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Comparison of flash glucose monitoring (FGM) and continuous glucose monitoring (CGM)

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

In addition to traditional self-monitoring of blood glucose (SMBG) through a glucometer, there are newer strategies to measure glucose levels, including the detection of interstitial glucose through Continuous Glucose Monitoring (CGM) or Flash Glucose Monitoring (FGM)

  • conventional glucometers determine blood glucose (SMBG), whilst sensors for CGM and FGM detect interstitial glucose levels.

FGM systems

FreeStyle Libre, is the first FGM system - note that FGM technology is sometimes referred to as intermittent continuous glucose monitoring (iCGM)

  • uses a wired glucose oxidase enzyme co-immobilized on an electrochemical sensor that is worn on the arm for up to 14 days (1)
    • patch sensor is about the size of a coin and has a short filament (4 mm long) that must be inserted into the subcutaneous tissue of the upper arm
    • FGM system does not require calibration with patient's SMBG
    • availability of glucose data is only on demand; in FGM, the glucose values are not constantly shown, and people can obtain real-time interstitial glucose values by placing a "reader" in proximity to the sensor
    • data are transferred from the sensor to the reader and recorded automatically every 15 min
      • also trends for the previous 8 h can be seen on the screen
    • the glucose change trend is indicated using an arrow
      • there are no alarms (in contrast to CGM) when defined values are exceeded or expected to be exceeded in the following minutes
    • FGM lacks connectivity with CSII devices
    • glucose values, which can be downloaded at any time, are presented in a simple form and include the Ambulatory Glucose Profile (AGP), which combines all the data from the sensor over a period of 14 days and gives a summarized visual display of glycaemic patterns
    • FGM is accurate, with reported overall mean absolute relative difference (MARD) of 11.4% for sensor results and stable over 14 days of use when compared with capillary BG reference values, and unaffected by body mass index, age, type of DM, clinical site, insulin administration, or A1C (1)
    • use of FGM has been associated with improvement in glucose control in both uncontrolled T2 and T1DM, and maintained for up to 24 weeks of using the device (1)
      • in well-controlled patients with T1DM, the use of FCGM reduced the time spent in hypoglycaemia by 38% at 6 months (intervention group) versus controls (SMBG) (1)
      • among participants with type 1 diabetes and high glycated hemoglobin levels, the use of intermittently scanned continuous glucose monitoring with optional alarms for high and low blood glucose levels resulted in significantly lower glycated hemoglobin levels than levels monitored by fingerstick testing (7)

CGM systems

  • has proven benefit in type 1 diabetes (2)
  • real-time CGM (RT-CGM) was then developed, showing glucose levels in real time (values are automatically displayed every 1-5 min), as well as their rate of change and glucose trends
    • there is however is a "lag time" between the plasma and interstitial fluid; therefore, interstitial glucose values do not correspond exactly to blood glucose concentration, which may cause a loss of accuracy for RT-CGM, especially during rapid glycemic excursions (3)
    • to maintain accurate sensor glucose readings, CGM systems require daily capillary blood calibrations (typically twice daily at stable glucose values)
      • at present, only the Dexcom G6 CGM system does not require fingersticks for calibration, and only the Dexcom G6 and G5 CGM systems can replace SMBG for insulin-dosing decisions (4)
        • other types of RT-CGM are approved as an adjunct to SMBG (5)
    • in real-time CGM systems, alarms can be programmed in case glycemic values are below or above a target range
      • feature is especially useful to detect unsuspected hypoglycaemia (such as during sleep)
    • RT-CGM can be associated with Continuous Subcutaneous Insulin Infusion (CSII), thus obtaining a Sensor-Augmented Pump (SAP), which in some cases includes algorithms capable of interrupting insulin infusion when the glucose concentration reaches or is expected to reach a defined level
      • significantly help to prevent hypoglycaemia, are only available with CGM and are an important step towards automated closed-loop systems and an artificial pancreas

    • Possible indications (6)
      • CGM is a useful tool in pump users, non-pump users with type 1 diabetes, and those with complex type 2 diabetes
        • most commonly used CGM is in combination with CSII - most modern pumps allow for CGM to be read in real time on the pump
        • if risk of severe nocturnal hypoglycaemia - CGM should be considered for high-risk patients, given the morbidity and possible mortality associated with severe nocturnal hypos
          • high-risk groups include patients with hypoglycaemia unawareness and particularly 'frequent flyers'with regular severe hypos
        • pregnant patient with type 1 diabetes who aims for meticulous glucose control
        • may be considered in patients who have occupations that require warning of hypoglycaemia such as professional drivers, or those working in remote or offshore environments

Comparison between features of CGM and FGM:


Glucose Measurement

User Calibration

Data Display

Trend Arrows

Alarms in Case of Hypoglycaemia or Hyperglycaemia

Maximum Duration of Sensor

Can be connected to an Insulin Pump

Adjustment of the Insulin Dose Based on Sensor Results




on demand



14 days





Yes, daily (except for Dexcom G6)

displayed automatically



Depending on the kind


Yes (presently, only Dexcom G5 and G6)


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