Continuous Glucose MonitoringCGM Systems Explained
Imagine being able to see your blood sugar at any moment—not just when you prick your finger, but all day long, even while you sleep. That's what continuous glucose monitoring (CGM) provides. By measuring glucose every few minutes and displaying it on your phone or a dedicated receiver, CGM gives you something finger sticks can't: a complete picture of your glucose patterns, trends, and the ability to see where you're heading, not just where you are.
For many people with diabetes, CGM has been transformative. Studies consistently show that CGM users achieve better A1C levels, spend more time in their target range, and experience fewer dangerous lows. The technology has improved dramatically in recent years, becoming smaller, more accurate, and more affordable. This guide covers everything you need to know about how CGM works, who can benefit from it, and how to get started.
How CGM Works
A CGM system consists of three main components that work together to deliver continuous glucose readings to your device.
The Sensor
The sensor is a tiny, flexible filament—about the width of a hair—inserted just beneath your skin using an automatic applicator. It sits in the interstitial fluid (the fluid between your cells) rather than in your blood. Glucose from your bloodstream constantly diffuses into this fluid, allowing the sensor to measure glucose levels indirectly. Sensors are typically worn on the abdomen or the back of the upper arm and last from 7 to 14 days depending on the system.
The Transmitter
Sitting on top of the sensor and adhered to your skin, the transmitter contains the electronics that power the sensor, process its readings, and wirelessly send glucose data to your display device. Some systems have the transmitter integrated into the sensor itself (making it fully disposable), while others use a reusable transmitter that you transfer to each new sensor.
The Display (Receiver or Smartphone)
Your glucose readings can be displayed on a dedicated receiver device, a smartphone app, or both. The display shows your current glucose level, a trend arrow indicating whether you're rising, falling, or stable, and a graph of recent readings. Most systems can also send alerts when your glucose goes too high or too low, or when it's changing rapidly.
Why CGM Changes Diabetes Management
The difference between CGM and finger stick testing isn't just convenience—it's a fundamentally different kind of information that enables better decisions and outcomes.
See Trends, Not Just Numbers
A finger stick tells you where your blood sugar is at one moment. A CGM tells you where it is, where it's been, and where it's heading. Those trend arrows—showing whether you're rising, falling, or stable—are incredibly valuable. A reading of 130 mg/dL means something very different if you're rapidly rising after a meal versus slowly falling several hours later.
Alerts That Catch Problems Early
CGM systems can alert you when glucose is heading too high or too low, often before you'd notice symptoms. You can set thresholds for urgent alerts (like impending hypoglycemia) and less urgent notifications (like trending high after a meal). Some systems even predict where your glucose will be in 10-30 minutes and alert you before you reach dangerous levels.
Understand Your Patterns
CGM reveals patterns that occasional finger sticks miss. You might discover that your glucose spikes after certain foods, drops predictably after afternoon exercise, or rises overnight due to the dawn phenomenon. These insights allow you to make targeted adjustments to food, medication timing, and activity.
Overnight Peace of Mind
One of CGM's most valued benefits is visibility into overnight glucose. Many people are surprised to discover what happens while they sleep—dramatic swings they'd never know about from morning fasting readings alone. Alerts can wake you or a caregiver if glucose drops dangerously low during the night.
Better Time in Range
CGM introduces the concept of "time in range"—the percentage of time your glucose stays within your target zone (typically 70-180 mg/dL). This metric captures glucose control more comprehensively than A1C alone. Studies show CGM users consistently improve their time in range, often by 15-25%.
Remote Monitoring and Sharing
Most CGM systems allow you to share your readings in real-time with family members, caregivers, or healthcare providers. Parents of children with type 1 diabetes can monitor their child's glucose remotely. Partners can receive alerts if glucose drops overnight. This shared visibility brings peace of mind to everyone involved.
Current CGM Systems
Several CGM systems are available, each with different features, sensor lifespans, and costs. Technology in this space evolves rapidly, so specific features may change.
| System | Sensor Duration | Key Features |
|---|---|---|
| Dexcom G7 | 10 days | Factory calibrated (no finger sticks needed), all-in-one disposable sensor/transmitter, 30-minute warm-up, smartphone display, share feature, integrates with insulin pumps for automated delivery |
| Freestyle Libre 3 | 14 days | Smallest sensor available, continuous real-time readings to smartphone, customizable alerts, no scanning required, lower cost than some competitors |
| Freestyle Libre 2 | 14 days | Scan-to-read with optional real-time alerts, very affordable, widely available, good entry-level CGM |
| Medtronic Guardian 4 | 7 days | Designed for use with Medtronic insulin pumps, predictive alerts, SmartGuard automation for automatic insulin adjustments |
| Eversense E3 | 6 months (implanted) | Implanted sensor placed by healthcare provider, removable transmitter, vibration alerts on the body, eliminates frequent sensor changes |
New systems and improvements to existing systems are regularly introduced. Check with manufacturers and your healthcare provider for the most current options.
Who Should Consider CGM?
CGM was originally developed primarily for people with type 1 diabetes, but its benefits extend to many others. Professional diabetes organizations now recommend considering CGM for a wide range of patients.
Strongly Recommended For
- Type 1 diabetes: Guidelines recommend CGM for virtually all people with type 1, regardless of age or insulin delivery method
- Type 2 diabetes on intensive insulin therapy: Those taking multiple daily injections benefit significantly from continuous glucose visibility
- History of severe hypoglycemia: CGM alerts can catch lows before they become dangerous
- Hypoglycemia unawareness: If you don't feel your lows, CGM provides critical safety alerts
- Pregnancy with pre-existing diabetes: Tight glucose control is crucial during pregnancy, and CGM helps achieve it safely
- Highly variable glucose levels: CGM helps identify causes and patterns of variability
Also Beneficial For
- Type 2 diabetes on basal insulin or oral medications: Even without mealtime insulin, CGM provides valuable insights into how food, activity, and medication affect glucose
- Anyone wanting deeper understanding: CGM reveals patterns that occasional finger sticks miss
- Athletes and active individuals: Real-time data during exercise helps prevent lows and optimize performance
- People who dislike or struggle with finger sticks: CGM dramatically reduces or eliminates routine finger pricks
- Shift workers and irregular schedules: CGM adapts to your schedule rather than requiring set testing times
- Parents of children with diabetes: Remote monitoring provides peace of mind
Understanding CGM Accuracy
CGM is remarkably accurate for a device measuring glucose indirectly through interstitial fluid, but understanding its limitations helps you use it appropriately.
The Interstitial Fluid Lag
Because CGM measures glucose in interstitial fluid rather than blood, there's an inherent delay of 5-15 minutes between changes in blood glucose and what the CGM shows. When your glucose is stable, this delay doesn't matter much. But when glucose is changing rapidly—like right after a meal or during exercise—the CGM reading may lag behind your actual blood sugar.
When CGM Is Most and Least Accurate
- Most accurate: When glucose is stable or changing slowly; in the normal-to-moderately-elevated range; after the first 24 hours of sensor wear
- Less accurate: During rapid glucose changes; at very high or very low levels; during the first 12-24 hours after sensor insertion while it stabilizes
When to Verify with a Finger Stick
Despite CGM's accuracy improvements, there are times when confirming with a finger stick blood glucose test is wise:
- When your CGM reading doesn't match how you feel
- Before treating hypoglycemia, if you have time and aren't severely symptomatic
- Before making significant insulin dosing decisions
- When the trend arrow shows rapid change and you need a current reading
- In the first 24 hours of a new sensor
Understanding Trend Arrows
Trend arrows are one of CGM's most valuable features. They tell you not just where your glucose is, but which direction it's heading and how fast. Different systems use slightly different symbols, but the concept is similar across all CGMs.
| Arrow | What It Means | What to Consider |
|---|---|---|
| ↑↑ Double Up | Rising rapidly (>3 mg/dL per minute, >180 mg/dL in 30 min) | Significant rise coming; may need correction if high; consider what's causing it |
| ↑ Single Up | Rising (2-3 mg/dL per minute, 60-90 mg/dL in 30 min) | Moderate rise expected; monitor closely; insulin may still be working |
| ↗ Angled Up | Rising slowly (1-2 mg/dL per minute, 30-60 mg/dL in 30 min) | Slight upward trend; usually manageable; observe |
| → Flat/Stable | Stable (<1 mg/dL per minute, <30 mg/dL in 30 min) | Glucose holding steady; current approach is working |
| ↘ Angled Down | Falling slowly | Slight downward trend; watch for continued drop, especially if already lower range |
| ↓ Single Down | Falling (2-3 mg/dL per minute) | Significant drop expected; have carbs ready; consider preventive snack if active |
| ↓↓ Double Down | Falling rapidly (>3 mg/dL per minute) | Urgent—eat fast-acting carbs now; potential severe low approaching |
When making decisions, consider both the number AND the arrow. A reading of 110 mg/dL with a double down arrow demands action (eat something), while the same reading with a flat arrow might be perfect.
Getting Started with CGM
Getting a Prescription
CGM requires a prescription from a healthcare provider. If you're interested in CGM, talk to your doctor or diabetes care team. They can determine if CGM is appropriate for your situation, help you choose a system, and complete the necessary paperwork for insurance authorization.
Insurance Coverage
Coverage for CGM has improved significantly in recent years, though specifics vary:
- Medicare: Covers CGM for people using insulin who meet certain criteria (including multiple daily injections or insulin pump use, and a history of hypoglycemia or problematic glucose control)
- Private insurance: Most major plans cover CGM, though prior authorization is often required. Coverage may be better for type 1 than type 2 diabetes.
- Medicaid: Coverage varies by state but is expanding
- Manufacturer programs: Most CGM manufacturers offer assistance programs for those without adequate insurance coverage
Training and Learning
When you start CGM, expect some learning curve. Helpful steps include:
- Watch the manufacturer's training videos on sensor insertion and app setup
- Work with your diabetes care team or a diabetes educator to understand how to interpret data and adjust treatment
- Start with wide alert thresholds to avoid alarm fatigue, then tighten them as you get comfortable
- Focus first on just watching and learning patterns before making treatment changes
Tips for CGM Success
Getting the most from CGM involves more than just wearing a sensor. These practical tips help maximize benefits:
Sensor Placement and Care
- Rotate sensor sites: Don't use the same location repeatedly; tissue irritation and scar tissue can affect readings
- Prepare skin properly: Clean with alcohol and let dry completely before insertion
- Avoid problem areas: Don't place sensors near scar tissue, areas where you inject insulin, or where clothing rubs constantly
- Secure if needed: Adhesive overlays (from manufacturers or third parties) can help sensors stay attached during sweating or swimming
Managing Alerts
- Start conservatively: Set initial alert thresholds wider than ideal to avoid constant alarms while you're learning
- Distinguish urgent from informational: Make urgent low alerts unmissable; other alerts can be less intrusive
- Adjust based on experience: Once comfortable, tighten thresholds toward your actual target ranges
- Avoid alarm fatigue: If you're ignoring alerts, they're probably set wrong
Using Your Data
- Review patterns, not just numbers: Look at the bigger picture—what times of day are problematic? What happens after certain foods?
- Share with your healthcare team: CGM data helps them make better treatment recommendations
- Focus on time in range: Aim to gradually increase the percentage of time your glucose stays in target
- Be patient: CGM reveals problems that were always there—now you can actually address them
Frequently Asked Questions
Can I swim, shower, and exercise with a CGM?
Most CGM sensors are water-resistant and can be worn during swimming, showering, and sweating. Check your specific system's guidelines for depth and duration limits during swimming. The transmitter and sensor should remain attached, though adhesive overlays can help keep everything secure during heavy sweating or extended water exposure.
Does inserting the sensor hurt?
Most people describe sensor insertion as a brief pinch—less painful than a typical finger stick. Modern applicators insert the sensor quickly with a spring-loaded mechanism. The sensor filament is extremely thin and flexible, so you generally don't feel it once it's in place. Some sites (like the arm) may be more comfortable than others.
Will CGM replace finger stick testing entirely?
For many people, CGM dramatically reduces finger sticks but doesn't eliminate them entirely. Factory-calibrated systems (like Dexcom G7 and Libre 3) don't require routine finger sticks for calibration. However, you may still want to verify readings when they don't match how you feel, during rapid glucose changes, or before critical decisions.
Why do my CGM readings sometimes differ from finger stick readings?
CGM measures interstitial fluid glucose, which lags behind blood glucose by 5-15 minutes. When glucose is changing rapidly, this lag is more noticeable. Additionally, both CGM and finger stick meters have acceptable accuracy ranges, so some variation is normal even when both are working correctly. Consistent trends matter more than perfect number-matching.
Is CGM only for people with type 1 diabetes?
No. While CGM was developed primarily for type 1 diabetes, it benefits many people with type 2 as well—especially those on insulin, those experiencing hypoglycemia, or anyone wanting better insight into their glucose patterns. Some people even use CGM intermittently to learn how their bodies respond to different foods and activities.
How does CGM work with insulin pumps?
Some CGM systems integrate directly with insulin pumps to enable automated insulin delivery. The pump uses CGM readings to automatically adjust basal insulin delivery, reducing highs and lows with minimal user intervention. This combination, sometimes called a "hybrid closed loop" or "artificial pancreas," represents the current pinnacle of diabetes technology.