Continuous Glucose Monitor

Aka: Continuous Glucose Monitor, Freestyle Libre, Dexcom G6, Medtronic Guardian, Sensonics Eversense, GlucoWatch Biographer

II. Indications

  1. Adjunct to Glucometer and Hemoglobin A1C in Diabetes Mellitus
    1. Optimize Blood Glucose management in those on Insulin
    2. Primarily indicated in Type 1 Diabetes Mellitus

III. Preparations: Needle based monitors

  1. Needle inserted subcutaneously and left in place for days
  2. Samples interstitial fluid Glucose levels (lags Blood Glucose by 20-40 minutes)
  3. Readings are transmitted via bluetooth to proprietary devices or smartphone applications
  4. Monitors will prompt users to check a fingerstick Glucose when sensor values are out of range and generate an alert
  5. Devices
    1. Dexcom G6
    2. Medtronic Guardian
    3. Sensonics Eversense
    4. Freestyle Libre and Freestyle Libre 2
      1. Lower costs than other sensor/reader combinations, but requires scanning sensor with reader
      2. Freestyle Libre 2 adds alerts for highs/lows to the prior Libre 1 model
  6. Sensor Site Care
    1. Wash sensor site with soap and water and dry
    2. Wipe site with Alcohol swab and allow to dry, then insert sensor
    3. If sensor does not stay in place, consider measures to keep in place (e.g. Tegaderm, Skin Tac)

IV. Management

  1. Time in Range (TIR)
    1. Measures percentage of time that a patient is in goal range (70 to 180 mg/dl)
    2. Prioritize correcting low Blood Sugars (Hypoglycemia) first
    3. Prevent Glucose <54 mg/dl and limit Glucose <70 mg/dl to less than 1 hour per day
    4. Goal Time in Range >70% for those with Hemoglobin A1C <7%
  2. Glucose Variability (GV)
    1. Measures how wide Glucose values swing (35% is considered to be stable variability)
  3. Glucose Management Indicator (GMI)
    1. Estimates Hemoglobin A1C based on the last 14 days of Glucose readings
    2. May vary from actual Hemoglobin A1C by more than 0.3%

V. Preparations: GlucoWatch Biographer (discontinued, listed for historical reasons and clever mechanism)

  1. Wristwatch (GlucoWatch Biographer) measured Glucose through intact skin (without needles)
    1. Contained Glucose oxidase gel discs and electrodes, measuring Glucose via reverse Iontophoresis
    2. Discs converted Glucose to Hydrogen Peroxide, with a small electric current applied
    3. Sensor detected electrons released and reported an estimated Glucose every 20 minutes
    4. Disposable auto-sensors applied to back of watch provided 12 hours of readings
  2. Device accuracy was variable
    1. Required calibration with Glucometer
    2. Accuracy and device function was altered by perspiration and device jarring
    3. Measurements were less accurate for Blood Sugars below 80 mg/dl
  3. Skin irritation occurred with even short-term use
  4. References
    1. (2001) Med Lett Drugs Ther 43(1104):42 [PubMed]
    2. Garg (1999) Diabetes Care 22:1708 [PubMed]
    3. Tamada (1999) JAMA 282:1839 [PubMed]
    4. Tierney (2000) Ann Med 32:632 [PubMed]

VI. Resources

  1. Continuous Glucose Monitoring for Patients (NIH NIDDK)
    1. https://www.niddk.nih.gov/health-information/diabetes/overview/managing-diabetes/continuous-glucose-monitoring

VII. References

  1. (2019) Presc Lett 26(9): 52
  2. (2020) Presc Lett 27(9): 53

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