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NICE - continuous glucose monitoring (CGM) in type 1 diabetes

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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Continuous glucose monitoring (CGM) in Type 1 Diabetes

NICE state (1):

  • offer adults with type 1 diabetes a choice of real-time continuous glucose monitoring (rtCGM) or intermittently scanned continuous glucose monitoring (isCGM, commonly referred to as 'flash'), based on their individual preferences, needs, characteristics, and the functionality of the devices available. See box 1 for examples of factors to consider as part of this discussion
  • when choosing a CGM device:
    • use shared decision making to identify the person's needs and preferences, and offer them an appropriate device
    • if multiple devices meet their needs and preferences, offer the device with the lowest cost
  • Factors to consider when choosing a continuous glucose monitoring device
    • Accuracy of the device
    • Whether the device provides predictive alerts or alarms and if these need to be shared with anyone else (for example, a carer)
    • Whether using the device requires access to particular technologies (such as a smartphone and up-to-date phone software)
    • How easy the device is to use and take readings from, including for people with limited dexterity
    • Fear, frequency, awareness and severity of hypoglycaemia
    • Psychosocial factors
    • The person's insulin regimen or type of insulin pump, if relevant (taking into account whether a particular device integrates with their pump as part of a hybrid closed loop or insulin suspend function)
    • Whether, how often, and how the device needs to be calibrated, and how easy it is for the person to do this themselves
    • How data can be collected, compatibility of the device with other technology, and whether data can be shared with the person's healthcare provider to help inform treatment
    • Whether the device will affect the person's ability to do their job
    • How unpredictable the person's activity and blood glucose levels are and whether erratic blood glucose is affecting their quality of life
    • Whether the person has situations when symptoms of hypoglycaemia cannot be communicated or can be confused (for example, during exercise)
    • Clinical factors that may make devices easier or harder to use
    • Frequency of sensor replacement
    • Sensitivities to the device, for example local skin reactions
    • Body image concerns

CGM should be provided by a team with expertise in its use, as part of supporting people to self-manage their diabetes


  • subcutaneous continuous glucose monitoring (CGM)
    • has proven benefit in type 1 diabetes (2)
      • various designs (3,4)
        • subcutaneous monitoring
          • involves the subcutaneous insertion of a glucose sensor attached to a transmitter that sends signals to either an insulin pump or a hand-held meter
          • worn for 7 days
          • sensor inserted into subcutaneous abdominal fat
          • devices generally require regular calibration and blood glucose testing about twice a day
          • subcutaneous glucose levels change more slowly than plasma glucose, and this may be an important limitation, particularly if glucose levels are changing rapidly
            • subcutaneous glucose levels, therefore, have a short time lag, compared with blood glucose measurements, and measurements may not always match blood glucose. Nevertheless, when worn regularly (changed every 7 days), they improve outcomes in terms of hypoglycaemia and hyperglycaemia
            • patients can be alerted to hypo - and hyperglycaemia, and take early action to correct blood glucose levels
          • some CGM devices if used with particular insulin pumps will shut the pump off if a low glucose level is identified and thus is designed to prevent hypoglycaemia

        • a non-invasive CGM device uses the principle of Raman spectroscopy where a painless pulse of monochromatic light is transmitted into the skin, and the scattered light is detected for the determination of glucose levels (4)
          • device can be worn on the abdomen like a band and measures blood glucose levels every five minutes
          • sensor transmits data to a smartphone which is also enabled with alarms during periods of glucose excursion

  • flash glucose monitoring (FGM)
    • 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 (5)
        • 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 (5)
        • 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 (5)
          • 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) (5)


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