Measuring Blood Sugar Levels with Continuous Glucose Monitor (CGM) or Glucose Meter | Yoonah Kim, PharmD | RxEconsult

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Measuring Blood Sugar Levels with a Continuous Glucose Monitor (CGM) Category: Diabetes by - February 14, 2015 | Views: 26924 | Likes: 2 | Comment: 1  

CGM versus Glucometer

What is a Continuous Glucose Monitor (CGM)?

A continuous glucose monitor (CGM) is an FDA-approved blood glucose monitoring system that uses a small sensing device inserted under the skin to measure blood sugar (glucose) levels. It detects glucose levels in the interstitial fluid (fluid around the cells) and automatically records blood glucose readings every 1-5 minutes. It provides a steady stream of information about glucose levels and gives real-time glucose monitoring for a more complete picture of diabetic management.

How is CGM used?

The CGM sensor can be self-inserted at home, stays in place for 3 to 7 days and then must be replaced. When the sensor detects glucose levels the measurements are transmitted to a wireless monitor device and displays on the screen. The device notifies patients when their glucose level is too high or two low.

Fingerstick glucose measurements taken every 12 hours should be used to calibrate the CGM system for optimal accuracy. CGM does not replace a conventional glucometer because it is less accurate than a fingerstick glucose blood test. CGM is intended to improve glucose control while decreasing the occurrence of hypoglycemia (very low blood sugar).

Also Read: Dexcom Share Mobile Apps Helps Patients and Caregivers Monitor Glucose Levels

How effective is CGM among different age groups?

A multicenter clinical trial was conducted to study the value of CGM in the management of type 1 diabetes mellitus. The study included 322 adults and children who were already receiving intensive diabetic therapy. Of those patients, 98 patients were 25 years of age and older, 110 patients were between 15 and 24 years old, and 114 patients were between 8 to 14 years old. They were randomly assigned into a group that used CGM monitoring or a control group that used a home glucose meter to measure blood sugar levels. The change in the hemoglobin A1C (HbA1C) levels from baseline after 26 weeks was determined for each group. The results of the study were as follows:

  • Patients 25 years of age and older had a significant decrease in hemoglobin A1C compared to the control group. 26% of patients who used CGM had more than a 10% decrease ion HbA1C compared to only a 4% of patients in the control group. In addition, significantly more patients in the CGM group achieved the target HbA1C level of less than 7.0%.

  • Among patients who were 15 to 24 years of age the mean decrease in hemoglobin A1C was approximately 0.2% in both groups.

  • Among patients who were 8 to 14 years of age the mean decrease in hemoglobin A1C was approximately 0.37% in the CGM group and 0.22% in the glucose meter group. The difference between groups was not significant. However, more patients in the CGM group had a relative reduction of 10% or more in HbA1C level from baseline. 

The study concluded that the benefit of using CGM was strongly related to age. Patients who were 25 years of age and older were able to achieve tighter blood glucose control using CGM. Further studies should be done to identify the reduced effectiveness of CGM in younger patients and address ways to improve its effectiveness in other age groups.

What are some advantages and disadvantages of CGM?


  • It can be beneficial for certain patient populations who require tight blood glucose controls such as pregnant women or patients who frequently experience hypoglycemia.
  • CGM shows the direction of glucose levels so patients can manage their diet, physical activities, and medication schedules more easily.
  • CGM records blood sugar levels throughout the day and night and sends an alarm when levels fall dangerously low or are too high. 


  • CGM is more expensive than glucose meters and is not covered by some insurance companies.
  • CGM does not replace a glucose meter. Patients are still required to check their blood sugar levels with a glucose meter and to calibrate the CGM device.
  • Patients wear the inserted sensor device all the time and replace it every 3 to 7 days.

Which CGMs are available?

A prescription is required to purchase a CGM. Some insurance plans may not pay for CGM devices. The FDA has approved 3 CGM devices. They are manufactured by Abbott, Dexcom, and Medtronic. It usually costs $1000 to $2000 initially to purchase a CGM device and additional costs for supplies may range between $350 to $450 dollars per month. Find out more about cost and how to get started from each company’s websites below:

MiniMed 530G approved September 2013

Dexcom G4 Platinum approved January 2015

FreeStyle Navigator II approved March 2008


Juvenile Diabetes Research Foundation Continuous Glucose Monitoring Study Group. Continuous glucose monitoring and intensive treatment of type 1 diabetes. N Engl J Med. 2008; 359(14):1464-76. 

FDA Approved Devices: FreeStyle Navigator, Dexcom G4 Platinum, and MiniMed 530G

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