Not All Continuous Glucose Monitors (CGMs) Are the Same
Continuous glucose monitors (CGMs) differ significantly in their technology, features, accuracy, and clinical applications. Different CGM systems have varying monitoring principles, accuracy levels, and features that make them distinct from one another 1.
Types of CGM Systems
There are three main types of CGM devices 1:
- Real-time CGM (rtCGM): Systems that measure and display glucose levels continuously
- Intermittently scanned CGM (isCGM): Systems that measure glucose continuously but require scanning for visualization and storage of values
- Professional CGM: Clinic-based devices placed on patients for a discrete period (typically 7-14 days) for assessment of glycemic patterns
CGM technology can also be categorized as 1:
- Retrospective CGM: "Holter-style" monitoring where results are obtained after completion of monitoring period
- Real-time CGM: Provides immediate glucose readings with alarms and predictive alerts
Technical Differences Between CGM Systems
Most glucose sensors are designed based on electrochemical principles, with the sensor implanted in subcutaneous tissue 1:
- The sensor uses an enzyme (typically glucose oxidase) that interacts with interstitial fluid glucose to generate electrical signals
- These signals are converted into glucose concentrations through specific algorithms 1
Sensor types vary between disposable (rtCGM and isCGM) or implantable (rtCGM) 1
Accuracy varies substantially between different CGM systems 2, 3:
- In a head-to-head comparison study, FreeStyle Libre 3 showed significantly lower Mean Absolute Relative Difference (MARD) compared to Dexcom G7 (8.9% vs 13.6%) 2
- In another comparative study, the Navigator outperformed other devices with an aggregate MARD of 11.8% compared to 16.5% for Seven Plus and 20.3% for Guardian 3
Clinical Applications and Limitations
CGM systems have different approval statuses for clinical decision-making 1:
- Some systems require confirmatory blood glucose checks before treatment decisions
- Only certain systems (like Dexcom G5 and G6) have FDA approval for decision-making without confirmatory blood glucose checks
CGM accuracy can be affected by different factors 1, 4:
- Potential interfering substances vary between systems and include acetaminophen, ascorbic acid, and certain medications
- Environmental factors like hypoxemia can affect accuracy of some systems
- Patients with advanced chronic kidney disease may experience different accuracy levels with various CGM systems
Special considerations for specific patient populations 1:
- In patients with end-stage kidney disease on peritoneal dialysis, certain glucose meters using glucose dehydrogenase pyrroloquinoline quinone methods can be affected by icodextrin-based solutions
- CGM is currently not approved for use in pregnant patients, critically ill patients, and patients on dialysis 5
Practical Implications
The choice of CGM device should be based on individual circumstances, preferences, and needs 1
Users should be educated on potential interfering substances and other factors that may affect accuracy of their specific device 1
Skin reactions due to irritation or allergy should be assessed and addressed to aid in successful use of devices 1
People with diabetes should have uninterrupted access to their supplies to minimize gaps in continuous glucose monitoring 1
Real-time CGM should be used as close to daily as possible for maximal benefit, while intermittently scanned CGM devices should be scanned frequently (at minimum once every 8 hours) 1
In conclusion, healthcare providers should be aware of the significant differences between CGM systems when recommending them to patients, and should consider the specific features, accuracy, and limitations of each system based on individual patient needs 1.