Continuous glucose monitoring

A continuous glucose monitoring device is a small device people with Type 1 or Type 2 diabetes can wear to measure their glucose levels every few minutes throughout the day and night. You can use CGM devices by themselves or in combination with a compatible insulin pump.

A CGM device has 3 main parts:

• The sensor which is inserted just under your skin. The sensor is disposable and needs to be changed after several days. Some CGM devices come with an applicator to make insertion of the sensor under your skin easier.
• The transmitter which for some CGM systems is reusable and attaches to each new sensor, and for other systems is already part of the disposable sensor. The transmitter sends the glucose readings wirelessly to a receiver. Both the sensor and transmitter are held in place by an adhesive (sticky) patch.
• The receiver then gets the blood glucose results and displays them. For some monitoring devices, your receiver can be a mobile phone, a smart tablet (eg, an iPad), or a compatible insulin pump.

With some CGM devices, you can share your results with multiple caregivers including your healthcare provider.

See a comparison of CGM devices in Aotearoa New Zealand(external link).

Image credit: Canva

When you’re wearing the sensor, the CGM device measures the glucose in the fluid surrounding your cells (interstitial fluid) every few minutes, 24 hours a day.

The monitoring can easily show patterns and trends in your glucose levels.

However, if your symptoms don't match the sensor readings it's important to check your glucose levels by using finger-prick tests. CGM allows users to set alarms for high and low glucose levels.

Blood glucose levels that go up and down a lot can be damaging for your body. Very high (hyperglycaemia) or low blood glucose levels (hypoglycaemia) can be serious.

To manage your diabetes you need to monitor your blood glucose levels. Most people use finger-prick tests with a blood glucose monitor. This gives you a one-off picture, like looking at 1 photo or reading 1 page of a book. How often you need to check your blood glucose depends on many factors.

The information gathered by CGM is usually presented as a graph. This shows a more complete picture of how your blood glucose levels change over time and how what you eat and drink, physical activity, stress, and being sick change your results. The information gathered by a CGM device lets you see how much time each day your blood glucose level stays within the ideal range, as well as recording times when it goes too high or too low.

The aim of diabetes management is to keep your glucose levels in a healthy range. Because CGM devices give you a real-time picture of your blood glucose levels, people who use a CGM have been shown to achieve improved blood glucose control compared to using finger-prick testing alone. The newer CGM devices allow people with diabetes to greatly reduce the need to monitor their blood glucose with finger-pricking. Some people use a CGM device for a short time (eg, a week or a month) to understand the patterns of their blood glucose levels. This could also be helpful for some people with type 2 diabetes.

Most CGM devices can send you or your caregiver an alert when your glucose levels get too high or low. This makes it much easier to treat these before they become a problem. It's important to double-check your blood glucose levels with a finger prick test before deciding to treat very high or very low blood glucose levels, and any time your symptoms don’t match the sensor readings.

Which CGM you choose depends on many factors, talk with your diabetes healthcare provider to discuss the options before deciding if using a CGM device is right for you.

You will usually have a training session with a diabetes nurse, or with someone from the company who provides the device, to show you how to use it and how to make decisions on managing your diabetes based on the results.

Finger prick readings are still very important in the management of your diabetes. Using a CGM device means you’ll need to do them less often.

Finger prick tests may be needed to help calibrate the CGM device (check it's reading accurately) and at times when blood glucose levels are changing quickly (eg, if you're dehydrated, after doing extreme physical activity, or after consuming certain foods or medicines).

In addition, you should still check your blood glucose level using a finger prick test when you are sick, and if:

• You have symptoms of low blood glucose but your sensor reads above 4 mmol/L check the result by doing a finger prick test: If your blood glucose level is below 4 mmol/L, you're having a hypo and must treat it immediately. Read more about how to treat hypoglycaemia (low blood glucose).
• Your sensor reads below 4 mmol/L but you don't have symptoms of low blood glucose check the result by doing a finger prick test: Only treat yourself for a hypo if your blood glucose level is below 4 mmol/L. 

Read more about:

• blood glucose self-monitoring
• sick day plans for people with type 1 diabetes
• sick day plans for people with type 2 diabetes.

Note: Finger prick readings measure the amount of glucose in your blood. CGM measures the amount of glucose in the fluid surrounding your cells which may lag 2 to 10 minutes behind your blood glucose levels. It’s normal for these results to be different, what's important is the pattern of changes in your readings.

CGM devices and insulin pumps are not the same. An insulin pump delivers a steady flow of insulin into your body depending on what range you set the pump to. You can use a CGM device or a pump alone, or both together.

Using a CGM device with a compatible insulin pump will mean your CGM device automatically tells your pump how much insulin to put into your body based on your glucose levels. This is an added level of safety if your glucose levels suddenly change.

You will have to pay for a CGM device yourself as they aren’t currently funded in Aotearoa New Zealand. These devices are a lot more expensive than using finger prick blood glucose meters.

The medicine funding agency in Aotearoa New Zealand, Pharmac, is currently seeking bids from suppliers of CGM and insulin pumps in the hopes of making them free of charge for people with type 1 diabetes in 2024.