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Continuous Glucose Monitoring: A Review and Comparison of the Existing Systems

19 Jan 2022 5:34 AM | Anonymous

By: Naphtali Eke, PharmD

Mentor: Justinne Guyton, PharmD, BCACP, St. Louis College of Pharmacy at University of Health Sciences and Pharmacy in St. Louis/St. Louis County Department of Public Health


According to the National Diabetes Statistics Report, published by the Centers for Disease Control and Prevention, approximately 34.2 million people (10.5% of the population in the U.S.) have diabetes, with 7.3 million of those people who continue to go undiagnosed.1 Blood glucose monitoring (BGM) allows patients to measure their glucose from a blood drop, commonly through a fingerstick. However, continuous glucose monitoring (CGM) allows patients to measure their glucose in the interstitial fluid, in a continuous manner providing results every 5 minutes. Interstitial glucose levels correlate well with blood glucose levels, although there may be a delay if the glucose levels change quickly.2 Capillary blood glucose measurement via the fingerstick method in comparison to interstitial fluid measurements via CGMs has shown to provide more accuracy during periods of rapidly changing glucose levels such as hypoglycemia or right after a meal or dose of bolus insulin.3 Despite this, use of CGMs provide a wide variety of benefits, such as no fingerstick, alarms during periods of trending hyper or hypoglycemia, 24-hour monitoring, increased quality of life, and the ability to share results with primary care providers and family. However, they do come with a few well known disadvantages such as increased costs, reimbursement from Medicare mainly for insulin-dependent type-1 diabetics, and the inconvenience of having to constantly wear a device for benefit.4 The role of CGMs has been well established in type 1 diabetes, and more recent data from a meta-analysis in patients with type 2 diabetes also found that the use of CGMs compared to BGM resulted in improvement in hemoglobin A1c, improved ability to detect hypoglycemic events and nocturnal hypoglycemia, decreased time spent in hyperglycemia, and improved patient satisfaction.5 The purpose of this review is to compare and contrast the different features of the most common CGM systems on the U.S. market today: Dexcom G6, Freestyle Libre 2, and Medtronic’s Guardian Sensor.

Dexcom G66

The G6 is Dexcom’s latest generation CGM system, the Dexcom G6 is one of the most popular and commonly used devices in the U.S. It consists of a sensor, transmitter, and a display device, which can either be a receiver, compatible smartphone or watch. Compatible with these devices through their “Dexcom G6” and “Dexcom CLARITY” apps, it automatically sends real-time glucose readings every 5 minutes and is considered a real-time CGM (rtCGM). A new sensor is applied to the skin every 10 days. Another feature is that it is factory calibrated, which means the patient does not have to calibrate the device, or obtain a confirmatory blood glucose reading to make clinical decisions, although the option to calibrate is still available if the patient were to need it. The system can be used alone, or in conjunction with the Tandem t:slim X2 or the Tubeless Omnipod insulin pump. It is FDA-approved for ages 2 and up, which is the lowest approved age out of the three. The Dexcom G6 is covered by Medicare for patients with type 1 or 2 diabetes on intensive insulin therapy through approved Durable Medical Equipment (DME) suppliers. Of note, Missouri Medicaid does cover this device for patients with type 1 diabetes based on certain approval criteria. 

Freestyle Libre 27

The Freestyle Libre 2 system is one of the latest systems from Abbott, and it consists of a sensor and a reader. It works by holding the reader over the sensor each time a reading is desired, and is considered an intermittent scanned CGM (is-CGM), with readings as frequently as every 1 minute. Patients should scan at least every 8 hours for a full dataset. A new sensor is applied every 14 days and is a much simpler process in comparison to other systems. Just like the Dexcom G6, calibration is not required and the readings can be seen with a phone app rather than the reader. Also similar to Dexcom G6, the Freestyle Libre is covered by Medicare for patients with type 1 or 2 diabetes on intensive insulin therapy through DME suppliers. A great benefit to using this sensor is that despite whether a patient has insurance or not, the cost is priced 70% lower than the list price of other CGM systems. The FDA has approved its use in patients ages 4 and up.

Medtronic Guardian 38

A lesser-known option, Guardian 3, from Medtronic is similar to the Dexcom G6 in that it is a rtCGM that will transmit glucose readings to either a compatible smartphone or integrated insulin pump. It is composed of just the sensor and a rechargeable transmitter, using a smartphone as a reader. A disadvantage of this sensor is that it must be calibrated twice daily via BGM to maintain accuracy, an unfavorable characteristic for patients who are trying to avoid doing so in the first place. Its sensor use has the least durability, lasting 7 days before needing to be changed. Medicare does not provide coverage for Medtronic CGM supplies, so patients can either enroll in their CGM Medicare Discount Access Program, use their secondary insurance, or pay the out-of-pocket cost. The FDA has approved its use in patients age 14-75 years.


There are many ways to compare these systems. The Medtronic seems to be the least favored due to its need for daily calibration, and lack of Medicare coverage. It also has the shortest sensor lifespan. Changing a sensor may not be bothersome to some patients, but can be an inconvenience to others. The Freestyle Libre 2 has the longest sensor lifespan, the shortest sensor warm-up time, and is the lowest cost option for uninsured patients. The sensor warm-up time is the time it takes for a newly inserted sensor to acclimatize to the body before a patient is able to obtain accurate readings. A couple of disadvantages include no alerts when the glucose levels are trending to abnormal ranges (only notifies when it is already out of range), and no integration with an insulin pump. The patient would have to scan the sensor at least every 8 hours with either the reader or smartphone to obtain the current levels. This can be an inconvenience for patients who would prefer to be more discrete. Lastly, the Dexcom G6 has the benefit of real-time monitoring on an Apple Watch, unique from the others. Another consideration between meters is the potential for drug interactions (e.g. uric acid, galactose, xylose, acetaminophen, L-DOPA, and ascorbic acid) that can affect readings and vary by monitor.2 Patient’s should be instructed to check the device manual when receiving a CGM.


With the known challenges of the fingerstick method, it is anticipated that the desire for CGMs will continue to be on the rise. It is important for pharmacists to be knowledgeable about the variations in these devices to provide information to patients and providers.  This brief review of the most commonly used CGM systems highlighted some of the key considerations patients and providers may inquire about when selecting a CGM. Overall, when it comes to which sensor is the best, it may simply come to patient or provider preference, insulin pump compatibility, and cost. With new models of these sensors in development, such as the Guardian 4 and the Dexcom G7, additional and improved features may affect the patients’ decision-making process.


  1. Centers for Disease Control and Prevention. National Diabetes Statistics Report, 2020. [cited 2021 Dec 27]. Available from: https://www.cdc.gov/diabetes/data/statistics-report/index.html
  2. American Diabetes Association Professional Practice Committee. 7. Diabetes Technology: Standards of Medical Care in Diabetes – 2022. Diabetes Care. 2022 Jan 1;45(Supplement 1): S97-112.
  3. Cengiz E, Tamborlane WV. A tale of two compartments: interstitial versus blood glucose monitoring. Diabetes Technol Ther. 2009 Jun;11 Suppl 1(Suppl 1):S11-6.
  4. Hermanides J, Phillip M, DeVries JH. Current application of continuous glucose monitoring in the treatment of diabetes: pros and cons. Diabetes Care. 2011 May;34 Suppl 2(Suppl 2):S197-201.
  5. Janapala RN, Jayaraj JS, Fathima N, Kashif T, Usman N, Dasari A, Jahan N, Sachmechi I. Continuous Glucose Monitoring Versus Self-monitoring of Blood Glucose in Type 2 Diabetes Mellitus: A Systematic Review with Meta-analysis. Cureus. 2019 Sep 12;11(9):e5634.
  6. Dexcom. Descom G6 continuous glucose monitoring system. [cited 2021 Dec 27]. Available from: www.dexcom.com.
  7. Abbott. Freestyle Libre CGMs. [cited 2021 Dec 27]. Available from: www.freestyle.abbott/us-en/myfreestyle.html.
  8. Medtronic. Guardian 3 Sensor. [cited 2021 Dec 27]. Available from www.medtronic.com
  9. Diabetesnet. Comparison of current continuous glucose monitors (CGMs). [cited 2021 Dec 27]. Available from: https://www.diabetesnet.com/diabetes-technology/meters-monitors/compare-current-monitors/.

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