Normal Ranges for Continuous Blood Glucose (CBG) Monitoring
For healthy, non-diabetic individuals aged 20-69 years, normal CGM parameters include a 24-hour mean glucose of approximately 5.4-5.8 mmol/L (98-104 mg/dL), with 96% of time spent in the range of 70-140 mg/dL (3.9-7.8 mmol/L). 1, 2
Normal Reference Values for Non-Diabetic Individuals
Mean Glucose and Time in Range
- 24-hour mean glucose: 98-99 mg/dL (5.4-5.5 mmol/L) for individuals under 60 years; 104 mg/dL (5.8 mmol/L) for those over 60 years 2
- Time in tight range (70-140 mg/dL): Median 96% (interquartile range 93-98%), equivalent to approximately 23 hours per day 2
- Time above 140 mg/dL: Median 2.1% (approximately 30 minutes per day) 2
- Time below 70 mg/dL: Median 1.1% (approximately 15 minutes per day) 2
Glycemic Variability
- Coefficient of variation (CV): Mean 17 ± 3% in healthy individuals 2
- The CV reflects the standardized measure of glucose dispersion from the mean and is a key metric for assessing glycemic stability 1
Target Ranges for Diabetic Patients
Standard Target Range (Time in Range - TIR)
- Primary target: 70-180 mg/dL (3.9-10.0 mmol/L) 1
- Goal TIR percentage: Achieving approximately 60% or more of readings within this range correlates with HbA1c <7.5% 3
- For patients achieving HbA1c <58 mmol/mol (<7.5%), approximately 57% of CBG readings fall within 4-10 mmol/L (72-180 mg/dL) 3
Tight Range Targets
- Time in tight range (TITR): 70-140 mg/dL (3.9-7.8 mmol/L) is increasingly recognized as a more stringent target 4, 2
- The relationship between TIR and TITR is nonlinear, with a TITR:TIR ratio of approximately 0.42 when TIR is 20%, increasing to 0.66 when TIR is 80% 4
Hypoglycemia Thresholds
- Level 1 hypoglycemia: <70 mg/dL (3.9 mmol/L) - alert value requiring action 1
- Level 2 hypoglycemia: <54 mg/dL (3.0 mmol/L) - clinically significant hypoglycemia 1
- Level 3 hypoglycemia: Severe hypoglycemia requiring external assistance, regardless of glucose value 1
- Prolonged hypoglycemia: CGM levels <3.0 mmol/L lasting more than 120 minutes 1
Hyperglycemia Thresholds
- Level 1 hyperglycemia: 181-250 mg/dL (10.1-13.9 mmol/L) 1
- Level 2 hyperglycemia: >250 mg/dL (>13.9 mmol/L) - requires prompt intervention 1
Clinical Context and Target Adjustments
Inpatient Settings
- Non-critically ill patients: Upper limit target of 180 mg/dL, with lower limits of either 100 mg/dL or 140 mg/dL depending on individual patient factors 1
- Moderate hypoglycemia definition: <70 mg/dL or <72 mg/dL 1
- Severe hypoglycemia definition: <54 mg/dL 1
Correlation with HbA1c
- The 24-hour mean glucose correlates with HbA1c using the equation: 24-hour mean glucose (mmol/L) = 1.198 × HbA1c (%) - 0.582 1
- When HbA1c is 6.0%, 6.5%, and 7.0%, corresponding CGM 24-hour mean glucose levels are 6.6,7.2, and 7.8 mmol/L respectively 1
Important Caveats
Glycemic Variability Considerations
- Target CV: ≤36% for most patients, though some studies suggest <33% provides additional protection against hypoglycemia for insulin-treated patients 1
- Higher CV or higher time below range results in greater TITR for a given TIR, indicating more glycemic instability 4
Population Differences
- Type 2 diabetes patients typically have higher TITR for a given TIR compared to type 1 diabetes patients, primarily due to lower CV and less time below range 4
- Older adults (≥65 years) have slightly higher mean glucose values (104 mg/dL vs 98-99 mg/dL) compared to younger individuals 2
Data Collection Requirements
- Minimum CGM wear time: 14 days of data recommended for reliable estimates of mean glucose, TIR, and hyperglycemia 1
- Active monitoring percentage: At least 70% of data from the 14-day period should be available 1
- For hypoglycemia and glucose variability assessment, 14 days or more provides better estimates 1
Permissive Hyperglycemia
- Patients achieving HbA1c <7.5% may have up to 30% of readings >10 mmol/L (180 mg/dL), which is acceptable and may reduce anxiety associated with out-of-range readings 3