Real-time vs. intermittently scanned glucose sensors

A real-time sensor is a sensor that displays your estimated glucose automatically and continuously, without you having to do anything. The value appears on its own on your phone or on a dedicated reader, and it updates by itself every 1-5 minutes. In this way you always have your current glucose estimate available, with no effort on your part.

This type of sensor can warn you through alarms when your glucose rises too high or drops too low, because the data reaches the app automatically. Alarms are the main advantage of a real-time sensor over one with intermittent scanning, although some recent models of that kind do have alarms as well. In practice, alarms help you react in time to extreme glucose values [1].

An intermittently scanned sensor measures glucose all the time, but it does not show you the value automatically. To see your estimated glucose, you have to scan the sensor yourself, using a reader or your phone. In other words, the information exists, but you only see it when you ask for it through a scan [2].

Even the name explains this: the data is read "intermittently", meaning only from time to time, whenever you choose. Between two scans, the sensor keeps measuring and storing the values in memory, but they only appear on the screen once you perform the scan. This way of transmitting the measured data is what sets an intermittently scanned sensor apart from a real-time one [3]. Some recent intermittently scanned models do, though, send the values to your phone automatically, just like a real-time sensor, as the figure below shows [3].

With an intermittently scanned sensor, the estimated glucose does not appear on the screen by itself; instead, you find it out when you scan the sensor. At the moment of scanning, you receive the current value, shown either in mg/dL or in mmol/L (depending on the setting). This way, you are the one who decides when you want to see your estimated glucose, according to your needs that day [2].

Along with the value at that moment, a scan also shows an arrow telling you whether your glucose is rising, falling or staying stable. At the same time you see a graph of the values from the last few hours. In this way, a single scan tells you not only what your glucose is now, but also how it has changed in the past and where it is heading [5]. If the arrow shows a rapid fall or you have symptoms, confirm the value with a glucometer; below 70 mg/dL (3.9 mmol/L), take 15 g of carbohydrate and recheck after 15 minutes.

Scanning an intermittently scanned sensor is simple and quick. You bring the reader or your phone within a few centimeters of the sensor, and it picks up the data wirelessly, in just one second, using NFC technology (communication over a very short range, just a few centimeters). There is no need to press hard or to touch the sensor; you only have to bring the device close to it [6].

One practical advantage is that scanning also works through thin clothing, so you can do it discreetly, without having to uncover your arm. You can scan the sensor as often as you like, with no limit at all, both day and night, whenever you want to check your glucose [2].

If you forget to scan, the sensor does not break and keeps measuring your glucose the whole time. The problem is that the sensor can only store the values from the last few hours. If too much time passes between two scans, the oldest values are deleted to make room for new ones. A gap will then remain in your graph for that period [2].

In practice, you do not lose the sensor, only the information about your estimated glucose from the interval you did not scan and the memory no longer covered. That is why, to have a complete picture of the whole day, it is good to scan the sensor regularly. This way you make sure you are not missing important data about how your glucose has changed [7].

An intermittently scanned sensor can keep the glucose values from approximately the last 8 hours in its memory. This means that a scan shows you not only your glucose at that moment, but also the trend over the previous interval. That record has no interruptions, as long as you scanned more often than once every 8 hours [2].

To have the complete record of an entire day, you need to scan the sensor at least once every 8 hours, that is, at least three times a day. The more often you scan, the better, because you make sure you do not lose any data. The important thing is not to let more than 8 hours pass between two scans, so that no gaps remain in your data [6] [7].