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The Basics of Continuous Glucose Monitoring (CGM)

This article reviews the basics of continuous glucose monitoring. Please also view the complete list of CGM articles on our Continuous Glucose Monitoring page.

How CGM Works

Continuous glucose monitoring (CGM) allows people to see glucose readings 24 hours a day while doing few or no fingerstick tests. CGM uses a small wearable device to measure interstitial glucose (glucose that has left the blood and moved into the tissues).

interstitial glucose cgm 4ahs


  • Interstitial glucose readings are “older” than blood glucose readings by 5-15 minutes. This lag in time varies and can be longer after eating or after treating a low blood sugar. Therefore, CGM results don’t always match fingerstick blood glucose readings.

  • A glucose sensor (small electrode) is inserted under the skin and measures interstitial glucose every 1-5 minutes. The readings are sent wirelessly to a device, either automatically or by manually scanning the sensor with a reader. There are two major types of CGM.

Types of CGM - iCGM and rtCGM


 Intermittently Viewed or "Flash" (iCGM or FGM)

  • An example of an intermittently viewed CGM (iCGM) also known as Flash Glucose Monitor (FGM) is Abbott’s Freestyle Libre. There is one product currently available in Canada under this category. 
  • The person with diabetes manually scans the iCGM sensor with a handheld reader to see current and stored results. iCGM has no transmitter piece. 
  • iCGM does not have automated alarms for when gluoce levels reach certain limits. 
  • Fingerstick testing is not needed for calibrations but is needed at other times. 


 Real Time CGM (rtCGM) 

  • Real-time CGMs (rtCGM e.g. Dexcom and Medtronic) have a transmitter attached to the sensor to continuously send glucose results to a reading device.
  • Dexcom sends results to a receiver or a smartphone app or the Tandem insulin pump.  Medtronic sends results to a Medtronic insulin pump.
  • Recievers and apps can be programmed to alarm when glucose levels reach certain limits or change too quickly. Fingerstick testing is needed to calibrate rtCGMs at least twice daily and at other time 

Benefits & Challenges (Brief)

cgm screen graph2

Benefits of CGM may include:

  • Improved glycemic control.
  • Improved ability to identify glucose patterns and make daily management decisions.
  • Less fingerstick glucose testing: The glucose readings from some CGM (Dexcom, Libre) are approved for use in calculating insulin doses so this can reduce fingerstick testing. 
  • More information than fingerstick tests:
    • On The Device: CGM devices show a current glucose reading, a graph of previous hours’ glucose levels and rate of change (ROC) arrows. More information can lead to different decisions. E.g.  A reading of 5.2 mmol/L ↓↓ (dropping quickly) would likely require action to prevent a low, whereas a 5.2 mmol/L → (stable) might not.
    • In Reports: CGM devices offer various reports, including some type of Ambulatory Glucose Profile (AGP). The AGP translates glucose data into an image to easily identify glucose patterns. The "Dark Blue River" in the AGP below represents 50% of all readings. The "Light Blue River" (between the top and bottom edges of the light blue) contains 80%.  A "flatter" and "narrower" river represents more desirable glycemic control. 
Abbott Libre AGP DO 2018

sensor lags behind bg

Challenges of CGM include:

  • Cost
  • Alarm fatigue, feeling overwhelmed by data (this could lead to too many insulin adjustments)
  • Unrealistic expectations
  • Skin irritation
  • The requirement for fingerstick glucose tests for some situations (all brands).
  • Lag time: Sensor glucose (SG) readings can lag behind blood glucose (BG) readings at times, particularly when glucose levels change quickly (after eating, bolusing, treating low blood sugars, exercise). 

Brand Information (Brief)

Below are brief differences between brands. Sensor wear length & cost ($80+/-) varies. For more information, visit individual company websites. 

  1. Abbott’s Libre 
  2. Dexcom G6
  3. Medtronic Guardian Connect CGM 
    • 2020 sensor wear time is 7 days. A separate rechargable transmitter is required and lasts a year or longer. A free smart phone app is required. Guardian sensors can be used with Medtronic insulin pumps however require a different transmitter than the Guardian Connect standalone CGM. 
    • Read a Medtronic CGM  summary on our CGM Brand Information page.

 For a list of all our CGM webpages, please visit Glucose Sensors and Continuous Glucose Monitors (CGM).