Continuous Glucose Monitors (CGM)

 

Benefits of CGM Monitoring​

While CGM data is NOT an accepted ADA criteria for staging of T1D, CGMs are easy to wear, and patients can be taught to place them on themselves, giving patients an opportunity to participate in monitoring and to feel empowered. Many patients find CGM monitoring more acceptable than OGTTs or intermittent home glucose testing. Another benefit of CGM assessment is the ability to share results with the patient/family and to obtain detailed information about blood glucose variation while the person is eating their usual diet and participating in their normal daily activities. Sometimes, evidence of increasing glucose levels on CGM can encourage patients to undergo an OGTT for formal staging and evaluation of eligibility for immunotherapies. CGMs have the potential to identify important glycemic changes earlier than the less sensitive HbA1c or OGTT tests which are not practical for frequent assessments. They can also be used during times of physiologic stress (e.g. fevers, illness) or use of medication which impacts glycemic excursions (e.g. corticosteroids) to evaluate transient changes in glucose values. A common metric which has been shown to be associated with progression to Stage 3 T1D is the percent sensor values above 140 mg/dL.(2)

Considerations for CGM use: 

One important caveat in the use of CGMs for monitoring is the potential for errors in calibration and tendency for less accurate data in the first 24 hours of wear. This can be addressed by obtaining a few fasting fingerstick blood glucose values.  

Blinded CGM wear:  

Historically, CGMs used for monitoring of early-stage T1D in research studies have been worn in a blinded state. Data from the blinded CGM can be downloaded by the health care team and can be used to evaluate overall trends in glycemic excursions. Families can be given data alongside interpretation following CGM wear. This approach has the advantage of giving families context for data which may be unfamiliar and anxiety-provoking. The time above 140 mg/dL measure as well as interval changes from previous wears and a printout of sensor tracings may be shared with the family. Post-measurement discussion of blinded CGM findings allows sharing of information in the context of education regarding its meaning and an opportunity to answer questions and provide support. 

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Unblinded CGM wear:

Unblinded CGMs provide patients/families with a constant input of data which many individuals are not equipped to interpret unless provided with education and support. Some individuals can experience psychological distress due to this constant ambiguous information and may become anxious if they see what they perceive to be abnormally low or high blood sugars.
      Even families with experience using CGM for management of Stage 3 T1D may not be aware of what constitutes a cause for concern in people who are not using insulin. For example, individuals who are using insulin are typically taught to treat blood glucose values less than 70 mg/dL. This threshold is deliberately higher than the threshold of 54 mg/dL, the level at which neuroglycopenic symptoms begin to be noted, including changes in cognition, behavior, and risk for adverse events. Healthy individuals who are not using exogenous insulin have intact counterregulatory response to falling glucose levels, thus do not typically require intervention or concern for glucose values in the low normal range. For example, healthy individuals 7-80 years of age spend a median of 1.1% of their day with glucose below 70 mg/dL.(5)  In healthy young children (2-8 years of age), with more limited hepatic glycogen stores, 9% of sensor values are below 72 mg/dL, particularly notable in the early morning hours before waking.(6) 

 

Providing education is ESSENTIAL before using unblinded CGM. Topics should include: 

• Limitations of CGM data, including less reliable results in first 24 hours 

• Normal values expected 

• Importance of verification of high/low results with fingerstick blood glucose 

• What to do/who to call when values are outside of expected range 

 

Blinded vs Unblinded CGM summary: 

It is recommended that unblinded CGM wear is used only in the context of education by experienced providers and availability of psychological support.  

      This approach was piloted in individuals with Stage 2 T1D as part of The Early Start Study (TESS) at our center. Initial participant feedback to this approach was overall positive, and patients were able to gain an understanding of factors that impacted glycemic patterns. While most participants elected to continue using unblinded CGM following this study, there were a few families who elected to only use blinded CGM in order to reduce worries and focus on blood glucose trends. Commercially available CGMs do not have an option for blinded wear. This can be addressed by linking the CGM to a smartphone or receiver device that is not with the person at all times, but is brought in proximity periodically to download data to app/receiver. Dexcom G7 stores 24 hours of data and the Libre 3 stores 14 days of data.