Greater Glasgow and Clyde Child & Adolescent Diabetes Service 

FAQ - Continuous Glucose Monitoring Systems

A topic of great interest to many people with diabetes is the use of Continuous Glucose Monitoring Systems (CGMS).  Here we try to answer some Frequently Asked Questions on the subject.  As always, the team are happy to discuss CGMS with you if you want to know more.


What is CGMS?

Continuous Glucose Monitors do exactly what they say they do - they measure glucose in the body and transmit that to a device (e.g. an insulin pump or a mobile phone) so that an individual can respond to that data in real-time.


If it's not measuring blood glucose, what is the point of CGMS?

The benefit of CGMS is in detecting trends so a rising or rapidly falling glucose can be acted upon.  It can also provide a lot of information between meals, overnight etc. which can be used to adjust insulin therapy.


How is CGMS used by patients?

It is possible to use CGMS on its own, i.e. in conjunction with injection regimens, but most use by patients is linked with an insulin pump.  There are two systems in use which work with different pumps but more are in development.  These are described here (in alphabetical order):


Dexcom G6

Dexcom produce their G6 sensor where data can either be sent to a handset or a compatible mobile phone.  This device can then set of an alarm when it detects the glucose reaches either high or low limits, or when it predicts the user may be about to become hypo.  If a mobile phone is used, other phones can be linked to receive data, meaning that parents can be alerted to potential problems even when their child is in another room.  In addition, the G6 can link to the Tandem t:slim pump and this can suspend insulin delivery to try to prevent hypoglycaemia.


Dexcom markets the G6 as requiring no finger-prick glucose tests, which has attracted many people to it.  There are times, however, when blood testing is necessary.


Medtronic Guardian

Typically used with a Medtronic insulin pump, the Guardian system does much the same as the Dexcom G6.  When used with older pumps, the Guardian can predict when a hypo is likely to occur and suspend the insulin delivery.  With the most recent Medtronic pump, the 670G, it can also adjust the background insulin to give more insulin when it sees the glucose is rising.


Unlike the G6, the data cannot be sent to a mobile phone at this time and it also still requires at least 2-3 calibration blood glucose tests each day.



CGMS sounds brilliant so why isn't everyone using it?

Like insulin pumps, CGMS may not benefit everyone.  However it's fair to say that one of the biggest reasons is cost.  NHS funding for CGMS is limited and we as a diabetes team have had to prioritise those patients we feel are at most clinical need.  This may seem unfair to families who work very hard to control the diabetes, but funded CGMS is simply not available for everyone at the moment.


Some families decide to pay for CGMS themselves.  If you are thinking about doing this, please discuss it with the team in advance.  It is very hard for us to put forward your case to management if you have already purchased it.


How much does CGMS cost?

This is not a straightforward question because it depends upon factors such as device and continuous versus intermittent use.  Both Dexcom and Medtronic have different price structures for NHS versus individual buyers, and deals for agreeing to use CGMS for longer periods.  However, a ballpark figure for continuous use for a year is £2000-3000.


Is the Freestyle Libre the same as CGMS?

The Freestyle Libre is similar to CGMS in that it captures glucose measurements from a sensor under the skin.  However, the readings are not sent to a screen until the user swipes their reader over the sensor worn on the upper outer arm.  This means that there are no warning alarms.  Also the device cannot be linked directly to a pump.  So while, the Libre is a welcome step forward in providing more information on which to base changes to insulin and food, it is different and not to be confused with CGMS.  Indeed, this type of technology is often called "Flash Glucose Monitoring" because the sensor is "flashed" by the scanner.


More information on the Libre can be found here.


How does CGMS work?

The system most commonly used in the UK involves a inserting a small sensor under the skin (placed exactly like a cannula for an insulin pump) and this uses a chemical reaction to measure the glucose level.  However it is very important to note that what is being measured is NOT the blood glucose but that of the interstitial fluid (the "tissues" that surround the blood vessels) because they are not the same.  The tissue glucose generally lags about 10-15 minutes behind the blood glucose but can be further affected by factors such as temperature, so a blood glucose test may give a different reading to tissue glucose measured at the same time.


Can I share CGMS data with the diabetes team?

Yes, and for us to look after patients as well as we can it is important that we have access to these data.


Click here for more information.


If I am admitted to hospital, can I keep using my CGM?

As long as you are well enough and do not need an X-Ray or other scan, you may keep your glucose sensor in place.  However you should know that results from it will not be used to make clinical decisions during your stay. Because of the possible differences between tissue glucose and blood glucose, especially during times of illness and stress, nursing staff will continue to use finger-prick blood glucose tests to decide on things like insulin doses.