Background

Continuous glucose monitoring devices provide a way for patients to approximate their blood glucose by testing the interstitial fluid via the chemical reaction of fluorescence, every 1 to 5 minutes, with minimal “fingerstick” point-of-care glucose testing. These devices are frequently referred to by the abbreviation “CGMs.” There are two major types of CGMs called “real-time CGMs” or “intermittently scanning CGMs.” The major difference between the two is related to how the data is transmitted between the sensor and receiver. The real-time CGMs automatically send glucose data to the receiver, allowing alerts to automatically be set for hyper- and hypoglycemia. Intermittently scanning CGMs require the patient to manually place the reading device near the sensor in order to receive glucose data, so no automatic alerts or alarms can be set.

How to Use It

CGMs are useful for patients with type 1 and type 2 diabetes mellitus to reduce hypoglycemia and improve hemoglobin A1c. They are especially helpful in patients who experience hypoglycemia unawareness. ^, The devices offer various ways in which data is presented for patients and healthcare providers. The devices display current glucose readings along with graphical presentation of the glucose trend throughout the day. They also have the ability to display to the user the future trend of their glucose, including the following: glucose remaining stable, slowly or rapidly rising, and slowly or rapidly decreasing. The real-time CGMs can be programmed with alarms to alert the user of any of these trends along with hyper- and hypoglycemia self-set alert values.

The American Diabetes Association (ADA) recommends considering CGMs in patients with type 1 diabetes who have not reached their blood glucose goals, have frequent episodes of hypoglycemia, or hypoglycemia unawareness. The ADA also recommends using CGMs in children and women who are pregnant with type 1 diabetes. In type 2 diabetes, the ADA states that CGMs can be helpful to reduce hemoglobin A1c or hypoglycemic events. All patients with diabetes should be counseled to use their CGMs daily to show benefit, and intermittently scanning CGMs should be scanned at least every 8 hours.

How It Is Done

The CGM device system consists of three major components: sensor, transmitter, and receiver (which can also be patients’ smart phones). The sensor is placed subcutaneously by either the patient or provider, and depending on the system and age of the patient, it will be placed in the abdomen, back of the arm, or top of the buttocks. ^, For example, the Senseonics Eversense® sensor needs to be implanted with an incision by the provider into the upper arm. The reusable transmitter is then attached to the outside of the sensor by inserting it directly into a clip on the outside of the sensor or placed directly over it with adhesive. In comparison, the Freestyle Libre® system has the transmitter built into the application of the sensor, so it is discarded along with the sensor each time it is changed. The life of the sensor is different for each system. Once the sensor and transmitter are in place, there is a “warm-up” period that ranges from 1 to 24 hours, depending on the system, which is followed by traditional fingerstick glucose calibrations. After initial calibration, the Dexcom G4® and G5® and Medtronic Guardian® systems must be recalibrated every 12 hours via fingerstick testing. ^{, ,} The reader will prompt the patient when it is time to calibrate the device or change the sensor. Once the sensor has been calibrated, the sensor reads the interstitial glucose every 1 to 5 minutes, and the real-time CGMs’ transmitter automatically sends glucose data every 5 to 15 minutes to the user’s receiver. An intermittently scanning CGM only transmits on demand, like the Freestyle Libre ® receiver, which must be placed within 1.5 inches of the sensor to transmit the data. One caveat of CGMs is that interstitial fluid glucose lags behind actual blood glucose concentrations by about 4 minutes and is further delayed when the glucose level is rapidly changing. Continuous glucose monitors can be placed in clinic for professional use for the provider to assess trends from 10 to 14 days’ worth of glucose data.

Medication Implications

• ■
  

  The use of CGMs does not require any premedication.

  
  
• ■
  

  There are substances that interfere with the readings displayed by CGMs, and each system may have different interfering substances that will affect the reliability of the readings. It is important to review each system individually, as each system may have different interfering substances.

  
  
• ■
  

  Acetaminophen

  
  
  
  
  • ■
    

    When acetaminophen is oxidized at the sensor, it produces an electrochemical signal that may falsely elevate patients’ glucose readings. ^, There is limited data regarding this effect.

    
    
  • ■
    

    One small pilot study enrolled 10 healthy patients who were each given 1 g of acetaminophen and measured their blood and interstitial glucose throughout the day, all fasting until the end of the study. Their blood glucose remained around 90 mg/dL throughout the day, while interstitial glucose readings averaged 30 mg/dL and 21 mg/dL higher in the Dexcom G4 Platinum® and Medtronic REAL-time Guardian® systems, respectively. The glucose levels on the CGM increased about 30 minutes after acetaminophen intake.

    
    
  • ■
    

    Another study recorded data from 40 patients who were given 1 g of acetaminophen at breakfast. This study showed a statistically significant difference in glucose levels from the CGM compared with fingerstick glucose levels for 8 hours post-acetaminophen ingestion. The mean increase was 61 mg/dL in the CGM arm, and some patients who experienced hypoglycemia had CGM readings of 138 mg/dL and above.

    
    
  • ■
    

    As of January 2020, the systems that are not affected by acetaminophen ingestion are the Dexcom G6®, Senseonics Eversense®, and Freestyle Libre®. ^{, , , ,}

  
  
  
  
• ■
  

  Albuterol

  
  
  
  
  • ■
    

    A small pilot study of 19 subjects saw some interference with a 4-mg oral dose of albuterol, but the response varied among subjects. Some readings were increased, decreased, or remained similar to blood glucose.

    
    
  • ■
    

    CGMs used in this study were Dexcom G4 Platinum® and Medtronic Guardian®.

  
  
  
  
• ■
  

  Ascorbic acid

  
  
  
  
  • ■
    

    Ascorbic acid at doses 500 mg or higher have been shown to falsely increase glucose readings on Freestyle Libre® systems. ^,

  
  
  
  
• ■
  

  Aspirin

  
  
  
  
  • ■
    

    Aspirin at doses 650 mg or higher have been shown to falsely lower glucose readings on Freestyle Libre® systems. ^,

  
  
  
  
• ■
  

  Beta blockers

  
  
  
  
  • ■
    

    A small pilot study of 19 subjects saw some interference with a 100-mg dose of atenolol, but the response varied among subjects. Some readings were increased, decreased, or remained similar to blood glucose.

    
    
  • ■
    

    CGMs used in this study were Dexcom G4 Platinum® and Medtronic Guardian®. More variability was seen with the Medtronic Guardian.

  
  
  
  
• ■
  

  Lisinopril

  
  
  
  
  • ■
    

    A small pilot study of 19 subjects saw some interference with a 20-mg dose of lisinopril, but the response varied among subjects. Some readings were increased, decreased, or remained similar to blood glucose.

    
    
  • ■
    

    In this study, five subjects had detectable interstitial fluid concentrations of lisinopril. The changes in glucose values on CGM seemed to align with the interstitial fluid concentrations of lisinopril.

    
    
  • ■
    

    CGMs used in this study were Dexcom G4 Platinum® and Medtronic Guardian®.

  
  
  
  
• ■
  

  Mannitol and sorbitol

  
  
  
  
  • ■
    

    Mannitol has been shown to cause falsely elevated readings with the Senseonics Eversence® CGM system when used intravenously, as an irrigation solution, and in peritoneal dialysis. ^,

    
    
  • ■
    

    Sorbitol is frequently used in artificial sweeteners, and dietary consumption is not thought to affect sensor readings.

  
  
  
  
• ■
  

  Tetracyclines

  
  
  
  
  • ■
    

    Tetracyclines have been shown to interfere with the Senseonics Eversence® CGM system by causing falsely low readings. ^,

  
  
  
  
• ■
  

  There may be times when patients suspect that their CGM glucose readings are inaccurate or that the patient feels symptoms of hypo- or hyperglycemia with a normal glucose reading on the CGM. If this occurs, the patient should test their fingerstick glucose with their home point-of-care device to compare against their GCM reading. If there is a discrepancy between testing methods, the patient should be instructed to treat based on their fingerstick blood glucose reading and recalibrate their CGM by following the instructions for their specific device, if applicable. The Freestyle Libre® device cannot be recalibrated, as this is done in the factory. The manufacturer recommends, if inconsistency occurs, to treat based on the fingerstick blood glucose. Dexcom G6® is also factory calibrated; however, patients have the option to recalibrate if needed. One study has shown that recalibrating Dexcom G6® may not improve its accuracy and may actually worsen its accuracy.