Making the most of your continuous glucose monitoring system
A Continuous Glucose Monitor, (CGM) provides a sensor glucose level, (SG) that can be used for clinical decisions in most situations.
For example, you can see what effect physical activity has on your SG or whether your SG is above or below your target.
Did you know your CGM is part of a Continuous Glucose Monitoring System (CGMS) that can provide helpful information in addition to your SG?
A CGMS includes the sensor and reader needed to check your SG. It also includes detailed reports that are accessed on your CGMS website and a CGMS app on a smart phone or tablet. If you are unsure how to access your reports, you can ask your diabetes health care team or the CGMS device company to help.
This additional information includes:
A summary of your glucose data over several days or weeks, known as the Ambulatory Glucose Profile (or AGP).
- How long your CGM sensor has been reading your SGs.
- The percentage of time your SGs have been within your target range over a selected time period. This is known as Time in Range (TIR).
- Your estimated HbA1c.
- Any trends or patterns that indicate you may need to change something in your management regimen
- A record of the events you have logged in your CGMS such as insulin doses or physical activity.
- How much your SG levels change over a selected time period. This is known as glucose variability. Overall, the aim is to minimise variability in glucose levels.
You can check variability in a few different ways:
- The AGP wave graph: If the wave across the graph has large peaks and troughs, there is high variability in SGs.
- TIR table: If you have SGs above and below your target for a greater percentage of time than the target time, there is high variability.
- Standard deviation (SD): The SD tells you how much difference there is between all the glucose levels in a selected date range, compared to the mean, or average, for that selected date range. If the SD is higher than the mean SG divided by 3, there is high variation. For example, if your average SG is 6.0 mmol/L and your SD is 3.0, then you have high variability. Not all CGMS report SD.
- The co-efficient of variation (%CV): To work out the %CV, the SD is divided by your mean SG then multiplied by 100. Your CGMS will work out this calculation. If the %CV is greater than 36%, there is high variability.
It takes time to become confident to assess variability. Practice and support from your diabetes health care team will help.
Understanding the data in the CGMS reports can help you self-manage your diabetes. You can use this information to decide if you need to make any changes. You can also use this information to discuss your management with your diabetes health care team.
The table below lists and explains the features of the CGMS report. It also provides a general target where it is relevant.
Features of a CGMS report
| Feature | Explanation | Target |
| Ambulatory Glucose Profile (AGP). | Summarises your glucose levels over several days. Can be generated from up to 90 days of information. Shows the glucose data as a wave form graph across a 24-hour period |
|
| Sensor usage. Also known as: - Time sensor is active - Sensor wear |
Shows how much time the sensor has been reading glucose levels. | - Greater than 70%. |
| Glucose variability. - Standard Deviation (SD) - %CV - AGP report - Time in range (TIR) |
Tells you how much your SG has changed during a selected time period. A reliable indication of glucose variability can be obtained with:
|
- SD less than the mean divided by 3 - %CV less than 36% - Small variation on AGP - Time spent above SG target range and time spent below SG target range are both within or close to the TIR target. |
| Time in range (TIR). | The percentage of time in target range across a selected date range. |
- 3.9 to 10.0 mmol/L* *See the coloured graph below for all TIR targets. |
| Estimated HbA1c also known as the Glucose Management Indicator (GMI) on the AGP report. | Your HbA1c gives you an idea of how likely you are to develop long-term health issues from diabetes. | - 6.0 to 7.0% |
| Glucose patterns. |
A pattern of out of target SGs can help identify where changes may be needed. - Daily graphs |
|
| Factors that can affect your SG. | Recorded as events in the CGM reader. Review of the AGP event markers help provide a complete picture of diabetes management. |
These are the TIR targets for most people, most of the time. Some people will have slightly different TIR targets. TIR targets need to consider factors such as your age and other health conditions. Women who are pregnant will have different targets specific for pregnancy. You can talk with your diabetes healthcare professional(s) about the TIR target range that is right for you.
It will take time, education, practice and support from your diabetes health care team for you to be confident to understand the data in your CGMS reports. To begin, you can use this table as a reference. Why not take a look at your reports now and compare your data to the table.
Follow these steps to help you understand your report:
1. Check your sensor usage. If you have at least 14 days of data and you have been wearing your sensor for at least 70% of that time, your reports will be reliable.
2. Check your glucose variability:
- Observe the AGP wave form graph for peaks and troughs.
- Check your TIR table
- Check your SD and %CV.
3. Check your estimated HbA1c.
4. Check your daily graphs, daily logbook and AGP graph. Can you see any patterns of out of target SGs? For example, are you above or below target around the same time from day to day?
If you identify any issues with your diabetes management and you aren’t sure of the next steps, speak with your diabetes health care team. You can get guidance for any changes you may need to make.
You can find out more about CGM on the NDSS website at: www.ndss.com.au/livingwithdiabetes/continuousglucosemonitoring
Michelle Cox
Registered Nurse and Credentialled Diabetes Educator
Michelle joined Diabetes Victoria 12 years ago. She has worked as a nurse in paediatrics, regional and remote nursing and in the community health setting. Michelle facilitates programs for people living with diabetes, including the OzDAFNE program. Michelle loves spending time in and around the ocean near her home and exercising with friends and family.
