Continuous glucose monitoring: what it is and how it works

Keeping track of your blood glucose levels is a key part of living with type 2 diabetes. Being able to do so automatically, without finger-prick tests, is a game-changer made possible by continuous glucose monitoring, often referred to as CGM. Whether you’re newly diagnosed or a seasoned pro, you might benefit from knowing a bit more about how this emerging health technology can help you manage your condition.
Allie Anderson
min read
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Quick summary

  • Continuous glucose monitoring enables you to quickly check your blood glucose and note patterns over time.
  • A tiny sensor is painlessly inserted just under the skin and measures your levels continuously, while a small transmitter that sits on the skin’s surface sends the data wirelessly to a digital device. 
  • The discreet device can stay in place for up to 2 weeks, and most people find them inconspicuous and don’t notice them.
  • You can use a continuous glucose monitor to look at your blood sugar levels over a period of time, making diabetes management more straightforward.
  • CGM isn’t as accurate as finger-prick testing, but it can help people to understand how different factors affect their levels and make appropriate changes. 
  • Widely used in type 1 diabetes, CGM is less common for people with type 2, but the evidence is mounting about the benefits.

If you have diabetes, the chances are you’ll know all about blood glucose and the importance of keeping it under control. A crucial part of diabetes management is understanding your own blood glucose levels, and to do that, you need to be able to monitor them. 

Not everyone with type 2 diabetes needs to measure their blood glucose levels regularly, but those who do typically do so at certain times of the day, every day, using finger-prick testing.[1]

This familiar method uses a small, handheld device that pierces the skin on your fingertip and collects a droplet of blood, which is then tested by an attached meter. The reading displays within a few minutes and gives a snapshot of your blood glucose levels at that moment in time. 

Finger prick testing is reliable and accurate, but if you’re not a fan of needles the prospect of doing it several times a day might fill you with dread! And—even though it takes just a few minutes each time—it can be a nuisance. 

CGM devices could be more beneficial for people who are on insulin treatment for type 2 than finger prick testing

Martens et al, JAMA

A more convenient solution could be continuous glucose monitoring (CGM), which has been a mainstay in type 1 diabetes management for a while. Those with type 2 diabetes are increasingly using CGM too, so it’s become somewhat of a buzzword. 

But you might be wondering what is it, how does it work, and would you benefit from it? 

What is continuous blood glucose monitoring?

Continuous blood glucose monitoring does exactly what it says on the tin! Using a sensor and transmitter worn on the skin, and a smartphone or monitoring device, CGM allows you to check your blood sugar levels at any time at the touch of a button. 

The readings are transmitted electronically via a wireless connection to your device, where you can collect and record information over a period of time. The advantage is that you can see what your blood glucose levels are doing long-term, identify patterns and trends, and ultimately better manage your condition

How does a continuous glucose monitor work?

The sensor is a tiny, thin, flexible wire—only a few millimetres in length—which is placed just beneath the skin using an applicator. It’s very quick and painless to apply one, and you can do it really easily yourself at home. 

It’s coated in enzymes that can ‘sense’ and measure the amount of sugar in the fluid surrounding the cells (called interstitial fluid) under the skin. Attached to the sensor is a small, white disc—the transmitter—that sits discreetly on the skin’s surface and is kept in place with an adhesive patch. Once it’s been applied, it doesn’t need to be changed for 2 weeks.

CGM sensors are typically worn on the flat outside part of the upper arm, but some types can be placed on the abdomen. You won’t feel the sensor under your skin, and you can bathe and shower as normal, do sports, sleep, and go about all your usual day-to-day activities without even noticing you’re wearing a CGM. If you have problems with the sensor falling off, you can secure it in place with special CGM adhesives. 

Because the sensor is worn all the time, it keeps constant track of your blood glucose and takes readings at intermittent intervals, usually every few minutes. The electronic transmitter sends that data wirelessly to whichever device it’s linked up with. Some people use a special handheld CGM monitoring device, sometimes called a receiver, while others use a smartphone and an app.

There is another type of CGM called a flash monitor. It’s essentially the same, but instead of the transmitter automatically sending readings wirelessly, you have to scan the sensor to obtain the data. 

Whether you have a CGM or a flash monitor, you can download all your data that’s been collected, store it on a computer or smartphone app, and share it with your specialist diabetes clinician or family members.

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Why use a continuous glucose monitor?

As mentioned, the great thing about CGM is that it enables you to monitor your blood sugar over time, which has a number of advantages. 

As well as requiring no needles, nor the process of actively pricking your finger to get a blood sugar reading several times a day, CGM means you can see trends and patterns in your glucose levels. This can help you identify any factors that affect your glucose levels—like when you eat, what you eat and how much, exercise, or periods of illness, for example—and gives you a bigger picture of your condition.

CGM can improve glycemic control by expanding the percentage of time your blood sugar level is within your ‘target’ range

Maiorino et al, Diabetes Care

Some CGM devices alert you if your blood sugar looks like spiking or dropping too low, meaning you can prevent it from happening and sidestep the associated effects. 

Having your own health data and metrics available to you at the touch of a button can be empowering for people with a long-term condition. In fact, patient empowerment underpins the NHS Long-Term Plan, which states that access to and management of digital tools could transform people’s experiences of health care.[2] 

When people with diabetes feel empowered and involved in their care, it can improve blood glucose control.[3] What’s more, studies have shown that using CGM can derive better health outcomes, for example by helping patients spend more time within their blood glucose target range.[4]

Are there any disadvantages to using continuous glucose monitoring?

Because the CGM sensor measures sugar in the fluid surrounding the cells, rather than in the blood itself, continuous blood glucose monitoring is less accurate than finger prick testing. So, the readings aren’t in real time, but represents your blood sugar levels around 15 minutes earlier. 

The difference between your actual blood sugar level and your blood glucose monitor reading can be marked around mealtimes and when exercising. So, it’s worth checking your levels with the occasional finger prick test too. 

Plus, CGM devices can be quite complex, requiring some time to learn how to work and use them, and calibrate them with your smartphone app or receiver device. 

Although most people don’t notice the CGM sensor and transmitter, some people find them irritating.[5] Others simply don’t like the idea that it’s there, because if it’s worn on the arm and you wear short sleeves people will see it—although it’s small and discreet. And if you’re not especially tech-savvy, you could find yourself swamped with data and find it overwhelming and confusing.

What does the evidence say?

The main problem with CGM is not with the technology itself or how it’s used, but how straightforward it is to access. Most people with type 1 diabetes can get CGM monitors on the NHS,[6] but they aren’t commonly used among the type 2 population.

Research into whether more widespread adoption of CGM by people with type 2 is emerging, though. For instance, a recent study found that CGM devices could be more beneficial for people who are on insulin treatment for type 2 than finger prick testing.[7]

But there is a lack of data from scientific studies looking at whether continuous glucose monitoring improves outcomes for people with types 2 diabetes in general, especially those who don’t take insulin. Some people suggest that, because most people with a type 2 diagnosis don’t need to self-monitor their sugar levels, unnecessarily doing so can actually have a negative impact on your quality of life.

The reported benefits are being recognised by health policymakers, meaning more people will be able to choose CGM if it’s right for them. National Institute of Health and Care Excellence guidelines published earlier this year recommend widening NHS access to continuous glucose monitoring, including for some people with type 2 diabetes who take insulin.[8] 

And in the future, there may be a growing interest in the use of CGM for lots more people with type 2 diabetes—those whose condition is controlled with oral medication and not insulin—in effecting behaviour change

Being able to see your blood glucose levels in motion over time can help you understand how your diabetes is impacted by your lifestyle, and giving you the tools to make healthy, sustainable choices. In the long term, that can only be a positive thing.


  1. Checking your blood sugar levels. Diabetes UK. Retrieved 4 May 2022. Accessible here.
  2. Empowering people. NHS Long-Term Plan, Chapter 5: Digitally-enabled care will go mainstream across the NHS. National Health Service. Retrieved 4 May 2022. Accessible here.
  3. Anderson, R., Funnell, M.M., Butler, P.M., et al. (1995). Patient empowerment. Results of a randomized controlled trial. Diabetes Care 18(7):943-9. Accessible here.
  4. Maiorino, M., Signoriello, S., Maio, A., et al. (2020). Effects of Continuous Glucose Monitoring on Metrics of Glycemic Control in Diabetes: A Systematic Review With Meta-analysis of Randomized Controlled Trials. Diabetes Care 43(5):1146-1156. Accessible here.
  5. Flash glucose monitors (Freestyle Libre) and continuous glucose monitors (CGM). Diabetes UK. Retrieved 4 May 2022. Accessible here.
  6. Continuous glucose monitoring (CGM) and flash. NHS. Retrieved 4 May 2022. Accessible here.
  7. Martens, T., Beck, R.W., Bailey, R., et al. (2021). Effect of Continuous Glucose Monitoring on Glycemic Control in Patients With Type 2 Diabetes Treated With Basal Insulin. A Randomized Clinical Trial. JAMA 325(22):2262–2272. Accessible here
  8. Type 2 diabetes in adults: management (NG28). National Institute of Health and Care Excellence. Retrieved 4 May 2022. Accessible here.

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