Continuous glucose monitoring (CGM)

Anyone living with diabetes mellitus (herein referred to as diabetes) will know that it can be challenging to control.

There are various approaches to help manage your condition, including different ways to monitor your blood sugar levels (also called blood glucose levels) and other methods for treating high and low blood sugar (hyperglycemia and hypoglycemia). 

Continuous glucose monitoring (CGM) became available as a treatment before the turn of the millennium in 1999. This first device that could read blood glucose levels continuously was a professional CGM and was called the Continuous Glucose Monitor System (CGMS).

The CGMS was manufactured by Medtronic MiniMed. Since then, newer models have shown improvements in accuracy and patient acceptance, and the treatment has become available to more and more people living with diabetes. 

This article will explore more about CGM technology, how it is used, and how it can benefit the user. We will also cover some of the drawbacks and explain how you may get a CGM. Read on to learn more about this approach to monitoring your blood sugar levels and learning how it may work for you. 

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What is continuous glucose monitoring?

Continuous glucose monitoring (CGM) is a way to collect information about your blood sugar levels that you can then review and share with your diabetes specialist medical team as needed. CGM can automatically track your blood glucose levels, day or night, making it beneficial to achieve tight glycemic control.

With CGM, you can review your results yourself and see how they change over time (hours or days), and you can also view glucose trends. This can be empowering as it can assist with decision making around balancing the food you eat, the exercise you do, and the medication you take. 

If your healthcare team can access your readings instantly, they can recommend adjustments to your treatment as needed. In the current age, we will all be aware that we are doing more of our appointments online. This includes healthcare appointments, and the use of CGM can help with the ease of sharing blood sugar levels virtually (1,2). 

How does it work?

A CGM is connected via a sensor that is inserted under the skin, usually on your belly or arm. As mentioned, a CGM continuously measures your blood glucose levels. Yet, it is more accurate to say that a CGM doesn’t quite literally measure your blood glucose levels; but instead, it measures the amount of glucose in the fluid surrounding your body’s cells. This fluid is called interstitial fluid.

The sensor connected to the CGM tests glucose levels every few minutes, and a transmitter sends the information to a display monitor. This monitor may form part of a device such as an insulin pump (integrated system), or a separate handheld device, also called a standalone CGM. Both of these types of display monitors can be carried upon your person along with your CGM. In addition, some CGMs send information directly to a smartphone or tablet device.

Learning about CGM’s can be complicated, although it can help to simplify things by remembering that a CGM has three parts:

  • A sensor 
  • Transmitter
  • A display monitor that shows your blood sugar level 

Most people wear their CGM’s for up to seven days, after which time it will need to be replaced. To ensure that your CGM is working accurately, you will need to calibrate it by checking your blood glucose level with a finger-prick test, and this is usually required to be done twice per day (1,2). 

What’s the difference between CGM and Flash Glucose Monitoring?

Additionally, to a CGM, you can also get a flash glucose monitor (FGM). An FGM is a small sensor that is worn just under your skin. An FGM can continuously record your blood sugar levels throughout the day and the night, and it does this by measuring the amount of glucose in your interstitial fluid.

If we recall back to what a CGM is, there are certainly some similarities between the two! Yet, they are actually quite different. When you are using an FGM, you have to scan the sensor to know your blood sugar levels, whereas, with a CGM, the readings are transmitted to a display via your sensor, which happens automatically throughout the day and night.

It is also important to note that your FGM reading will not always mirror that of a finger-prick test, as there will be a slight time delay after checking your reading, for example, after eating or during exercise (1,3).

Therefore it is important to keep this in mind if you use your FGM during these times, and if you need to change your treatment for any reason (such as a hypo), always do a finger-prick test.

In 2014, medical devices company Abbott released the Freestyle Libre to the market, and it was undoubtedly a new, revolutionary way to monitor glucose for those people living with diabetes. The Freestyle Libre is an FGM, and to date, remains the only model on the market (3,4). 

This is a testament to what complex technology an FGM is, although it also makes it difficult to make this treatment available to lots of people with diabetes. In the UK, FGM’s became available on the NHS in 2019, although there are certain criteria you need to meet to be eligible.

See below for some examples of the criteria that people living in England must meet if they were to get an FGM, free of charge, on the NHS (note you will need to meet one or more of the below criteria, as an example).

  • Being pregnant and having type 1 diabetes

  • Having cystic fibrosis-related diabetes and taking insulin injections

  • Having diabetes and also needing hemodialysis due to chronic kidney disease

  • Having type 1 or type 2 diabetes, using insulin, and having a learning disability Having type 1 diabetes and:

I. Needing to check your sugar levels 8 times per day or more

II. Having severe episodes of hypoglycemia and/or have reduced awareness of your hypos 

III. Being unable to carry out regular tests due to learning disability (3). 

Note that the above is not an exhaustive list, and different criteria depend on where you live in the UK or elsewhere. If you are eligible, you will also need to commit to learning how to use the device, engaging closely with your diabetes specialist team members, and ensuring regular blood sugar level testing. 

Diabetes UK has a comprehensive list of the criteria required to be met depending on where you are in the UK. 

It is worth noting that you can buy a flash system yourself. According to the National Health Service (NHS), it costs roughly:

  • £150 for a starter pack (reader and 2 sensors)
  • £50 for a sensor
  • £50 for a reader (5)

As always, when you’re thinking of making changes to your diabetes treatment, speak to your diabetes team if you’re thinking of buying an FGM (Freestyle Libre).

Advantages and disadvantages of a CGM

Like any system used for monitoring blood glucose levels, CGM has its advantages and its disadvantages. These include the ease of use and a rich variety of useable data you can gain from a CGM, and how this can be of benefit, to how potentially this can also be a disadvantage, depending on the person. S

ee below for a summary table of the advantages and disadvantages of using a CGM ((1) adapted from Diabetes UK).

In a 2011 study looking at the meta-analysis of results from randomized control trials using individual patient data, the glycaemic control in type 1 diabetes during real-time CGM, compared with self-monitoring of blood glucose, was reviewed. Among the findings were that CGM was associated with a significant reduction in HbA1c percentage. This reduction was most significant among those with the highest HbA1c at baseline and among those participants who most frequently used their sensors.

The study also found that the use of CGM leads to reduced episodes of hypoglycemia. The study concluded that the most cost-effective or best-placed use of CGM is likely to aim at people with type 1 diabetes, particularly those who have ongoing issues with poor control throughout their standard insulin therapy. CGM was also found to be most effective among those who were willing and able to use it frequently (6). 

An example of a CGM model that is approved for use in making treatment decisions is the Dexcom G5 Mobile. This means that the CGM results can then go on to inform and action changes to your insulin. Other models that are not eligible for making treatment decisions will need to have the CGM readings confirmed with a finger-prick blood sugar reading before taking insulin or managing a hypo (2). 

How can I get a CGM?

Depending on your intentions for a CGM, you have a few options. You can:

  • Borrow one from your diabetes specialist team if you only want to use it for a short while, e.g., to see trends in blood sugar levels and perhaps inform a decision about permanent use of a CGM

  • Get one for free (certain criteria must be met)

  • Buy one

The criteria for a free CGM differs between children and adults. Below are a few points that are taken into consideration when making decisions about who is eligible for the treatment. Speak with your dedicated diabetes team for more information.


  • Can be considered for CGM when they have hypo unawareness and/or have had a severe hypo more than once per year with no clear cause

  • Are eligible if they have a fear of hypos 

  • Must be prepared to use a CGM at least 70% of the time for maximum efficacy

  • Can be considered for CGM if they have regular hypos that affect their life significantly but have no symptoms 

It is important to note that CGM is not considered appropriate for all adults. Any use of CGM should be supported by an expert center, with the aims of improving the user’s HbA1c, quality of life, and reducing episodes of hypos. 


CGM should be offered to children who:

  • Have hypo unawareness and/or frequent, severe hypos 

  • Struggle to communicate about hypos due to disability or learning difficulties 

Additionally, consideration should be given to children who:

  • Play a lot of sport

  • Experience hyperglycemia despite good support 

  • Are under school age or have other issues such as eating disorders that make treatment of diabetes more challenging 

As mentioned, you can also buy a CGM if you do not qualify for a free one. You would still need to speak with your specialist team for their support as training will be required for effective use.

In the UK, a CGM will cost around £1,000 for a standalone system or about £500 if you are already receiving insulin pump therapy. Additionally, you will need a sensor that is around £60, and these do require replacing around every seven days (1). 


Some people are living with diabetes will be eligible for a CGM free of charge alongside their other medications, whereas some people may not meet these criteria. The latter group of people can purchase one, and this may be advisable if they have been advised or strongly feel as though it would help them improve their glycemic control.

There is evidence of the benefit of using a CGM, although there are also reported drawbacks as with all treatments. Whatever you choose, ensure that you discuss the options in detail with your specialist diabetes team.

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  1. Diabetes UK (2020). Continuous Glucose Monitoring. (Online). Available: accessed on 08/12/2020
  2. National Institute of Diabetes and Digestive and Kidney Diseases (NIH) (2017). Continuous Glucose Monitoring. (Online). Available: accessed on 10/12/2020
  3. Diabetes UK (2020). Flash Glucose Monitoring. (Online). Available: accessed on 10/12/2020
  4. Heinemann L, Freckmann G. CGM versus FGM; or, continuous glucose monitoring is not flash glucose monitoring. J Diabetes Sci Technol 2015. b;9:947–50. 10.1177/1932296815603528
  5. National Health Service (NHS) (2018). Flash Monitoring (Freestyle Libre). (Online). available: accessed on 09/12/2020
  6. John C Pickup, Suzanne C Freeman, Alex J Sutton. Glycaemic control in type 1 diabetes during real time continuous glucose monitoring compared with self monitoring of blood glucose: meta-analysis of randomised controlled trials using individual patient data. BMJ. 2011; 343: d3805. Published online 2011 Jul 7. doi: 10.1136/bmj.d3805

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