How does Continuous Glucose Monitoring Work?

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How does Continuous Glucose Monitoring Work?

Continuous glucose monitoring tracks your blood glucose levels, without frequent finger-pricking tests, consistently throughout the day. It allows you to see a graph of your levels rather than a single measurement, and can also anticipate highs and lows. 

Seeing glucose levels in real time allows you to make decisions about your diet and lifestyle accordingly. There are many different continuous glucose monitors available, each with different customisable features. A specialised healthcare professional can help you decide which one is best for you. 

On this page, you’ll learn how continuous glucose monitoring actually works, what it does, how to use it, and the advantages and disadvantages. 

What is Continuous Glucose Monitoring? (CGM)

Checking your blood sugar frequently is an important part of diabetes management. A continuous glucose monitoring device is wearable technology that allows you to access your blood sugar levels at a glance – either via a handheld reader or a compatible smartphone.  

Continuous glucose monitoring allows you to access data in real-time or retrospectively. It can be used as a standalone device or as part of an integrated solution with your insulin pump. 

A CGM works by measuring the amount of glucose in your interstitial fluid – the fluid between your cells. Unlike a finger-prick test which is an accurate snapshot of your blood sugar level at a single point in time, the blood glucose reading from your interstitial fluid has a slight delay. 

How does it work?

Continuous glucose monitoring works by measuring glucose levels in the body’s interstitial fluid (the fluid between tissue cells).  

Every few minutes, the sensor takes a reading and the attached transmitter sends them to a receiver. The receiver could be a separate device or an app on your smartphone or smart watch. Most CGMs take readings between every 2 to 5 minutes. 

It is important to note that interstitial fluid sugar readings are a few minutes behind your blood sugar levels, which means that you will still need to do finger-prick checks every now and then. You may also need to use a fingerstick to calibrate the device. 


Each device is slightly different, but works in similar ways 

A small sensor is placed just under your skin, usually on your arm or belly. An applicator makes this part quick and easy to do. You can do it yourself or ask a nurse or loved one for help. Adhesive tape holds the sensor in place. 

The sensor measures the glucose levels in the fluid under your skin. This sensor will need to be changed regularly  ⁠— usually every 7 to 14 days depending on the device. There are some long-term implantable CGM devices available, which have to be replaced by your healthcare provider. 

All CGM systems use a transmitter to wirelessly send your glucose data from the sensor to your monitor or smart device. The transmitter is usually reusable and attaches to each new sensor. 

You can download your CGM data anytime, and can share the information with your healthcare provider. If you are caring for a child, older adult or person with a disability, you can have their glucose data sent straight to you. 

Are CGM Devices Easy to Use?

Using a glucose monitoring device can be a little tricky at first, so a specialist nurse or doctor will show you how to use it the first time. 

Before using a CGM, you will need to learn to: 

  • insert the sensor properly 
  • calibrate the device with fingerstick blood glucose readings (some devices skip this step) 
  • set device alarms 
  • transfer data to a computer or your phone for long-term analysis 
  • respond to and make changes to your care plan based on the collected data 


What are the Benefits of a CGM?

Using a CGM can make it easier to manage type 1 and type 2 diabetes. Using a CGM long term will: 

  • show you a bigger picture of how diabetes affects you 
  • help you adjust your insulin doses 
  • alert you to highs and lows, sometimes before they happen 
  • reduce your finger-stick checks (though they will still be necessary) 

What are the Limits of a CGM?

Researchers are working hard to make CGMs as accurate as possible, but you can’t solely rely on CGMs to make treatment decisions. As the results show a lag, it is important to regularly do finger-stick glucose tests in order to check the accuracy of your CGM device. 

Wearing a CGM device may also irritate your skin or cause bruising. Changing it regularly and switching locations can help reduce discomfort. Most people find CGMs more comfortable than doing frequent finger-pricking tests, however. 

A CGM system is also more expensive than a standard glucose meter, and are also not right for everyone. Be sure to speak with your doctor to determine if you are eligible. 

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Carol Willis

Carol Willis - Diabetes Clinic Facilitator

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