”Medical Disclaimer: This content is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Always seek the advice of your physician or qualified health provider with any questions you may have regarding a medical condition.
You have a continuous glucose monitor (CGM), but looking at all those numbers can feel overwhelming. What does that trend arrow actually mean? Is Time in Range really more important than A1C?
Your CGM provides more data than ever before—but data only helps if you know how to use it.
Here's how to interpret your CGM data and turn it into better diabetes decisions.
The CGM Display Explained
Understanding Your Screen
| Element | What It Shows | Why It Matters |
|---|---|---|
| Current glucose | Your glucose right now | Immediate decision-making |
| Trend arrow | Direction and rate of change | Predicts where you're heading |
| Trend graph | Glucose over last 1-24 hours | Shows patterns |
| Alarms/alerts | Hypo or hyperglycemia warnings | Safety alerts |
The Trend Arrow: Your Crystal Ball
The trend arrow is one of the most valuable CGM features:
| Arrow | Meaning | Action Consideration |
|---|---|---|
| ↓ Straight down | Dropping rapidly (2-3 mg/dL/min) | May need to treat |
| ↘ Down diagonally | Dropping (1-2 mg/dL/min) | Watch closely |
| → Straight across | Stable | Continue monitoring |
| ↗ Up diagonally | Rising (1-2 mg/dL/min) | Rising slowly |
| ↑ Straight up | Rising rapidly (2-3 mg/dL/min) | May need to treat |
Key principle: The trend arrow predicts where you'll be in 15-30 minutes. Use it to take action before problems occur.
Key CGM Metrics Explained
Time in Range (TIR)
What it is: Percentage of time glucose is between 70 and 180 mg/dL
| TIR | A1C Approximate | Target For |
|---|---|---|
| > 70% | ~7.0% | Most adults with diabetes |
| > 50% | ~8.0% | Older adults, high risk |
| <50% | > 8.0% | Suboptimal |
Why it matters:
- Correlates with A1C but adds information about variability
- Directly tied to diabetes complications risk
- More actionable than A1C alone
- Captures both highs and lows
Example: Two patients with A1C 7.0%—one with TIR 80% (stable), one with TIR 60% (lots of highs and lows). Same A1C, very different risk.
Time Below Range (TBR)
Level 1 hypoglycemia: Glucose ＜ 70 mg/dL Level 2 hypoglycemia: Glucose ＜ 54 mg/dL
| Target | Level 1 | Level 2 |
|---|---|---|
| Most patients | ＜ 4% | ＜ 1% |
| High risk | ＜ 1% | ＜ 0.5% |
Why it matters:
- Hypoglycemia is dangerous and potentially fatal
- Repeated hypoglycemia causes "hypoglycemia unawareness"
- Level 2 (＜ 54) is associated with cardiac arrhythmias
Time Above Range (TAR)
What it is: Percentage of time glucose > 180 mg/dL
| Category | Threshold | Target |
|---|---|---|
| TAR | > 180 mg/dL | ＜ 25% |
| Very high TAR | > 250 mg/dL | Minimize |
Why it matters:
- Indicates hyperglycemia burden
- Correlates with long-term complications
- Very high TAR (> 250) may indicate ketone risk in type 1
Glucose Variability
Coefficient of Variation (CV) = (Standard deviation ÷ Mean glucose) × 100
| CV | Interpretation |
|---|---|
| <36% | Stable (target) |
| ≥ 36% | High variability (increased risk) |
Why variability matters:
- High variability = more oxidative stress
- Increases complications risk even at same A1C
- Associated with hypoglycemia risk
- Quality of life impact (glucose "rollercoaster")
AGP: Ambulatory Glucose Profile
What Is AGP?
AGP is a standardized report that combines multiple days of CGM data into a single visualization. Most CGM systems and apps can generate AGP reports.
The AGP Display
| Section | What It Shows |
|---|---|
| TIR | Percentage in target range |
| Median glucose | Middle value of all readings |
| Interquartile range | Middle 50% of readings |
| Glucose profile | Pattern over 24 hours |
| Daily patterns | Weekday vs weekend differences |
| TIR targets | 70-180 mg/dL (shaded area) |
Using CGM Data for Decision Making
Before Eating
| CGM Reading + Trend | Action |
|---|---|
| 70-100, flat/↑ | Eat normally, consider pre-bolus if using insulin |
| 70-100, ↓↓ | May need small snack before activity or insulin |
| 100-140, flat/↑ | Eat normally |
| 140-180, ↑↑ | Consider pre-meal walk, smaller portion, or medication |
| > 180, ↑↑ | Significant intervention may be needed |
| <70, any trend | Treat hypoglycemia first |
Before Exercise
| Pre-Exercise CGM | Consideration |
|---|---|
| 100-180 mg/dL | Generally safe to exercise |
| <100 mg/dL | Have 15 g carbs before exercise |
| <70 mg/dL | Treat hypoglycemia before exercising |
| > 250 mg/dL | Check ketones (type 1), consider postponing vigorous activity |
Before Bed
| Bedtime CGM | Action |
|---|---|
| 100-140, flat | Generally safe for sleep |
| 70-99 | Have small snack (7-15 g carbs) |
| > 180 and rising | May need correction |
| <70 | Treat hypoglycemia before sleep |
| <100 with downward trend | Monitor closely, may need snack |
When Driving
| CGM Threshold | Action |
|---|---|
| <100 mg/dL | Wait to drive until > 100 |
| 70-99 with stable trend | May drive with caution |
| Trend arrows ↓↓ or ↑↑ | Consider waiting until stable |
Interpreting Patterns
The Dawn Phenomenon
Pattern: Glucose rises in early morning hours (4-8 AM)
| Time | Typical Pattern |
|---|---|
| 3 AM | Normal or slightly low |
| 6 AM | Rising (even before eating) |
| 8 AM | Elevated fasting glucose |
Management:
- Exercise in evening (not too late)
- Adjust evening medication timing
- Consider overnight basal insulin adjustment
- Eat earlier, lighter dinner
Post-Meal Spikes
Pattern: Glucose rises excessively after eating
| Mealtime | Target Rise | Problematic |
|---|---|---|
| 1-hour post-meal | ＜ 180 mg/dL | > 180 indicates problem |
| 2-hour post-meal | ＜ 140 mg/dL | > 140 indicates problem |
Management:
- Eat vegetables/protein first
- Reduce carbohydrate portion
- Take a post-meal walk
- Adjust mealtime medication timing
Late-Night Hypoglycemia
Pattern: Glucose drops in overnight hours
| Time | Low Reading Pattern |
|---|---|
| 11 PM - 3 AM | ＜ 70 mg/dL |
| 3 AM - 6 AM | May rebound high (Somogyi effect) |
Management:
- Check 2-3 AM glucose occasionally
- Adjust evening/nighttime insulin or sulfonylurea
- Have bedtime snack if needed
- Review medication timing
Setting Alarms and Alerts
Recommended Alarm Thresholds
| Alarm | Setting | Avoid Setting Too Low Because... |
|---|---|---|
| High alarm | 250-300 mg/dL | Alarm fatigue if set too low |
| Low alarm | 70 mg/dL | Gives time to act before severe hypo |
| Urgent low | 55 mg/dL | Action required immediately |
| Rate of change | 2-3 mg/dL/min | Catch rapid changes early |
Minimizing Alarm Fatigue
| Strategy | How |
|---|---|
| Set appropriate thresholds | Personalize to your patterns |
| Use different tones | Distinguish high vs low alarms |
| Enable snooze | Short break between alerts (not too long) |
| during predictable patterns | |
| Adjust for activity | Turn off alerts during exercise if safe |
Sharing CGM Data
Remote Monitoring
CGM data can be shared with:
| Person | Why Share |
|---|---|
| Partners/family | Safety during hypoglycemia |
| Caregivers | Support for children/elderly |
| Healthcare providers | Data-driven treatment decisions |
| Diabetes educator | Pattern identification and education |
Following Sharing
Some CGM systems allow "followers" who can receive:
- Text alerts for hypoglycemia
- Real-time glucose data
- Trend information
This is particularly valuable for:
- Parents of children with diabetes
- Partners of those with hypoglycemia unawareness
- Caregivers of elderly patients
CGM Data Reports
AGP Report Sections
| Section | What It Tells You |
|---|---|
| Overlay | 14 days overlaid on one 24-hour graph |
| Daily summaries | Each day's average TIR/TAR/TBR |
| Weekly summaries | Day-by-day comparison |
| Logbook | Meals, exercise, medication notes |
Review Frequency
| Situation | Review Frequency |
|---|---|
| Making medication changes | Weekly |
| Stable, at goal | Monthly |
| With healthcare provider | Quarterly or more often |
Common Mistakes to Avoid
Mistake 1: Ignoring the Trend Arrow
Problem: Only looking at the current number
Better approach: Use the trend arrow to predict and prevent
Mistake 2: Over-Treating Based on One Reading
Problem: Treating a single high or low without context
Better approach: Look at the pattern—treat trends, not individual points
Mistake 3: Not Acting on Data
Problem: Collecting data but not using it
Better approach: Review weekly, identify patterns, make changes
Mistake 4: Calibration Errors (if applicable)
Problem: Calibrating when glucose is changing rapidly
Better approach: Only calibrate when stable (flat trend arrow)
Mistake 5: Sensor Complacency
Problem: Assuming sensor is always accurate
Better approach: Confirm with fingerstick if symptoms don't match CGM reading
CGM for Different Populations
Type 1 Diabetes
| Focus | Why |
|---|---|
| Preventing hypoglycemia | Major safety concern |
| Insulin dosing | Trend arrows guide dosing |
| Exercise management | Adjust for activity |
| Sick days | Monitor closely |
Type 2 Diabetes
| Focus | Why |
|---|---|
| Post-meal spikes | Identify problematic foods |
| Medication timing | Optimize effectiveness |
| Lifestyle patterns | See impact of food/activity |
| Progress monitoring | Track improvement over time |
Pregnancy (with Diabetes)
| Target | Why More Stringent |
|---|---|
| TIR | > 70% (aim higher if possible) |
| TAR | Minimize (hyperglycemia harms fetus) |
| TBR | ＜ 1% (hypoglycemia harms fetus) |
| Mean glucose | Aim for 95-115 mg/dL |
Key Takeaways
- Time in Range > 70% is the primary CGM goal for most adults
- The trend arrow predicts where glucose is heading—use it!
- AGP reports consolidate data into meaningful patterns
- Set alarms appropriately to prevent alarm fatigue
- Share data with healthcare providers for data-driven decisions
- Review regularly to identify patterns and make adjustments
- CGM complements A1C—it provides information A1C cannot
FAQ Section
What is a good Time in Range?
For most adults with diabetes, a good Time in Range is > 70% (glucose between 70-180 mg/dL). For older adults or those at high risk of hypoglycemia, a target of > 50% may be appropriate. TIR correlates with A1C but provides additional information about variability and hypoglycemia.
What does the trend arrow mean on CGM?
The trend arrow shows the direction and rate of glucose change. A straight up arrow (↑) means glucose is rising rapidly (2-3 mg/dL/min). A straight down arrow (↓) means glucose is dropping rapidly. Use the trend arrow to predict where your glucose will be in 15-30 minutes and take action early.
How often should I calibrate my CGM?
If your CGM requires calibration (Dexcom G6, Guardian 4), calibrate 2x daily when glucose is stable (flat trend arrow). Do NOT calibrate when glucose is changing rapidly (after meals, exercise, or medication). Factory-calibrated systems (Dexcom G7, Libre 3) do not require calibration but fingersticks can be used for verification.
What is the difference between TIR and A1C?
Time in Range (TIR) and A1C are related but different. A1C is a 3-month average of glycated hemoglobin. TIR shows daily glucose patterns—percentage of time in target, highs, and lows. Two people can have the same A1C with very different TIR patterns. TIR provides actionable information for daily management.
How do I interpret my CGM graph?
Look for patterns across the day: fasting patterns (dawn phenomenon), post-meal spikes (problematic foods), overnight lows (medication timing), and exercise effects. Compare weekdays vs weekends. Look for consistent patterns rather than individual readings. Share AGP reports with your healthcare team for interpretation.
Sources:
- American Diabetes Association. Standards of Care in Diabetes—2025. Diabetes Care 2025; 48(Suppl 1)
- Battelino T, et al. Diabetes Care 2019;42:236-243 (TIR clinical consensus)
- CGM Interpretation Guide