CGM Data and Report Interpretation

The health care professional interprets the personal CGM reports retrospectively and evaluates for glycemic excursions above/below target range, seeking to identify patterns and potential causes for these excursions with the person with diabetes.  

Device download reports vary by manufacturer and include differing data presentations; however, there has been a recent push to focus on use of the Ambulatory Glucose Profile (AGP) report, which was developed by the Park Nicollet International Diabetes Center (IDC) in Minneapolis, Minnesota27. The use of one report would aid in standardization of care and would help to make interpretation more accurate and efficient. The AGP report includes summary statistics, a glucose profile graph and an insulin profile graph or glucose daily calendar graphs. 

Key Metrics 

The 2017 International Consensus on Use of Continuous Glucose Monitoring report published in Diabetes Care provides a detailed description of the 14 key metrics that can be analyzed when reviewing retrospective data. More recently, the International Consensus on Time in Range identified standardized clinical targets for CGM data interpretation, as follows:  

The first priority is to reduce the time spent below range (work to eliminate hypoglycemia) and then focus on decreasing time above range or increasing time in range.  

Number of days CGM is worn 14 days is recommended 

Percentage of time CGM is active  70% of data from 14 days is recommended 

Mean glucose Glucose Management Indicator (GMI)  This used to be called the estimated A1C (eA1C) but now uses an updated formula for converting CGM-derived mean glucose to an estimate of current A1C level. 

Coefficient of Variation (CV)  This is a measure of glycemic variability. A CV of less than or equal to 36% is considered acceptable, >36% is considered unstable and intervention is needed. 

Very High Time Above Range (TAR)  % of readings and time >250 mg/dl  

High Time Above Range (TAR) % of readings and time 181-250 mg/dl 

Time In Range (TIR)  % of readings and time 70-180 mg/dl 

Low Time Below Range (TBR) % of readings and time 54-69 mg/dl  

Very Low Time Below Range (TBR)  % of readings and time <54 mg/dl 

Glucose Time In Range Measure  

Glucose time in target range (TIR) is another important measure and Dr. Richard Bergenstal, in 2018, identified correlations of TIR and HbA1C as follows: (Used with permission from Sage Publications. Reference Guide fontegrating Continuous Glucose Monitoring Into Clinical Practice31) 

Step 1 Confirm that adequate data are available . For Current CGM users, a minimum of 70% of 2 weeks of data is recommended.34 Fewer days are needed when professional CGM systems are used. 

Step 2 Print out the AGP and ask patients to describe their daily self-management . When are they taking their insulin and how much? When do they wake? When do they eat? Do they exercise and, if so, what type of exercise and when are they doing it? Document this information on the AGP printout. 

Step 3 Ask the patients what they see in the AGP and why they think it may be important . Then listen. Interactive discussion with patients allows them to better understand how their insulin, food and other factors affect their glucose levels and also helps clinicians identify knowledge deficits or behaviors that may not support glycemic goals. 

Step 4 Look for problematic glycemic patterns in the following order of priority: 

Review the overall glucose profile (initial view) to determine the time of day when patterns are occurring, then review the daily graphs to double-check patterns to see if they are clustered on certain days. 

Step 5 Encourage patients to reflect on what they think may be causing the problem and discuss potential solutions.

Step 6 Collaboratively develop an action plan. Make sure patients fully understand the changes they will be making and that they have the knowledge/skills to implement the plan. 

Step 7 Make a copy of the marked up AGP printout for the patient and enter the original into the electronic medical record (EMR). If electronic entry is not possible, copy and paste the AGP into the EMR as a progress note.