How glucose sensor data is transmitted and stored

The sensor contains a small transmitter. It sends the values wirelessly, usually over Bluetooth radio waves, to the device where you see them — your phone or a dedicated receiver. On the vast majority of today's sensors, the "real-time" ones, this transfer happens automatically, every 1–5 minutes, without you pressing any button.

Some systems instead use a different technology, called NFC, which has a very short range. You have to bring the phone close to the sensor to communicate with it and see your values [1]. Systems with continuous transmission display glucose in real time and can alert you at any time. Scanning systems show the values only when you bring the phone close to the sensor, so between scans only the models with optional alarms warn you. Continuous monitoring generally leads to better glucose control, as the figure below shows [2]. Intermittent scanning has, in fact, since been replaced by continuous transmission [3].

Your data is kept in several places at once, for safety. The sensor has its own internal memory, which retains the values when the phone is not nearby. On most systems it covers about 24 hours. On some newer models it can cover much longer, even the entire wear period [4].

The device on which you see the results — the phone or the receiver — in turn keeps a much longer history of your glucose values. Many systems then automatically send a copy of the data to a secure online storage space, called the cloud. You reach it through the personal account you create in the sensor app. That copy stays available regardless of what later happens to your phone. Anyone else — your doctor or your family — can see this data only if you choose to share it.

No, it is not lost. The Bluetooth link between the sensor and the phone works over a distance of about six meters in open space. Walls, other obstacles and even your own body — for example when the phone is on the opposite side from the sensor — can shorten this distance considerably. If you move too far away, the display device temporarily loses the signal. You usually do not lose the values, though. The sensor keeps them in its internal memory and fills them in automatically in the app when you come back within range [4]. While the link is down, however, the app receives no new values, so it cannot trigger hypoglycemia or hyperglycemia alarms. These resume as soon as the signal returns.

The situation is different with NFC scanning systems. They have no continuous link with the phone, so only the models that have alarms can warn you, and only while the sensor is connected. These systems also keep the data for only about eight hours. You have to bring the phone close to the sensor before this interval passes, otherwise the older values are lost for good [1].

Yes, you can see past values at any time. The app keeps a detailed history of your glucose values by day, week and month, which helps you follow how the values have changed over time.

Many apps also generate a standardized report, called the ambulatory glucose profile (AGP). It summarizes your glucose patterns — for example time in range, the percentage of the day your glucose stays within the target range — in an easy-to-follow format. The report is used both by you and by your medical team [5].

If your data is saved online, through a personal account, you do not lose it when you change your phone. You install the app on the new phone and sign in with the same account. Your history then reappears automatically. That is why it is useful to create an account and not change it later.

Data kept only locally, on the old phone, without an online copy, can however be lost when you switch. In addition, the sensor usually connects to a single phone at a time. Once you move to the new phone, it will send its values only to that one.

Yes, you can easily share your values with your doctor. The app generates a standardized report — the ambulatory glucose profile (AGP) — which can be saved or printed and which presents your glucose values in a clear, easy-to-interpret format [5].

Many systems also allow the data to be sent directly over the internet, to the platform used by your doctor. They can then see your values before or during the consultation. This overview helps them adjust your treatment more effectively [4].

No. If the phone or receiver battery runs out, the screen goes dark, but the sensor keeps working on its own. It continues to measure the glucose concentration and keeps the values in its internal memory.

Once you recharge the battery and reopen the app, the missing values are filled in automatically, and the copy saved online also stays safe. A dead battery therefore does not erase your glucose history. If the values do not appear, close and reopen the app and check that Bluetooth is turned on. If the app still does not reconnect to the sensor, contact the manufacturer's support service or your medical team.

No, the sensor works without an internet connection. The link between the sensor and the phone is made over Bluetooth (or NFC scanning), which does not need access to a data network. You can therefore see your glucose even in an area with no signal [4].

You only need the internet for certain additional features, such as saving the data to the cloud or sharing the values with your doctor or your family (remote monitoring). The basic functions — measuring glucose and displaying the estimated values — work without the internet too.