Components of a glucose monitoring system

A continuous glucose monitoring system has three parts that work together:

• the sensor — measures the concentration of glucose beneath the skin;
• the transmitter — sends the data wirelessly;
• the display device — on which you see the results [1].

In most modern systems, the sensor and the transmitter are joined together into a single piece at the factory, as the figure below shows.

These components form a simple and efficient chain: the sensor gathers the information, the transmitter sends it, and the display device shows it to you as values and a graph. In practice, you always have the estimated glucose level in front of you, without pricking your finger every time to get a reading with the glucometer.

The sensor is the part that measures the concentration of glucose. It has a very thin, flexible filament, inserted under the skin, which stays there for the entire duration of use. The tip of this filament comes into contact with the fluid between the cells (the interstitial fluid) and measures the glucose level in that space almost non-stop [1] [3].

The sensor is small and light, and after application you usually no longer feel it. It is the active part of the system, that is, the one that produces the information about the glucose estimate. The rest of the components only transmit, interpret, and display what the sensor measures [3].

The transmitter takes the values measured by the sensor and sends them wirelessly to the device on which you will see them. It is the bridge between the sensor and your phone or dedicated receiver. Without the transmitter, the information would remain trapped in the sensor.

The transmitter sends the data automatically, usually every few minutes, using a wireless connection similar to the one between a phone and headphones. In some systems it is a separate piece. In modern systems it is hidden inside the same plastic housing as the sensor, so you don't even notice it as a distinct element.

The display device is the screen on which you read the glucose estimate. It can be a dedicated receiver, received together with the system, or even your phone, on which an app dedicated to this purpose runs. There, the current value appears, along with a graph of the trend over the last few hours and an arrow (sometimes several) that shows the direction of the glucose [4].

The display device is also what triggers the alarm when glucose drops or rises too much. For the alarm to be heard, the device needs a charged battery, the volume turned on, and the app's permissions active. On silent or in "Do Not Disturb" mode, an alarm can go unnoticed. The display device does not measure anything on its own; it only receives, interprets, and presents to you the data sent by the transmitter, in an easy-to-understand form.

Yes, with most current systems you can use your phone as the display device, through a dedicated app, usually free. The phone receives the data from the transmitter and shows you the estimated glucose values together with the graph and triggers the alarms, exactly like a dedicated receiver [5].

Using the phone is convenient, because you have it with you all the time anyway. A separate receiver remains a useful alternative for those who prefer a dedicated device or as a backup solution. It is important to check whether your phone model is compatible with the sensor's app [5].

It depends on the system. In some older models, the transmitter is a separate piece, which you attach to the sensor after application and reuse with every new sensor. In newer models, the sensor and the transmitter are joined together into a single piece at the factory, which you wear as one whole.

Both options do the same thing, and the difference is a practical one. A separate transmitter means one more piece to manage and reuse. The combined version is simpler, because you throw everything away at once at the end of wear.

The adhesive patch keeps the sensor stuck to the skin for the entire duration of wear. It prevents the sensor from moving or falling off during daily activities, in the shower, or during sleep, so that the filament under the skin stays well positioned.

Good fixation is important, because a sensor that comes off can no longer measure correctly and cannot be put back in place. Sometimes an extra strip can be added over the patch, for more security. In some people, the adhesive can irritate the skin, and this should be discussed with the doctor [6]. If the sensor comes off before the end of its wear time, you apply a new one in a different place, and until the new sensor starts showing values you check your glucose with the glucometer. To replace the lost sensor, you can contact your medical team or the manufacturer.

The applicator is the device with which you place the sensor on the skin. You position it on the chosen spot and give it a single press. The sensor is then fixed to the skin, and its thin filament goes under the skin, in the right spot, quickly and with minimal discomfort [7].

The applicator is used to place the sensor, and is then thrown away or kept, depending on the model. Its role is to make the application simple and safe, so that you can place the sensor yourself at home, without the help of another person [7].

No. The sensor is changed most often, at the end of its wear duration. The display device — that is, the receiver or the phone — stays the same for a long time, and you reuse it with every new sensor.

The transmitter is what makes the difference between systems. If it is a separate, reusable piece, you keep it for several sensors (generally one year, though the duration differs from one system to another). If it is combined with the sensor, it is changed together with it. Thus, the replacement schedule depends on the type of system you use [2] [8].

If you lose the display device, the sensor on your body continues to measure glucose. It does not stop and does not break. Only the display of the values is interrupted, that is, you can no longer see them until you re-establish the connection with the sensor from another device. In the case of the phone, you reinstall the app on another compatible phone and reconnect [4].

Many systems keep the data from the last few hours in memory and recover it after reconnection, so that you don't completely lose the information from that period. Recovering this data is, however, more problematic when you change the receiver than after simply going out of its range. It is very likely that you will no longer be able to recover the data from the period when you were without a connection to the replaced device. On the same device, by contrast, you do recover it on reconnection [4].

Yes. The connection between the sensor, the transmitter, and the display device is wireless. Real-time sensors communicate through a wireless technology (Bluetooth LE), while intermittently scanned sensors use an NFC (Near Field Communication) connection [1] [4]. Bluetooth LE is similar to the link between a phone and wireless headphones, and NFC to the one used for contactless payments at the POS.

This wireless communication leaves you complete freedom of movement. For the data to arrive correctly, the display device must be close enough to the sensor. If you move too far away, the connection is temporarily interrupted and resumes when you come back within range [4].