| Measurement Site | Interstitial fluid (subcutaneous) |
| Reading Frequency | Every 1β5 minutes |
| Physiological Lag | 5β15 minutes vs. blood glucose |
| Typical Sensor Lifespan | 7β15 days |
| Accuracy (MARD) | 7β9% (current generation) |
| Glucose Range | 40β500 mg/dL depending on device |
| Connectivity | Bluetooth to smartphone or receiver |
Key Takeaways
- CGMs measure glucose in interstitial fluid, not blood β readings lag blood glucose by 5β15 minutes
- Most sensors last 7β15 days and transmit wirelessly to a smartphone or receiver
- Alarms for high/low glucose enable proactive management, reducing dangerous episodes
- Calibration requirements vary by device β some are factory-calibrated and need no fingersticks
What Is a Continuous Glucose Monitor?
A continuous glucose monitor (CGM) is a wearable medical device that automatically measures glucose levels throughout the day and night. Unlike traditional blood glucose meters requiring a fingerstick, CGMs use a tiny sensor inserted just beneath the skin to sample glucose from interstitial fluid every 1β5 minutes.
How CGM Sensors Work
The Electrochemical Sensing Mechanism
Most CGM sensors use an enzymatic electrochemical reaction. A glucose oxidase enzyme coating on the sensor tip reacts with glucose molecules in interstitial fluid, producing hydrogen peroxide. This triggers a small electrical current proportional to glucose concentration, which the sensor electronics convert into a mg/dL or mmol/L reading.
Interstitial Fluid vs. Blood Glucose
Because CGMs sample interstitial fluid rather than capillary blood, there is a physiological lag of approximately 5β15 minutes. This matters most during rapid glucose changes β such as after eating or intense exercise β when CGM readings may trail actual blood glucose levels.
CGM System Components
- Sensor: A flexible filament (0.3β0.4 mm wide) inserted subcutaneously, typically on the abdomen or upper arm
- Transmitter: Clips onto the sensor; sends encrypted data via Bluetooth to a display device
- Receiver/App: Displays real-time glucose value, trend arrows, and historical graphs
CGM Device Comparison
| Device | Wear Duration | Calibration | Range |
|---|---|---|---|
| Dexcom G7 | 10 days | Optional | 40β400 mg/dL |
| FreeStyle Libre 3 | 14 days | None required | 40β500 mg/dL |
| Medtronic Guardian 4 | 7 days | None required | 40β400 mg/dL |
Accuracy and MARD
CGM accuracy is measured by Mean Absolute Relative Difference (MARD). Lower MARD indicates greater accuracy. Current generation devices achieve MARD values of 7β9%, compared to 10β12% for older sensors. Factors affecting accuracy include sensor placement, hydration, certain medications (acetaminophen), and the first 24 hours after insertion (warm-up period).
Alert and Alarm Features
CGMs allow customizable alerts for hypoglycemia (typically <70 mg/dL), hyperglycemia (>180 mg/dL), and rapid rate-of-change. Predictive low alerts can warn users 20 minutes before glucose is projected to drop below threshold β a critical safety feature, especially overnight.
Choosing the Right CGM Supplies
Consistent sensor performance depends on quality application and compatible adhesive patches. MDS Diabetes stocks a full range of CGM sensors, transmitters, and skin-prep accessories at mdsdiabetes.com, including extended-wear adhesive overlays to keep sensors secure during exercise or swimming.
Pros and Cons of CGM Technology
Advantages
- Continuous real-time data reduces dangerous hypoglycemia events
- Trend arrows enable proactive insulin and lifestyle adjustments
- Eliminates most routine fingerstick testing
Limitations
- Interstitial lag can mislead during rapid glucose swings
- Sensor cost and insurance coverage vary significantly