Continuous Glucose Monitoring (CGM)

Web Resource Last Updated: 28-01-2020



Continuous glucose monitoring (CGM) is a way you can automatically measure your glucose levels every few minutes throughout the day and night. It involves using a system that comprises a sensor, a transmitter and a handheld monitor on which you can read the results.

The CGM allows you to monitor your glucose levels and assess how they change while you are sleeping, after you eat, during exercise or when you are ill.

How does the CGM system work?

  • The sensor is actually a small electrode that is inserted under the skin. Your healthcare professional will use a special insertion device to do this. The sensation is similar to having an insulin injection.
  • The sensor measures your interstitial glucose levels.
  • The sensor is connected to a transmitter which sends the data wirelessly to your handheld monitor.

These are some of the features of the CGM:

  • A visual display of your current interstitial glucose level
  • A trend arrow indicating whether your glucose level is falling or rising
  • A trend graph showing results over the last three, six or 24 hours
  • An alarm setting to alert you when your glucose level is high or low
  • An option to download data so that you can review it with your diabetes care team

What is interstitial glucose?

  • Interstitial fluid is a thin layer of fluid around your tissue cells. Glucose levels in the interstitial fluid lag behind glucose levels in your blood by up to 15 minutes.
  • The lag time is longest if your blood glucose level is changing rapidly, after eating or if you are exercising.
  • Due to the lag time, you should also do a finger prick blood glucose check if you are considering additional insulin or treatment for hypoglycaemia. 

What is the difference between CGM and flash glucose monitoring?

CGM monitors your glucose level continuously and sends data to your display device on an ongoing basis. You can set it to alert you when your reading is high or low and you can easily view trends and patterns. With flash glucose monitoring, it is only when you scan your sensor that you can take a reading and see trends.  

Advantages of using a CGM

  • You can track your glucose levels throughout the day and night.
  • You can see what your levels are like at times when you don’t normally test, e.g. during the night.
  • You can see trends: when your glucose levels are starting to rise or fall, so you can take action earlier and avoid a hypoglycaemic episode (a ‘hypo’).
  • Fewer finger prick checks are necessary.
  • It helps you achieve and maintain your target HbA1c level.
  • You can set it to alert you when your glucose level is high or low.

Disadvantages of using a CGM 

  • It is easy to become overwhelmed with the amount of data.
  • You may find wearing the sensor uncomfortable or feel that it is unattractive.
  • You need to be motivated to benefit from the data provided.

How do I get a CGM?

It depends on whether you want to use the CGM for a short period (7–14 days) or long term. If you want to wear one for a week or two to help you look at your glucose trends, your clinic might be able to lend you one for that length of time. Then you can look at the data with your diabetes care team and decide whether you need to make any changes to the way you manage your diabetes.

If you want to use a CGM long term, it will need to be funded in some way. A CGM is sometimes available on the NHS, but it is not possible for everyone to receive one. There are strict criteria set by NICE (the National Institute for Health and Care Excellence) around who is eligible, and you need to meet these criteria in order to qualify for funding.

Can I buy a CGM for myself?

Yes, but prices can vary, depending on which CGM you choose, and they tend to be very expensive. If you do go ahead and purchase your own, remember that you are likely to benefit most from the system if you get support in using it, so ask what support the company and your diabetes care team can give you.

Useful resources

There are 4 main manufacturers of CGM systems used in the UK:

  • Medtronic 
  • Animas 
  • Abbott 
  • Dexcom  

To learn more about blood glucose and interstitial glucose, watch the video below:

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