Continuous glucose monitoring (CGM)

CGM gives information about glucose levels every few minutes, allowing the user to see a graph of glucose levels rather than just a single measurement at a given point in time. The user or their carer inserts a sensor that measures the glucose level in the tissue just below the skin. The CGM sensor connects to a transmitter that beams the glucose data to a receiver, which can be either a handheld device or an insulin pump. The sensor is worn for 6 or more days and then replaced with a new one.

Dexcom receiver

Dexcom receiver

Costs and systems available in the UK – Abbott, Dexcom, Medtronic
NHS funding
CGM research
– Glucose sensing but not CGM – Abbott FreeStyle Libre

Introduction

CGM allows you to track whether your glucose is high or low, stable, rising or falling. You can also see how your glucose levels vary. For example you can track what happens while you are sleeping, after you eat, when you exercise, or when you are feeling unwell. The information is particularly helpful if you take insulin as you can match your insulin more easily to your needs.

There is a growing body of research that suggests CGM can help reduce A1C without increasing the risk of hypoglycemia. (See our research section for more information). In October 2011 the Endocrine Society issued a new clinical guideline. They concluded that there is high quality evidence that CGM can be a beneficial tool to help maintain target blood glucose levels, and limit the risk of hypoglycaemia in children and adolescents with type 1 diabetes who are at least eight years old, and in adults with type 1 diabetes as well if used on a daily basis.

CGM is a relatively new technology. Results are not 100% accurate, and consequently it is important to also use blood stick testing. CGM is more expensive than existing systems which monitor blood glucose, and is not generally available on the NHS.

CGM is potentially useful to anyone with diabetes but is especially useful if you use insulin. CGM can be very helpful to people on multiple daily injections as well as to pump users. Some CGM users report their CGM is as beneficial to their glucose control as having a pump. It helps if you are motivated as CGM takes a while to get used to, and is not always straightforward to use. It’s also worth remembering that you’ll still have to do finger stick blood tests as well. Indeed some people using CGM say they do more finger stick tests than they used to!

How does it work?

CGM works by sensing glucose levels in the body’s interstitial fluid. (Interstitial fluid is the fluid between tissue cells). These are tested every few minutes or so and the results sent to a receiver. In most systems the results can be seen immediately.

CGM has three parts:
1. Sensor: this is inserted into the skin and worn for a few days, then disposed of. It senses how much glucose there is in the interstitial fluid. A sensor is usually around 5mm long.

2. Transmitter: the sensor connects to a transmitter worn on the skin which communicates with the receiver (usually wirelessly).

3. Receiver: the receiver records results and in most cases displays them immediately on a pager sized device often worn on a belt or carried in a handbag. Systems that display immediate results are often called ‘real-time’.

CGM systems also come with software to enable you to analyse results. You can for example look at patterns at certain times of the day or after eating or when exercising.

If you’re using a real-time system you can use the information to act swiftly to help avoid high or low blood sugar. If you download results for viewing on a PC later it can help you identify trends. It is particularly helpful for looking at night-time patterns, when it’s difficult to do blood stick tests.

Important

CGM sensors measure glucose levels in interstitial fluid and not blood glucose. Glucose levels in interstitial fluid lag behind glucose levels in blood. As a result CGM results may be inaccurate, particularly when glucose levels are changing rapidly. For this reason it’s important to take a blood stick test to confirm the reading before taking action for example taking more insulin. CGM systems also have to be regularly calibrated with blood glucose levels (typically twice a day). CGM systems are most useful for viewing the glucose trend rather than the actual result, ie is your glucose level stable, rising or falling.

Drivers – DVLA says that drivers using CGM must also monitor blood glucose levels

Read our other CGM pages