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Continuous Glucose Monitor

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Continuous Glucose Monitor, Continuous Glucose Monitoring, Freestyle Libre, Dexcom G6, Medtronic Guardian, Sensonics Eversense, GlucoWatch Biographer, Stelo CGM, Lingo CGM, Libre Rio, CGM

  • See also
  1. Blood Glucose Meter (Glucometer)
  • Indications
  1. Adjunct to Glucometer and Hemoglobin A1C in Diabetes Mellitus
  2. Consider in all patients with Diabetes Mellitus (including DM2 not on Insulin, and Gestational Diabetes)
    1. Insurance coverage may require at least one daily Insulin injection or severe Hyperglycemia history
    2. ADA recommends initiating at the time of initial Type 2 diabetes diagnosis
  3. Optimize Blood Glucose management in those on multiple Insulin injections per day
    1. Had primarily been indicated in Type 1 Diabetes Mellitus
    2. Consider in Type 2 Diabetes Mellitus with multiple daily Insulin injections or history of Hypoglycemia
  • 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. Insurance covers in most indicated cases (multiple daily Insulin injections or history of Hypoglycemia), but may require prior auth
  6. Prescription CGM Devices (preferred)
    1. Dexcom G7 (D6 is discontinued in July 2026)
      1. Dexcom G7 is similar to Freestyle Libre 3 (both sync to smartphones, G3 syncs to smart watches)
      2. G7 Sensor changed every 10 days (with a 15-day sensor also available)
      3. In 2023, Dexcom sensors cost $350/month (contrast with $130/month for Libre 3)
    2. Freestyle Libre, Freestyle Libre 2 and Freestyle Libre 3 (for age >=4 years old)
      1. Lower costs than other sensor/reader combinations, and Libre 3 plus is a small device
      2. Sensor (specific to each Libre model 1,2 or 3) worn on upper arm and changed every 14 days
      3. Freestyle Libre 2 adds alerts for highs/lows to the prior Libre 1 model (both 1 and 2 require scanning sensor with reader)
      4. Freestyle Libre 3 ($130/month) adds smaller size, high accuracy and transmits to smartphone app without scanning
    3. Other devices
      1. Medtronic Guardian
      2. Sensonics Eversense
        1. Eversense 365 sensor is imserted by medical provider and stays in place for 1 year
  7. Over-the-counter CGM Devices (e.g. Stelo, Lingo, Libre Rio)
    1. Less expensive ($100/month in 2025) than prescription devices ($150 to 400/month)
    2. Limited Glucose monitoring range (from a low of 55-70 up to a high of 220-250 mg/dl)
    3. Lack alarms to norify of low or high Glucose levels
    4. Approved for age >=18 years and not on Insulin, and not at risk of severe Hypoglycemia
    5. Worn on the upper arm
    6. Presc Lett 32(5): 25
  8. 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)
  • Management
  1. General
    1. Aim for CGM active use >70% of the 10-14 days device duration for best efficacy
  2. Abnormal readings
    1. Confirm with finger stick Blood Sugar from a Glucometer if prompted by CGM or does not fit situation
    2. Falsely low Glucose may be triggered by sleeping on the sensor
    3. Falsely high Glucose may occur with high dose Acetaminophen or Vitamin C
  3. 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%
  4. Glucose Variability (GV)
    1. Measures how wide Glucose values swing (35% is considered to be stable variability)
  5. 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%
  • Efficacy
  1. CGM use is associated with an increased Time in Range (TIR) and reduced Hypoglycemia episodes
  • 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]
  • Resources
  1. Continuous Glucose Monitoring for Patients (NIH NIDDK)
    1. https://www.niddk.nih.gov/health-information/diabetes/overview/managing-diabetes/continuous-glucose-monitoring
  • References
  1. (2019) Presc Lett 26(9): 52
  2. (2020) Presc Lett 27(9): 53
  3. (2022) Presc Lett 29(10): 57
  4. (2026) Presc Lett 33(4): 2-4