Continuous Glucose Monitoring

What is a continuous glucose monitor?

Fewer than half of Americans with diabetes achieve goals for blood glucose management as set by the American Diabetes Association. Failure to reach these goals increases risk of complications including cardiovascular disease, amputation, impaired vision, kidney disease, and premature death. People with diabetes typically monitor their blood glucose by collecting a drop of blood from a finger stick, applying the blood to a test strip then inserting the strip into a blood glucose analyzer. Continuous glucose monitors (CGMs) offer an alternative means to monitor glucose.

A CGM consists of a tiny metal probe that is inserted into subcutaneous fat typically in the upper arm and a transmitter that sends interstitial (not blood) glucose data wirelessly to a receiver. Studies show that interstitial glucose concentrations closely match blood glucose concentrations. CGMs are designed to be used for a finite period of time (for example, a week or a few months) before they’re replaced. CGMs can also be part of a system that organizes, reports, and displays glucose levels, provides warnings of high or low glucose levels, and links to an app that provides health behavior feedback and guidance. CGMs are meant to be used on a daily basis and can replace the traditional finger-stick method of monitoring blood glucose.

Advantages of CGMs relative to traditional blood glucose monitoring

CGM’s have several positive attributes. First, they can eliminate the need to use finger sticks and blood glucose measuring units to directly measure blood glucose. Second, CGMs can provide frequent (for example, every 5 minutes) glucose levels that can help people visualize the glucose increase after eating certain foods, or after exercise, or while being sedentary. Connecting personal actions to glucose levels can help users identify how their actions affect their glucose levels and possibly motivate healthy choices. Third, CGS can link to mobile apps that provide behavioral health information and insights that can improve glucose management. Fourth, CGMs are increasing covered by health insurance. Fifth, CGMs can quickly detect excessively low glucose levels (hypoglycemia), which can lead to major adverse outcomes such as a coma.

Results of an early CGM trial

An early randomized controlled trial with 224 participants with an average age of 59 years and with long-term type 2 diabetes compared the effectiveness of managing glucose with the standard finger-stick method or a CGM (FreeStyle Libre by Abbott). This CGM unit was worn on the back of an arm for 14 days collecting glucose data every 15 minutes before being replaced. Participants came from 26 European diabetes centers. HbA1c is a protein that integrates blood glucose levels over 2-3 months and is used to determine if a patient has type 2 diabetes (above 6.5 percent) or not. After 6 months, HbA1c levels for the CGM group declined by 0.28 percentage points from 8.65 to 8.37 percent. For the control group, HbA1c levels declined by 0.41 percentage points from 8.75 to 8.34 percent. The differences between the groups were not statistically significant. Change in time in range (glucose between 70 and 180 mg/dL) was not statistically significant different between the groups. Other measures of glucose, such as glucose variability and hypoglycemia, were either non-significantly different between the groups or tended to favor the CGM group. Members of the CGM group reported significantly improved treatment satisfaction compared to the control group. No serious adverse events related to the CGM units or the study procedure occurred. This study suggested that CGMs can safely and effectively replace standard blood glucose monitoring.

Are CGMs effective for patients with type 1 diabetes?

Researchers with the Diamond Study Group tested whether CGMs would lead to better glucose monitoring and lower HbA1c levels. Participants (N=152) with an average age of 48 years with long-term type 1 diabetes were randomized in a 2:1 ratio to either the CGM group or the control group. Those in the CGM group received instructions to use and calibrate their CGMs daily. Those in the control groups were asked to check their blood glucose with a finger stick 5 times daily. After 24 weeks, participants in the CGM group and in the control group reduced their HbA1c levels by 1.0 percentage point and 0.1 percentage point, respectively. After adjusting for confounding factors, the CGM group showed a statistically and clinically significant 0.6 percentage point drop in HbA1c compared to the control group. In addition, the CGM group had significant improvements in time in range (glucose between 70 and 180 mg/dL), high glucose, low glucose, and glucose variability compared to the control group. Participants in the CGM group rated their experience with the CGMs as favorable. This study suggests that CGMs can help persons with long-term type 1 diabetes manage their glucose better than usual care.

Are CGMs effective for patients with type 2 diabetes and insulin injections?

Researchers in the Diamond Study Group also conducted a parallel study of participants with type 2 diabetes. One hundred fifty-eight participants with an average age of 60 years and with long-term type 2 diabetes and who received multiple daily insulin injections comprised the study subjects. Half were randomized to the CGM group and half to the control (usual care) group. Participants in the CGM group used the Dexcom G4 Platinum CGM system to monitor glucose, while those in the control group used finger-sticks to monitor blood glucose. Baseline average HbA1c was 8.5 percent for both groups. After both 12 and 24 weeks, the CGM group reduced HbA1c significantly by 0.3 percentage points more than the control group. In addition, more CGM participants significantly reduced their HbA1c levels by more than 0.5 percentage points, or lowered their HbA1c to 7.0 percent, or spent more time in range (glucose between 70-180 mg/dL) compared to the control group. The groups did not differ significantly in quality of life measures. The CGM group members reported that they were highly satisfied with CGM use, in line with their near-daily use of the CGM. The above results applied across a broad range of HbA1c levels, age, education, and numeracy skills. The Dexcom CGM improved glucose management compared to usual care for persons with type 2 diabetes who receive multiple daily insulin injections. Note that Dexcom, Inc. a manufacturer of CGM units, funded the study, and a Dexcom employee served as a co-author.

What about poorly controlled type 2 diabetes with basal insulin?

The Mobile Study Group conducted a randomized controlled trial of CGM effectiveness, this time with 175 middle-aged persons (average age 57 years) with poorly controlled type 2 diabetes and using basal insulin therapy. Participants were recruited from 15 primary care clinics around the US. Half of the participants were randomized to the CGM group, with the other half randomized to the control group (usual care). Members of the CGM group received a Dexcom G6 unit for glucose monitoring. Members of the control group received a OneTouch Verio Flex and test strips to monitor their blood glucose using tiny blood samples from finger sticks. From baseline to 8 months, both groups lowered their HbA1c levels, with the CGM lowering HbA1c significantly more. In addition, CGM group members had significantly more time in range (glucose between 70 and 180 mg/dL), significantly less time with glucose above 250 mg/dL, and significantly lower average glucose, among other glucose measures, compared to the control group. Severe adverse events occurred equally in the two groups. Ninety-three percent of the CGM group members and 97 percent of the control group completed the 8-month study, suggesting that the CGM was well tolerated. Persons with long-standing type 2 diabetes and receiving basal insulin therapy showed better glucose control in the CGM group compared to the control group over 8 months. This study was funded by Dexcom and one of the co-authors was a Dexcom employee.

What about the new CGM user experience?

Dexcom, Inc., a company that manufactures continuous glucose monitoring devices, developed a study to determine how new users rated their experience with the Dexcom G6 CGM. Participants (N=248) with either insulin-treated type 1 or type 2 diabetes who used a Dexcom CGM unit were recruited by Dexcom and followed for 3-5 months. From baseline to 24 weeks HbA1c levels fell significantly from 8.2 to 7.1 percent. Declines in HbA1c were larger for participants with higher baseline levels. Glucose variability, as measured by its standard deviation, fell by 41 and 50 percent for those with type 1 and type 2 diabetes, respectively. The 17-item Diabetes Distress Scale (DDS) and the 14-item Hypoglycemic Attitudes and Behavior Scale (HABS) measured participant quality of life. For DDS and HABS overall, participants reported significant improvements from baseline to 24 weeks. For the DDS and HABS subscales, participants reported significant improvements in 4 of 5 subscales and all 3 subscales, respectively. Ninety-three percent of participants were very satisfied or satisfied with their CGM unit.  The take-home message: Participants new to CGMs significantly and meaningfully lowered their HbA1c over three months and reported improved quality of life, all with no or minimal assistance from Dexcom. Sounds good to me!

What about combining a CGM with a mobile app?

The often-poor management of type 2 diabetes prompted entrepreneurs to combine continuous glucose monitoring with self-management behavior interventions. The BlueStar program from Welldoc, Inc. is one example. Scientists with Welldoc and Dexcom tested the program over three months with data from 52 patients with type 2 diabetes and with an average age of 53 years in three healthcare clinics. Participants received Dexcom continuous glucose monitors and instructions on their use plus access to and instructions for using the Welldoc mobile app. From baseline to 3 months, participants in the CGM plus mobile app group improved their glucose control metrics compared to participants using a control app. Specifically, participants in the BlueStar program reduced their average glucose from 164 to 155 mg/dL, with greater improvements for participants with baseline glucose higher than 180 mg/dL. Also, participants in the BlueStar program whose baseline glucose exceeded 180 mg/dL increased their time-in-range (glucose between 70 and 180 mg/dL) from 21 to 36 minutes per day, and reduced their time above 250 mg/dL from 25 to 11 minutes per day, both being statistically significant and clinically meaningful improvements. As an added bonus, BlueStar participants engaged with the Welldoc app with respect to medications, exercise, food, weight, sleep, and blood pressure. Insights from the continuous glucose monitor data plus help from the app in solving problems and applying effective self-care behaviors likely contributed to better glucose management

What to do

If you have type 1 or type 2 diabetes (or if you’ve been diagnosed with pre-diabetes), check out continuous glucose monitors. Evidence suggests that CGMs could help you lower your average blood sugar levels and reduce blood sugar spikes. Ask your primary care provider if your health insurance will pay for a CGM. Even if your insurance won’t pay, consider purchasing a three-month CGM to see how the foods you eat and your exercise (or lack thereof) affect your glucose levels.

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