From left: Medtronic Guardian, Abbott Freestyle Libre, and Dexcom G6 CGM systems (click on image to zoom)
The Diabetes Control and Complications Trial (DCCT) demonstrated the importance of frequent glucose monitoring for people with type 1 diabetes. In recent years, continuous glucose monitoring (CGM) systems have been developed to provide better glucose monitoring in terms of real-time data, ease of monitoring, and the use of trend arrows to predict future glucose directions.
Earlier CGM systems were retrospective by design, i.e. data from these systems can only be accessed retrospectively. Newer CGM systems feature real-time glucose monitoring (rt-CGM) or glucose readings that can easily be accessible by the user at any time of the day (flash CGM). The currently available systems include Medtronic Guardian (rt-CGM) and Abbott Freestyle Libre (flash CGM). Dexcom, another main rt-CGM system, is currently available in many countries but not in Malaysia.
Real-time CGM or flash CGM helps people with diabetes in two ways. First, they provide continuous and easily accessible glucose data for the users. This helps people with diabetes make better decisions regarding their insulin dosing, carbohydrate counting and dietary intake, sick day monitoring, exercise, sleep, and other daily activities. The trend arrows features of most CGM systems also help their users to predict glucose readings and take pre-emptive measures to prevent hyper- or hypoglycaemia before they happen.
Second, CGM data is summarized in a standard report featuring time in range (TIR), time below range (TBR), time above range (TAR), glucose management indicator (GMI), glucose variability (%CV), ambulatory glucose profile (AGP), as well as daily glucose profiles. Some CGM systems also provide analysis on the most common periods for hyper- or hypoglycaemia in an average day. These data allows the people with diabetes and their diabetes care providers to make better treatment decisions, especially on insulin dose adjustments as well as behavioural modifications to prevent hyper- or hypoglycaemic episodes.
Example of a CGM report. From Battelino et al. Diabetes Care. 2019 Aug;42(8):1593-1603. (click on image to zoom)
One of the most important paradigm shift in diabetes management with CGM is the use of time in range (TIR) instead of HbA1c as the treatment target. TIR is defined is the time spent within the sensor glucose range of 3.9 to 10.0 mmol/L. A TIR of more than 70% have been shown to correlate with HbA1c of less than 7%, and is the recommended treatment goal for most people with type 1 and type 2 diabetes. A lower TIR target of more than 50% corresponds to HbA1c of less than 8%, and is the recommended goal for high-risk individuals, especially the elderly, frail, those who are at high-risk for hypoglycaemia, and those with advanced comorbidities. A TIR goal of more than 60% is recommended for children, adolescents and young adults less than 25 years old.
In pregnant mothers, TIR is defined as the time spent within the sensor glucose range of 3.5 to 7.8 mmol/L. For pregnant mothers with type 1 diabetes, the TIR goal is > 70%, while for pregnant mothers with gestational diabetes or type 2 diabetes, the TIR goals have not been objectively defined.
Another important treatment parameter is the times below range (TBR) defined as time spent within sensor glucose range between 3.0 to 3.9 mmol/L (level 1) and time spent within sensor glucose range of less than 3.0 (level 2). A total TBR goal of less than 4% is recommended for most people with type 1 and type 2 diabetes, whereas a total TBR goal of less than 1% is recommended for high-risk individuals.
CGM targets for different diabetes populations. From Battelino et al. Diabetes Care. 2019 Aug;42(8):1593-1603. (click on image to zoom)