How does a flash glucose monitor and CGM work?
Flash glucose monitors and continuous glucose monitors let you check your sugar levels without you having to prick your fingers.
You wear a small sensor on your body day and night that reads your sugar levels so you can see the information on your mobile, or other device. If someone helps you look after your diabetes, their mobile can be linked up too.
With a CGM and a flash glucose monitor (FreeStyle Libre2), you can also set an alarm to sound if your sugar levels go too low or too high.
This can also be set up to sound on someone else’s mobile, for example, a parent or carer’s.
One of the other main benefits of a flash glucose monitor and CGM is being able to review what your sugar levels do every minute of the day and night. With the charts and graphs, you can start to see and understand how food, activity, and other things affect your sugar levels.
"It's a real mental health break knowing what your blood sugar levels are doing at any one time." Matt
Find out the difference between a flash glucose monitor and a CGM. You can now choose to use your FreeStyle Libre2 as a CGM - so your readings show up automatically on your smartphone, without having to scan your sensor with your phone. See how to do this on our news story.
Find out about using a smartwatch to track your blood sugar levels.
Time in range
As you can share your data online with your healthcare team, they can adjust your treatment. They can also suggest things to help increase time in range – which is shown every day on your device.
Devices that work with a flash glucose monitor (Abbott website)
Devices that work wth a CGM (Dexcom website)
Watch Matt's video about his experience of using a Freestyle Libre 2
A description of what happens on screen in the video can be downloaded (Word, 12KB)
Videos on how to use a Freestyle Libre
Help and tutorials from the Freestyle Libre manufacturer Abbott.
With a CGM your latest sugar levels show up on device or mobile automatically, transmitted by Bluetooth. With a flash glucose monitor, it’s only when you wave (scan) your device over your sensor that you get your sugar readings.
But unlike Flash, some CGMs can ‘talk’ to an insulin pump, which is important if you want to use a closed loop system.
Your diabetes healthcare team should be able to support you to make the decision on which device will be best for you.
How do I get a flash glucose monitor or CGM?
If you’re in England, Wales or Scotland, check the latest guidelines on who should be offered a flash glucose monitor or CGM on the NHS. If you’re in Northern Ireland, you’ll need to check with your healthcare team.
Generally people with type 1 diabetes should be offered a CGM or flash glucose monitor and some people with type 2 diabetes may also be offered a CGM or flash glucose monitor to help manage diabetes.
Getting free Flash sensors and CGM sensors
If you get a Freestyle Libre or a CGM on the NHS, you should also get the other things you need like sensors and replacement parts.
It’s also worth knowing that sensors only work for individual products, so sensors for a Freestyle Libre won’t work for a Freestyle Libre 2 or another CGM.
“Using a Freestyle Libre helps me prevent hypos and it’s much more sociable because when I'm out, I don’t have to find somewhere convenient and clean to do a finger prick check. I’m lucky I can afford to self-fund.”
Margaret, 73, who has type 2 diabetes and uses insulin.
If you don’t qualify for a flash glucose monitor or CGM, your main option is to self-fund a device. It’s expensive so if it’s something you’re thinking of doing, it’s worth speaking to your healthcare team first to see if they think it’s something that would suit you.
There is only one flash glucose monitor that is available to buy in the UK at the moment, the Freestyle Libre 2. This model has alarms for predicted high or low blood sugar levels. But when the alarm does sound you still have you scan your arm to check your blood sugar level.
Both a CGM and a flash glucose monitor are offered on the NHS to people who qualify and are also available to buy.
The cost of Freestyle Libre 2 sensors is around £50 every 2 weeks. If you have a compatible smartphone you can just download the app and use your phone to scan the sensor to see the readings. But if you don’t, you can buy a small device called a reader for around £50 and this is a one-off purchase. Speak to your healthcare professional about where you can buy a reader from – they can be available direct from the manufacturer (Abbott), from a pharmacy or online but this can change.
Cost of buying a CGM
As well as the sensors which cost between £40 to £70 each and usually last for 10 to 14 days. Sometimes you’ll also need to buy a transmitter and a reader, sometimes known as a receiver, which is a one-off cost. They are often sold in ‘starter pack’ bundles along with some sensors which cost around £200. You can usually buy these directly from the manufacturer's website.
The Freestyle Libre 3 is a CGM which is available on the NHS but not yet available to buy.
These are some of the other CGMs available to buy or available on the NHS.
Dexcom G7 licensed for those aged two and over
Dexcom One licensed for those aged two and over
Dexcom G6 licensed for those aged two and over
Dexcom G5 licensed for those aged two and over
Glucomenday licensed for those aged six and over
Guardiam Connect (Medtronic) no age restriction
GlucoRx Aidex (licensed for those aged 14 and over)
Some CGMs work with insulin pumps - see information on these integrated systems.
How finger prick checks differ to CGM/Flash readings
A finger prick check tells you what your blood sugar level is at that moment.
With a flash glucose monitor or CGM, what’s being measured is the amount of sugar in the fluid surrounding your cells. This is called interstitial fluid. It’s not quite as accurate as a finger prick test as it lags behind blood sugar levels by up to 15 minutes. And the difference between the blood sugar and CGM or flash reading is more likely to be greater when you’re eating or exercising.
So even if you’re offered a flash glucose monitor or CGM by your healthcare team, it’s important that you still get your diabetes kit on prescription including your blood glucose meter to let you do finger prick checks.
"I found that pricking my finger up to ten times a day was challenging and sometimes painful so I got the freestyle libre 2 sensor which has helped a huge tonne. Alfie, 15
Why you still need some finger prick checks
You may need to do a finger prick check, for example, if what you’re feeling doesn’t match what your blood sugar reading says. Or if you’re treating a hypo, you may want to do one to get the most accurate result.
With some CGMs, you may also need the result of a finger prick check every day to reset your device.
With a flash glucose monitor, sensors (small white discs) should be worn on the arms only. And we recommend that they aren’t placed over areas with tattoos as this could impact your results. With a CGM, you can wear the sensor on different parts of the body, such as your abdomen.
The sensors don’t normally need to be taken off. You can usually wear them in the bath, shower and during sports. But some people do have problems with them falling off. There are adhesives you can buy to keep them in place.
You cannot remove a sensor for a while – once it has come off you need to replace it with a new one.
How often you have to change the sensor will depend on the type of model you’re using and the manufacturer’s instructions. You’ll usually need to change it at least once every 14 days.
It’s quick and painless to put on a sensor. You insert them just under the skin using an applicator.
Diabetes technology is a wonderful thing, but it certainly isn’t for everyone.
Some people feel uneasy about getting lots of data, and for some people it gives them more confidence. We’ve got more information to help you work through these different feelings about using diabetes tech. And you can always call our helpline to ask questions or just to talk it through.
Advantages of Flash and CGM
Disadvantages of Flash and CGM
You don’t need to do so many finger-prick checks and you can set alarms if your blood sugar goes too low or too high (unless you’re using a Freestyle Libre 1).
It’s not always as accurate as doing a finger prick check and it can take a while to get used to using the technology
You can see trends, like when your sugar levels are starting to rise or drop, so you can take action earlier.
You can get overloaded with data, which can confuse or worry some people.
You can get your sugar levels in your target range more often as you have more information about when you’re high and low.
You may find wearing the sensor irritating, you might not like the look of it, or showing people that you have diabetes.
You can see what your levels are like at times when you don’t normally test, like during the night (either by looking back at your own data or scanning someone else’s sensor while they’re asleep).
Your glucose levels can be shared with your diabetes team, so they can review and adjust your diabetes management. It also means that the information can be shared easily during virtual appointments.
Learning to use your CGM or Freestyle Libre
Your diabetes healthcare team will set you up with your CGM or Freestyle Libre and show you how to use it if you’re getting it for free on the NHS. Sometimes manufacturers will help you set up the technology – this is fine and can help your healthcare team get more people on to the technology more quickly.
Abbott, which supply the Freestyle Libre, also provide free online learning for people with diabetes using the technology to help them get the most from it.
You may also benefit from going on a diabetes education course if you use Flash (Freestyle Libre) or CGM. Ask your GP or other diabetes healthcare professional to refer you.
If you want to share your experiences about using tech or find out information by asking others using it, go to our forum.
To scan your Freestyle Libre 1 or Freestyle Libre 2 sensors from a smart mobile phone, download the Freestyle LibreLink app from the app store on your phone.
Sharing data with friends and family
You can also share the data from your Freestyle Libre with friends, family or carers via the Libre Linkup app. They will need to download the LibreLinkUp app and create an account. You can then give them access to your data using your own LibreLink app and their email address.
If it’s your first time opening the app, then you may need to set up an account. Once you’ve done that, hold your phone or device up to the sensor and you should feel it buzz twice. The sensor has been scanned and the app requires 60 minutes to calibrate – so you’ll need to wait that long and then you can use it.
If your sensors, reader or CGM transmitter are faulty, you can send them back to the manufacturer and request replacements. It may help to send example readings, compared with finger prick checks.
You can also report any concerns to the MHRA yellow card scheme.
Getting insurance for your CGM or Freestyle Libre
Like all types of tech, looking after it and things like making sure the batteries are charged is important. Because electronic devices are expensive, you may wonder whether to get insurance to cover it in case you lose or break it. Before taking out separate insurance, it’s worth seeing if you can have it as a named item on your household insurance. This might be a cheaper and simpler option.
Travelling abroad with a CGM
You'll need to let your airline know if you're travelling ny plane with a CGM and will need to switch to checking your blood sugar levels manually. Read our guidance.
Driving and checking your blood sugars using a CGM or Flash
Some drivers can use a flash glucose monitor or CGM to check their sugar levels when driving, but you must confirm your levels with a finger-prick test if:
- your blood sugar level is 4 mmol/l or below
- you have symptoms of a hypo
- your monitor gives a reading that’s not consistent with the symptoms you’re getting – for example, if you feel like you’re having a hypo but the reading doesn’t show this.
You can swim with all CGMs and flash sensors available in the UK (at the time of writing March 2023) to a depth of 3 feet and for 30 minutes, but some sensors may allow you to swim deeper or further than this. For more information on swimming with your sensor, check the instructions in your user guide.
It is generally recommended that sensors are secured with extra tape during water activities.
We know that lots of people have been reporting issues they've been having with the Freestyle Libre 2.
We previously collated some of these issues and fed this back to Abbott who have provided us with some 'Top Tips' for issues with Libre 2 alarms.
If you have any concerns or specific issues with your Freestyle Libre 2, please contact Abbott directly via their Libre2 support pages. Or use the Abbott guides we're sharing below.
Can I use a smartwatch to check my blood sugar levels?
You may be able to link a smartwatch to a CGM or flash glucose monitor (read more further down). But we would not recommend any of the smartwatches that are intended to be used without a sensor attached to the body. They are sometimes called non-invasive blood glucose smartwatches.
Although there has been some promising research from companies developing prototypes that may become available in the future, we’ve read about cheaper smartwatches that claim to measure blood glucose being sold online. These do not have to go through the same testing in trials as medical devices so we don’t know that they are accurate and they aren’t medically approved, so we would not recommend them.Join the discussion on our forum.
If you use some devices, like the Freestyle Libre 2, Dexcom One, G6 and G7, you can use a standalone reader instead of a phone to check the readings. However, these aren’t usually included as standard and you’d need to contact the manufacturer to get one or speak to your healthcare professional.
Another option could be to use a smartwatch to check the CGM readings if you have compatible devices and software. For some CGM devices you can use the official app to connect the sensor to a phone and smartwatch, but for others you’d have to use a third-party app which can be more complicated to set-up. However, you’d still also need to have your phone within range of the watch and the sensor to connect them and see your readings or get alerts – and to scan if you’re using Flash.
Join the chat on smartwatches and devices on the forum
Still have more questions or things you're not sure about on CGMs or flash glucose monitors after reading this page? Call our helpline on 0345 123 2399.