What is a continuous glucose monitor (CGM)?
A continuous glucose monitor is a small device that you wear just under your skin. It measures your glucose (sugar) levels continuously throughout the day and night, letting you see trends in your levels and alerts you to highs and lows.
A CGM doesn’t actually measure your blood glucose levels, it measures the amount of glucose in the fluid that surrounds your body cells – called interstitial fluid. There is a small time delay when checking this fluid, especially after eating or if you're exercising. So your CGM result isn't always exactly the same as your finger-prick result. This means you'll still need to do a finger-prick test if you’re thinking of changing your treatment at any point, like if you need to take more insulin or if you're treating a hypo, so you can get the most accurate result.
There are two types of CGM:
- Real time: you can check your sugar levels yourself at any time, as well as being able to download them.
- Retrospective: you can’t see your sugar levels in real time but you can look back at results by downloading them.
There are a few different types of CGM available.
How does it work?
A CGM has three parts:
- A sensor that sits just underneath your skin and measures your sugar levels.
- A transmitter that’s attached to the sensor and sends your levels to your display device.
- A display device that shows you your sugar level. This might be a separate hand held device (known as “standalone” CGM) or a pump (known as an “integrated system”).
CGM comes with software so you can analyse your results and see patterns in your sugar levels over time.
You generally wear a sensor for up to seven days, and after that you need to replace it. When you change your sensor, you reattach the transmitter to your new sensor.
You need to calibrate a CGM by checking your finger-prick blood glucose levels, generally twice a day.
What’s the difference between CGM and Flash Glucose Monitoring?
CGM monitors your sugar levels continuously and sends data to your display device (a hand held monitor or pump). So you can set alerts for high, low or rate of change. With flash glucose monitoring it’s only when you scan your sensor that you get your reading and trends.
Advantages of using a CGM
- You can track your sugar levels all through the day and night.
- You can see what your levels are like at times when you don’t normally test, eg during the night.
- You can see trends: when your sugar levels are starting to rise or drop, so you can take action earlier.
- Generally, you don’t need to do so many finger prick checks.
- It can help improve you HbA1c level as you can tailor your insulin doses more carefully.
- It can help reduce hypos as you can see a downward trend before you actually go hypo.
- You can set it to alarm at high and low levels.
Disadvantages of using a CGM
- You can get overloaded with data, which can confuse or worry you.
- You still need to do some finger prick checks.
- You may find wearing the sensor irritating or unsightly.
- You need to be motivated to use the data it gives you to get the best diabetes management.
Using a CGM and driving
If you usually monitor your blood sugar levels, you must check them before driving and every two hours during a long journey.
You can use a continuous glucose monitor to check your sugar levels before you drive. These rules recently changed, meaning drivers with diabetes now have more choice in how they check their sugar levels. Read more about driving and diabetes.
How can I get a CGM?
It depends on whether you want to use CGM for a short while or long term. If you want to wear one for a week or two to help you look at your sugar trends, your clinic might well be able to loan you one for that length of time. Then you can look at the data with your doctor or nurse and decide on whether you need to make any changes to how you look after your diabetes. Your clinic might use real time or retrospective CGM. As long as your clinic has a CGM it can loan out like this you don’t need to worry about getting any funding, as the CGM is already owned by your clinic. But you might have to wait until the CGM is available.
If you want long term CGM, it will need to be funded it in some way. You can get CGM on the NHS, but it’s not available for everyone. There are strict criteria set out by NICE around who is eligible, and you need to meet this criteria in order to get funding for a CGM.
What NICE say about CGM for adults:
- They don’t recommend it for all adults with Type 1 diabetes.
- They say it should be considered for people who have:
- had more than one severe hypo a year with no obvious cause
- complete hypo unawareness
- more than 2 hypos a week, with no symptoms and which affect day to day life
- extreme fear of hypos
- an HbA1c level of 75mmols/mol despite testing at least 10 times a day.
- A person must be prepared to use it at least 70 per cent of the time.
- Real-time CGM should be provided by a centre who are expert in using it, as part of strategies improve a person’s HbA1c levels and reduce hypos.
What NICE say about CGM for children:
- It should be offered to children who:
- have frequent, severe hypos
- have hypo unawareness with serious consequences (eg fits, anxiety)
- can’t recognise, or tell somebody about hypo symptoms (eg because of developmental or neurological issues).
- It should be considered for children who:
- are under school age
- play high levels of sport (eg compete at regional or national or international level)
- have other issues that make diabetes management more difficult (eg anorexia or steroid treatment)
- have high blood sugar levels despite a lot of support and insulin adjustment.
Can I buy a CGM for myself?
Yes you can, but you'll need to speak to your diabetes team first and get their support. You can expect to pay around £1,000, for a standalone system or about £500 if you already have a pump and want an integrated system that works with that. Sensors will be about £60 each.
Given that CGM is very expensive, and you’ll get the best out of it if you look at the data it gives you with your doctor or nurse, so it’s always best to speak to them about how CGM would help you and whether you’re eligible for it under the NICE criteria before you go out and buy one for yourself.