What is CGM?
Continuous glucose monitoring (CGM) devices are small wearable systems that measure and display glucose levels throughout the day and night. They can be programmed to sound alarms and send warnings if glucose levels move outside an individually set target range. CGM devices also display arrows to show whether the glucose level is currently rising, falling or remaining steady. The graphs on the CGM receiver can be reviewed to look for patterns and trends in glucose levels.
CGM devices have three main parts:
- The sensor sits on the skin with a small electrode inserted just under the skin on the tummy. It measures the level of glucose in the fluid between your cells. A new sensor needs to be inserted every 6 to 7 days, depending on the device.
- The transmitter is attached to the sensor and sends glucose readings to the wireless receiver, insulin pump or compatible smartphone. Transmitters are reusable but need to be replaced every 3-12 months, depending on the device.
- The receiver allows you to view your glucose data. The receiver may be a standalone device, insulin pump or compatible smartphone via an app. The receiver also stores glucose data, which can be uploaded for you and your diabetes health care team to review, to help in making decisions about changes to insulin doses or pump settings.
Unlike a finger prick blood glucose reading, CGM is continually measuring your glucose levels. These readings are displayed on the receiver, along with arrows which tell you the direction your glucose levels are heading—i.e. whether they are rising, falling or remaining steady.
CGM has a number of benefits
- 24/7 readings. CGM allows you to see glucose levels across the day, rather than just at a single point in time. The CGM graphs may help in learning how different things, such as food and exercise, affect glucose levels. Reviewing these graphs for patterns may help you to balance food, exercise and insulin doses.
- Trend arrows. As well as showing glucose levels at any point in time, CGM shows whether glucose levels are rising, falling or remaining steady, and how fast they’re changing.
- Alarms. You can set the CGM device to sound an alarm on your receiver or smartphone, if glucose levels rise above or drops below individually set target levels. The alarm allows you to act before glucose levels rise too far above or drop too far below your target range. The alarms may be especially useful for people who can’t always tell when they’re having a hypo (hypoglycaemia or low blood glucose level).
- Overnight monitoring. CGM devices measure glucose levels throughout the night without the need to wake up and do finger prick checks. Calibrations may sometimes be needed overnight but can usually be timed to avoid this. Alerts can be set on your receiver or smartphone, to wake you if glucose levels drop below or rise above your target range and need treatment.
- Reduced need for finger prick checks. CGM doesn’t completely replace the need to do finger prick checks but it does reduce the number needed. The devices still need to be calibrated at least twice per day (according to the manufacturers’ instructions) by doing a finger prick blood glucose check and entering the result. This is important to ensure the accuracy of the readings from the CGM device. It is also recommended that readings above or below your target range are confirmed with a finger prick blood glucose check before treatment.
- Peace of mind. Being able to see glucose levels at any time, and having high and low glucose alerts if levels go outside your target range, can provide reassurance and reduce anxiety.
- Data sharing. Some CGM devices have the option of sharing glucose data with up to five other people via an app on their smartphone or smart device, or SMS messages to notify them of alerts and alarms. This can be particularly useful if you are the carer of a young child as it allows you to monitor your child’s glucose levels when you are not with them. Data can also be downloaded to share with your diabetes health care team.
- Insulin pump integration to prevent hypos. Some CGM devices work with a compatible insulin pump. They can temporarily suspend insulin delivery via the pump if glucose levels drop below your target range or if the sensor predicts the glucose level will become too low. Insulin delivery from the pump starts again once glucose levels start rising again. This may help to prevent hypos or to make them easier to manage. Less food may also be needed to treat a hypo.
Potential downsides of CGM
While most people find that the benefits of CGM outweigh any negatives, there are a few things to consider before you decide to get started.
- CGM doesn’t replace finger prick blood glucose monitoring. While using CGM can reduce the number of finger prick checks needed, CGM devices still need calibrating at least twice per day, by entering a finger prick glucose reading. Finger prick checks are also recommended when glucose levels are changing rapidly, to confirm a hypo, before giving a correction dose for high glucose levels, and when symptoms don’t match the CGM reading.
- Lag time. CGM devices measure glucose levels in the interstitial fluid (the fluid between your cells) rather than the blood. Because glucose travels to the blood first and then to the interstitial fluid, CGM and blood glucose levels won’t usually be exactly the same. When glucose levels are stable, the readings should be close, but there will be a greater difference when glucose levels are rising or falling quickly.
- Being attached. Some people don’t like wearing the sensor and transmitter, particularly if they are using an insulin pump, as they then have two different devices attached to their body. In children, it can also be difficult to find suitable sites to insert the sensor, particularly if they don’t have much body fat. Your health professional can help with working out the best sites for wearing the sensor.
- Staying attached. It can be difficult for some people to keep the sensor and transmitter attached, particularly if they spend a lot of time in water and/or sweat a lot during exercise. The sensor might also be knocked off while playing or during sport and sometimes while sleeping. Unfortunately, if they come off they can’t be reused but this can usually be prevented by using extra tape to keep the sensor attached. Your health professional can provide advice on the available options and which tape might be best for you.
- Information overload. While the additional information CGM provides can be useful, it can also be overwhelming to see what glucose levels are doing all the time. Your health professional can help you to learn how to use and interpret this information so that it can be used to improve diabetes management rather than becoming something else to worry about.
- Alarm fatigue. While CGM alarms can be very helpful, if they occur often, some people find them annoying and disruptive, and may even start to ignore some of the alarms. For this reason, it’s important to talk to your health professional about setting the right glucose targets for you or your child. They should be set so that alarms only sound at times where you would want to take action to address high or low glucose levels.
- Cost. For those who aren’t eligible for subsidised CGM products through the NDSS, CGM is costly and is not covered by private health insurance. You can find out more about the CGM subsidy here.
- You need to stay close to the receiver. Your smartphone, pump or receiver needs to be within six metres of your sensor/transmitter to receive data.
How can I find out more?
You can find out more about CGM and Flash GM, the CGM Initiative, the available devices and their compatibility with your pump or smartphone and smart devices, by checking out:
- Continuous glucose monitoring: a guide to using CGM for children and young people with type 1 diabetes booklet
- Continuous glucose monitoring fact sheet
- CGM and Flash GM device summary and compatibility chart
We also encourage you to talk with your diabetes health care team who can explain more about CGM and Flash GM and help answer any questions you may have.
If you have more questions, you can also call the NDSS Helpline on 1800 637 700. The NDSS Helpline operates between 8:30am to 5pm Monday to Friday and from 9am to 12pm on Saturdays and national public holidays.