Normal Capillary Blood Glucose (CBG) Levels
For individuals without diabetes, normal fasting CBG is 70-100 mg/dL (3.9-5.6 mmol/L), with postprandial values remaining below 140 mg/dL (7.8 mmol/L) 1-2 hours after meals. 1
For Individuals WITHOUT Diabetes
Fasting Values
- Normal fasting range: 70-100 mg/dL (3.9-5.6 mmol/L) 1
- Impaired fasting glucose (prediabetes): 100-125 mg/dL (5.6-6.9 mmol/L) 2, 1
- Diabetes threshold: ≥126 mg/dL (7.0 mmol/L) on two separate occasions 2, 1
Postprandial Values (1-2 hours after meals)
- Normal: <140 mg/dL (7.8 mmol/L) 1
- Impaired glucose tolerance: 140-199 mg/dL (7.8-11.0 mmol/L) 2
- Diabetes threshold: ≥200 mg/dL (11.1 mmol/L) 2
Continuous Glucose Monitoring Data in Healthy Individuals
Recent large-scale studies using modern CGM technology provide important context for what constitutes "normal" glucose fluctuations throughout the day:
- Healthy individuals spend approximately 96% of time between 70-140 mg/dL (3.9-7.8 mmol/L) 3
- Mean average glucose in healthy adults: 98-99 mg/dL (5.4-5.5 mmol/L) for those under 60 years 3
- Mean average glucose in adults >60 years: 104 mg/dL (5.8 mmol/L) 3
- Time spent >140 mg/dL: median 2.1% (approximately 30 minutes/day) 3
- Time spent <70 mg/dL: median 1.1% (approximately 15 minutes/day) 3
A larger community-based study confirmed that normoglycemic adults spend 87% of time in the 70-140 mg/dL range and more than 15 minutes/day above 180 mg/dL, with approximately 3 hours/day above 140 mg/dL 4. This demonstrates that even healthy individuals experience significant glucose excursions beyond traditional "normal" ranges.
For Individuals WITH Diabetes
Target Ranges (American Diabetes Association)
- Preprandial (fasting/before meals): 80-130 mg/dL (4.4-7.2 mmol/L) 2
- Peak postprandial (1-2 hours after meals): <180 mg/dL (10.0 mmol/L) 2
- A1C target: <7.0% (53 mmol/mol) 2
Hypoglycemia Thresholds
These thresholds apply to both diabetic and non-diabetic individuals:
- Level 1 hypoglycemia: <70 mg/dL (3.9 mmol/L) but ≥54 mg/dL (3.0 mmol/L) - requires attention and treatment with 15-20g of fast-acting carbohydrates 2, 5, 1
- Level 2 hypoglycemia: <54 mg/dL (3.0 mmol/L) - neuroglycopenic symptoms begin, requires immediate action 2, 5, 6, 1
- Level 3 (severe) hypoglycemia: altered mental/physical status requiring assistance, often <40 mg/dL (2.2 mmol/L) 2, 1
Hyperglycemia Thresholds
- Random glucose ≥200 mg/dL (11.1 mmol/L) with classic symptoms (polyuria, polydipsia, weight loss) confirms diabetes without repeat testing 2, 1
- Renal threshold for glucosuria: >180 mg/dL (10.0 mmol/L) - glucose begins spilling into urine 1
Critical Clinical Considerations
Treatment of Hypoglycemia
When CBG is <70 mg/dL (3.9 mmol/L):
- Administer 15-20g of fast-acting carbohydrates (glucose preferred) 2, 6
- Recheck CBG in 15 minutes; repeat treatment if still low 2, 6
- Once trending up, provide meal/snack to prevent recurrence 2, 6
- Glucagon should be available for patients at risk of Level 2/3 hypoglycemia 2, 6
Common Pitfalls
- Random CBG alone has limited diagnostic value - sensitivity and specificity are only 75-80% for detecting diabetes, and results must be interpreted according to age and time since last meal 2
- A single low glucose on oral glucose tolerance test does NOT diagnose reactive hypoglycemia - diagnosis requires documented hypoglycemia during spontaneous symptomatic episodes with symptom resolution after treatment 5
- Stress hyperglycemia in acute illness does not necessarily indicate diabetes - particularly in children, this warrants endocrinology consultation rather than immediate diabetes diagnosis 1
Cardiovascular Risk Considerations
Even glucose levels in the "normal" range carry prognostic significance: