagnostic decisions should be based on laboratory plasma glucose measurements, not point-of-care glucose meters, which have greater analytical variability 1.