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.
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.