Continuous Glucose Monitors (CGMs) The Ultimate Guide 2024

*Disclaimer: All content and information in this blog is for informational and educational purposes only. This article was written by Pete Lomas (T1D) and medically reviewed by Rachael Baker, BNg, CDE, MNgPrac and Adil Maqbool, MD

Last updated on 08/02/24.

This is a complete guide to the wonderful, the magical, the incredible Continuous Glucose Monitors (CGMs) in 2024. As we’re sure we all agree, CGMs are pretty life-changing. Whether you have T1D yourself, or know someone who does, let’s give a round of applause to these little devices that do a lot of the T1D hard work for us.

In this in-depth guide you will find:

• • What are Continuous Glucose Monitors
  • Short history of CGMs
  • How do Continuous Glucose Monitors work
  • How accurate is a CGM
  • Continuous Glucose Monitors Review
  • Who qualifies for CGMs
  • CGM system cost. Does insurance cover CGMs?
  • How to remove a CGM sensor
  • How to wear a CGM sensor
  • Where can you get a CGM system
  • How to choose the right CGM system for you

   

  What are Continuous Glucose Monitors?

  There is a wide variety of sensors for blood glucose monitoring available on the market today. CGM and FGM (flash glucose monitoring) systems give us the opportunity not only to see our BG in real-time but also what happened before (graph) and what may happen later (trends).

  Sensors are electrodes that, placed under the skin, measure the blood glucose level from the interstitial fluid (not the blood). The concentration in the interstitial fluid is delayed by about a dozen or so minutes compared to the concentration in the blood. The electrode is very thin (slightly thicker than a hair) and you do not feel it after applying the sensor. The needle is only used to insert the electrode under the skin (depending on the system it has to be pulled out or it retracts itself after application).

  Although there are different products on the market, all CGM systems broadly work in the same way. They do what they say on the tin – they continuously monitor glucose levels throughout the day.

  Why is this great?

  Well, unlike finger prick testing, which gives you one solitary reading with no indication of which direction your blood sugar is going in, CGMs can help you and your diabetes team see:

  • If blood sugars are going up or down;
  • How your levels change over time;
  • What happens to your blood sugars overnight;
  • Patterns that occur after specific meals or during exercise;

  Short history of CGMs

  The first professional CGM device made by Medtronic MiniMed was approved by the FDA (Food and Drug Administration) in the United States in June of 1999. This sensor could be worn only up to 72 hours and the patient was blinded to the data (it was reviewed in HCP’s office).

  Glucowatch Biographer, made by Cygnus, Inc., was the first real-time, non-invasive CGM. It was approved by the FDA in March of 2001. The device, in form of a wristwatch, measured glucose from the subcutaneous fluid through the skin with an electrode. Unfortunately, the method used to stimulate the secretion of subcutaneous fluid under the skin (“reverse iontophoresis”) caused irritation, even though it was non-invasive.

  In 2004 Medtronic introduced a new CGM system, Guardian REAL-Time, that had a hypo and hyper warning alarms. After that, in 2 years’ time, Medtronic launched the very first pump-sensor integrated system.

  A new CGM system, that made it to the market in 2006, was Dexcom STS (Short-Term Sensor), with a 3-day wear sensor. In 2012 Dexcom introduced G4 Platinum, and in 2015 G5 Mobile which was the first one to allow data to be transmitted to user’s phone.

  In 2008, Abbott launched its FreeStyle Navigator in the United States, and Medtronic introduced the iPro which was a next-gen professional continuous glucose monitor system. After launching first pump-sensor integrated system in 2006, Medtronic made improvements in their technology and made the second-generation device available in 2009.

  In 2013, Medtronic’s Enlite sensor was the first one to be part of the integrated system (with Medtronic MiniMed 530G) that had “threshold suspend” function for hypoglycemia events. 4 years later Medtronic introduced Guardian Sensor 3 that was a part of the first hybrid closed-loop system.

  FreeStyle Libre Pro (professional CGM – data was reviewed in HCP’s office) and FreeStyle Libre (CGM for patients) hit the market between 2016 and 2017 (depending on the country). They were the first CGM systems that didn’t require calibrations and had such long wear time (14 days).

  The highly anticipated Dexcom G6 was launched in 2018 and was the very first CGM that didn’t require calibrations and a separate device for receiving data.

  After less than 2 decades, CGMs changed diabetes management forever. Today, the diabetes community is even more excited for the new CGM models, among others, FreeStyle Libre 3, Dexcom G7, or Medtronic Synergy. Let’s see what else the future of diabetes will bring.

  How do Continuous Glucose Monitors work?

  • A small sensor sits underneath the skin that reads how much sugar is in the interstitial fluid;
  • A transmitter, which is attached to your sensor, sends data wirelessly to a receiver;
  • The receiver displays your blood sugar levels. This can either be a handheld device, a compatible smart device like a phone or watch, or an insulin pump; 
  • A small sensor sits underneath the skin that reads how much sugar is in the interstitial fluid;
  • A transmitter, which is attached to your sensor, sends data wirelessly to a receiver;
  • The receiver displays your blood sugar levels. This can either be a handheld device, a compatible smart device like a phone or watch, or an insulin pump; 

  CGM systems also come with software so you can analyze results, look for patterns and help to better plan treatment decisions in the future.

  Without further ado, let’s introduce the Continuous Glucose Monitors systems.

  How accurate is a CGM

  The performance of the CGM system is measured in mean absolute relative difference known as MARD. MARD is the average of the absolute error between all CGM values and matched reference values. The lower the MARD number is, the better the accuracy of the CGM system. Currently, MARD is the most common metric to assess the CGM system performance. Manufacturers usually share an average MARD of their CGM system, but some provide more detailed description on their websites. You can find some examples below:

  Dexcom G6 MARD†

  • Overall – 9.0%
  • Day 1 – 9.3%
  • Day 2 – 8.4%
  • Day 4-5 – 9.4%
  • Day 7 – 8.7%
  • Day 10 – 9.0%

   

  Medtronic Guardian MARD†

  7-13 years old:

  • Abdomen – 9.84% (3-4 calibrations/day) 10.46% (2 calibrations/day)
  • Buttocks – 8.79% (3-4 calibrations/day) 9.14% (2 calibrations/day)

  14+ years old:

  • Abdomen – 9.6% (3-4 calibrations/day) 8.7% (2 calibrations/day)
  • Arm – 10.6% (3-4 calibrations/day) 9.1% (2 calibrations/day)

   

  Without further ado, let’s introduce the Continuous Glucose Monitors systems.

  Continuous Glucose Monitors Review

  Dexcom

  Dexcom’s latest product is the Dexcom G7  (following on from the Dexcom G4, G5 & G6),  which is supposedly 60% smaller than the G6.

  The Dexcom G6 offers an easy and (mostly) pain-free insertion and a sensor that lasts up to 10 days. The company places emphasis on ‘continuous’, explaining that the data can tell a story for the wearer. It can be used by anyone above 2 years old, no matter what insulin pump or pen you use. The sensor can be worn on the belly or on arm, you can shower with it, exercise with it, swim with it (or, as some of the NJaP team like to do – take hour-long baths with it on!).

  The dedicated Dexcom app has arrows that show the speed and direction your blood sugar is going in. A double down arrow, for example, shows that your blood sugar is falling quickly.

  You can set high- and low-level ranges, so when your glucose reading is at or below a threshold, your Dexcom receiver can alert you. This means you can treat before a hyper or hypo hits.

  The Dexcom G5 and G6 systems are the only ones licensed to make treatment decisions without confirmatory finger-prick tests. 

  Pros:

  • No need to calibrate (G6)
  • Proven to lower HbA1c
  • Sensor lasts for 10 days (G6)
  • Extending the life of the sensor is possible
  • You can use reader device or an app
  • Additional platform for data analysis (Dexcom CLARITY)
  • Hypo or hyper warning alarms

  Cons:

  • Can be prohibitively expensive
  • Adhesive on the sensor is poor (this is where our Dexcom overlay patches, for G4, G5 & G6 comes in!)
  • Only lasts up to 7 days (G5)

   

  Medtronic

  The Medtronic Enlite CGM System is part of a larger integrated automatic insulin delivery system with the Medtronic pump (Paradigm, MiniMed 630G, or MiniMed 640G) and Contour glucose meter. Every 5 minutes, the blood sugar level is automatically sent from the sensor to the pump, which alerts you to rapidly changing blood glucose levels and in the case of hypoglycaemia and hyperglycaemia (when measuring blood sugar, the meter reading is also automatically sent to the pump).

  The sensor kit includes a transmitter (Guardian 2 Link), patches, serter and charger. The sensor works for 6 days. The sensor is applied using a reusable serter. The start-up of the sensor takes 2 hours. To start the sensor, 1 calibration is required immediately after start-up, then 1 calibration after 6 hours, and then calibration is required at least once every 12 hours. Medtronic recommends 3-4 calibrations per day for the most accurate readings.

  The Medtronic Guardian CGM System gives options to patients who use insulin pump therapy or who use MDI therapy or who take oral diabetes medication.

  Guardian Sensor 3 can be used as a part of a larger integrated automatic insulin delivery system with the Medtronic pump (MiniMed 630G, MiniMed 640G, MiniMed 670G, or MiniMed 780G) or as part of a stand-alone Guardian Connect System (not compatible with insulin pump) that connects with the smartphone app.

  Guardian Sensor 3 works with SmartGuard technology when patient is using a pump, and with Sugar.IQ personal diabetes assistant when patient is using a smartphone.

  The sensor kit includes a transmitter (Guardian Link 3 for pumps and Guardian Connect for stand-alone), patches, serter and charger. The sensor works for 7 days. The sensor is applied using a reusable serter. The start-up of the sensor takes 2 hours. To start the sensor, 1 calibration is required immediately after start-up, then 1 calibration after 6 hours, and then calibration is required at least once every 12 hours. Medtronic recommends 3-4 calibrations per day for the most accurate readings.

  Medtronic have developed exclusive SmartGuard technology that protects against hypos. Insulin delivery in a pump is automatically stopped when glucose is predicted to approach a pre-set low limit and then insulin resumes when levels recover. Wearers can set multiple low limits for different times of the day. These are the only CGM systems that can alert you of potential highs and lows up to 60 minutes in advance (with Enlite sensor – 30 minutes, with Guardian sensor – 60 minutes).

•  

  The Medtronic MiniMed 780G is the latest addition to the renowned Medtronic series, dedicated to enhancing the quality of life for people living with diabetes. This innovative system combines an insulin pump, continuous glucose monitor (CGM), and intelligent algorithms in a hybrid closed-loop design.

• Read more on Medtronic MiniMed 780G

  Pros:

  • Sensor is now 80% smaller in size compared to a former version
  • Small and flat transmitter
  • Environment friendly
  • Mobile app available (Guardian)
  • Hypo or hyper warning alarms

   

  Cons:

  • Only lasts up to 6 days (Enlite) and 7 days (Guardian)
  • Medtronic pump required (Enlite)
  • Lack of mobile app (Enlite)
  • Extending the life of the sensor is not possible
  • 2-4 calibrations daily are required
  • Adhesive on the sensor is poor (this is where our Medtronic patches come in!)

   

  Accu-Check Eversense XL

  Like other Continuous Glucose Monitoring systems, a smart sensor placed under the skin reads glucose in the interstitial fluid. A smart transmitter that is attached to the skin with a double-sided patch is wirelessly powering data to  the phone.

  The main difference with the Eversense XL is that a new sensor is fully inserted under the skin for up to several months at a time. As such, the Eversense XL is the first implantable CGM.

  Now we get onto some technical stuff, as explained by the company themselves:

  “Glucose reversibly binds to the sensor’s polymer coating and the amount of light emitted by the polymer coatings rises and falls with changing glucose levels. Photodetectors in the sensor measure the emitted light and wirelessly send these measurements to the transmitter, which calculates glucose concentration, rate of change and direction.

  This glucose data is sent wirelessly to the smartphone app that provides real-time tracking, intuitive displays to help identify patterns, and information to help stay in range. The smart transmitter also provides on-body vibration alerts even if your phone is not nearby or is switched off.”

   

  Pros:

  • The sensor can last up to 180 days
  • Hypo or hyper warning alarms
  • The sensor is made of bio-compatible materials
  • On-body vibrations from the transmitter

   

  Cons:

  • The implant requires a medical professional
  • 2 calibrations daily are required
  • 24-hours warm up
  • Sensor can be placed only on arm
  • Transmitter needs daily charging (10-15 minutes)

  Learn more: All about the Eversense CGM system 

  Freestyle Libre

  FreeStyle Libre is an FGM (flash glucose monitoring) system, i.e., a Flash type system. This means that to see the blood sugar level, we have to scan the sensor with a reader or phone with a dedicated app (it does not happen automatically as in the case of CGM sensors).

  How does Freestyle Libre work? The system consists of a sensor (electrode and transmitter in one that is pre-installed in an applicator), a mobile application and/or a reader. The sensor must be scanned at least once every 8 hours to retrieve data from it (the memory covers the last 8 hours). The sensor works for 14 days and is currently the sensor with the longest (factory) life on the market. The sensor is applied with a disposable applicator.

  A flash system is made up of:

  • A sensor you stick to your arm (about the size of a £2 coin)
  • A reader (a small device you use to scan the sensor to see your sugar levels. You can also use a smartphone app to scan the sensor.)

  Check our Freestyle Libre patches.

  Pros:

  • More affordable than a CGM
  • No calibrations required
  • Sensors last 14 days
  • Small and flat
  • Only 1-hour warm up

  Cons:

  • FGM technology
  • No hypo or hyper warning alarms
  • Requires scanning

   

  The Freestyle Libre 2

  The new version of the Libre system, rolled out in 2018 in Germany and across Europe, can alert users in real-time of hypos and hypers. The glucose monitoring system Libre 2 uses Bluetooth technology and blood sugar alarms. This is making Freestyle Libre more competitive with Dexcom and other ‘true’ CGM companies, but it is not more expensive than the original Freestyle Libre.

   

  Pros:

  • Despite better functionality, it remains at the cheapest price-point
  • No calibrations required
  • Only 1-hour warm up
  • Optional alerts

  Cons:

  • Not widely available across the world
  • Requires scanning

  Also, Abbott is expanding its global bio-wearables presence by launching a new product line called “Lingo,” which mirrors the style of its Freestyle Libre Sensors. Lingo will utilize established diabetes monitoring technology but adds functionality to monitor glucose, ketones, alcohol, and lactate levels through a smartphone interface.

  Learn more about Lingo: Abbott announces the future of Bio-Wearables with Lingo 

•  

  MiaoMiao 1 & 2

  MiaoMiao is a smart reader. It sits on top of the Libre sensor and turns it into a CGM (hurrah!) sending your blood sugars to your phone or watch every 5 minutes. 

  Users will need compatible apps and sensors to use the MiaoMioa 1 and MiaoMiao 2 systems. Currently most of the popular CGM applications, like xDrip+, Tomato, Glimp and Spike all work fine. However, different applications work for different sensors.

  MiaoMiao 2 is slimmer and lighter and has better connectivity than MiaoMiao 1. It’s also easier to attach with FreeStyle Libre sensors.

  Check our MiaoMiao patches.

  Pros:

  • You just need to buy one MiaoMiao – it can last for years
  • More affordable than a ‘true’ CGM system
  • It is rechargeable
  • Battery lasts up to 14 days (FSL sensor life)

  Cons:

  • Anecdotal reports of people having skin reactions to the adhesive
  • Long arrival time and extra VAT costs (shipping from China)
  • It can easily rip off (this is where our MiaoMiao patches come in!)

  So there we have it – our round-up of the current CGMs on the market!

  And whilst all these CGMs are the real star of the show, a big hand needs to go to our NJaP patches – these little pops of color that ensure your CGM can do the best job possible. We like to think of our patches as the quiet achievers. Dependable, reliable and always there to get your back. Have you seen our latest range of diabetic sensor patches?

•  

  Who qualifies for CGMs

  CGM sensors have become almost essential diabetes technology as they enable patients to track and analyse blood sugars. CGMs are relatively easy to use, but they still require technical knowledge, data analysis understanding, and regular interactions. Some patients can improve their diabetes management by using a CGM system, but this is not a solution for everyone as many factors comes into play.

  There are many aspects of wearing a CGM to be thought out and discussed:

  • Physical aspects
  • Safety aspects
  • Wearing CGM in public
  • Insertion sites
  • Skin reactions

  It is important to consider all pros and cons of a CGM system and discuss the possibilities with healthcare provider.

   

  How to wear a CGM sensor

  Each manufacturer of the continuous glucose monitor system indicates the places for the sensor application. Most of the CGMs are designed to be worn on arm and it is proven to be the most effective spot for most systems.

  Official recommendations can be seen in the table below:

  CGM system	Upper arm	Abdomen	Upper bottocks
  Dexcom G5	❌	✔️	❌
  Dexcom G6	✔️	✔️	✔️ (2-17 y.o.)
  Medtronic Enlite	✔️	✔️	✔️
  Medtronic Guardian	✔️	✔️	✔️ (7-13 y.o.)
  Accu-Chek Eversense XL	✔️	❌	❌
  FreeStyle Libre 1 & 2	✔️	❌	❌
  MiaoMiao 1 & 2 (as FSL)	✔️	❌	❌

   

• How to remove a CGM sensor

  Removing a Continuous Glucose Monitor (CGM) sensor involves a few simple steps to ensure safety and comfort. First, wash your hands thoroughly to prevent any infection. Dispose of the used sensor according to the manufacturer’s instructions. Removal procedures might vary slightly between different models and brands. We wrote down specific instructions and gave some great tips on how to keep your CGM sensor from falling off .

Sensor removal can be tricky, especially when patches are too sticky, or there is just simply too many of them. This is where the NJaP patches come to the rescue! Our CGM patches are not only useful to keep the sensor in place, but also to easily remove it after its job is done. The latest innovation from Not Just a Patch, the AIR CGM patches, stand out as the lightest, most durable, and skin-friendly option on the market, thanks to their innovative design and the use of advanced materials. This product underscores the company’s commitment to improving the lives of individuals with diabetes by offering a more comfortable, reliable, and user-friendly solution for continuous glucose monitoring. 

What about skin irritations caused by CGMs?

Continuous Glucose Monitors are composed of a collection of materials, including, but not limited to, plastic, metal, cloth, and various lubrication and adhesive components. Any, or all, of these elements can be irritating to the biological components which make up the human body, including the skin. The most common allergen found in CGM devices involves the adhesive substance used to keep the device attached to the skin. Mild-to-moderate allergies can usually be treated with anti-histamine and/or anti-inflammatory creams and topical ointments. For more severe cases, seeking a healthcare provider’s intervention is recommended. 

Learn more about tips to help avoid CGM skin irritation: CGM’s and skin reactions Guide

CGM system cost. Does insurance cover CGMs?

The cost of a CGM system widely varies depending on its features, but also on your location and type of insurance you have. To learn if there is a reimbursement program in your country and if you are eligible for full or partial CGM reimbursement, contact your health insurance company or national health fund accordingly to healthcare system in your country.

In many countries where a reimbursement program is available, eligibility usually depends on:

• Age
• Type of diabetes
• HbA1c level
• Hypoglycemia awareness

If you are not eligible for reimbursement or there is no such program in your country, but you still want to use CGM system, to reduce costs, you can:

• Use a free CGM app instead of a reading device
• Extend sensors if possible (it is not officially recommended by the manufacturers)
• Seek organizations/foundations that are supporting PWD who cannot afford CGM

Where can you get a CGM system

CGM systems are typically available to buy in local diabetes stores or pharmacies (offline and online) for a full price and for a reduced price if a reimbursement program applies. In some countries, fully reimbursed sensors are only available at hospitals/diabetes clinics and distributed to patients by diabetes nurses.

How to choose the right CGM system for you

When it comes to technology, decisions are never easy. When you add diabetes to this, it gets even harder. There is no perfect sensor, each has both advantages and disadvantages. The most important thing is that the system meets YOUR needs and expectations. The impressions of one person about a given system do not have to coincide with yours – everyone has different impressions about each CGM system, some functions are more important than others.

Answering the questions below might be helpful:

• Do you want to use a pump dedicated CGM system or a standalone one?
• Do you mind calibrating the sensors or do you prefer no fingerpricks?
• Where would you like to see your readings? (phone, reader device, insulin pump)
• Do you mind pulling out the needle yourself or do you prefer an automatic application?
• Are you ready for significant additional costs or are you on a budget?

In some countries it is possible to request a free sample from a manufacturer or test the system for a lower cost from a local representative.

We hope that the information on these systems collected here will help you better understand how CGM work, but it is YOU who will decide which system is best for YOU. We wish you the best of luck in finding the perfect diabetes technology for yourself.

More about Continuous Glucose Monitors

• How a Continuous Glucose Monitor changed my T1D Life
• How to loop in diabetes 
• The future of CGM technology looks bright 
• All about nutrition and CGMs

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About Pete Lomas, T1D 

Pete is the CEO and Founder of a diabetes technology start-up called Balance Health and the CEO of Not Just a Patch. As a person with T1D himself, he sees the opportunity in improving patient experiences via technology, product and brands. He is driven to make a difference in the lives of people with diabetes via design driven products and solutions.

About Rachel Baker, BNg, CDE, MNgPrac

Rachael Baker is a leading Nurse Practitioner Credentialed Diabetes Educator (CDE) in Brisbane and is passionate about staying up to date with Diabetes Technology and optimising these tools to be implemented in a way that is helpful and practical for people living with Diabetes. Using a holistic approach, Rachael focuses on self-empowerment for individuals living with Diabetes and supports them to live a life without limits.

About Dr. Adil Maqbool

Dr. Adil has been a speaker at various medical conferences and health awareness campaigns, where he shared his research and findings, and advocated for accurate medical knowledge. He has been featured in publications such as Cardiology & Vascular Research, The Journal of Physical Fitness and Sports Medicine, and The Lancet.