What a glucose sensor is and how CGM works

A glucose sensor is a small medical device that you wear directly on your body and that automatically estimates your glucose, day and night. It is usually applied on the back of the arm or on the abdomen. It has a very thin, flexible filament that stays under the skin, while the rest of the device sits on top, held in place by an adhesive patch. In some models, the sensor is entirely under the skin and is inserted by a doctor [1].

The main advantage is that, once applied, it works automatically, without you doing anything to obtain estimated glucose values. The sensor is only one component of a system. The system also includes a part that transmits the data and a device on which you see the results — a phone, a dedicated reader, or an insulin pump. In this way, measuring glucose becomes a continuous process that runs on its own, instead of an action you repeat manually [2].

A glucose sensor measures the concentration of glucose in the interstitial fluid, that is, in the thin layer of fluid that surrounds the cells beneath the skin. It does not measure glucose directly from the blood. The glucose in the interstitial fluid follows blood glucose very closely, however, so the sensor can give you a good picture of your values [3]. There are moments when the value on the sensor and the one in the blood can differ, especially when glucose is rising or falling rapidly [3].

The result is displayed in mg/dL (milligrams per deciliter) or in mmol/L (millimoles per liter), two different units of measurement for the same value, and the device shows it in whichever unit was selected at setup. For example, the same glucose level is written either as 180 mg/dL or as 10.0 mmol/L. A value of 70 mg/dL (3.9 mmol/L) is the threshold below which glucose is considered low (hypoglycemia). This lets you quickly see whether you are in a safe zone or whether your glucose is too high or too low.

Continuous glucose monitoring is a method by which your glucose is tracked automatically, without interruption, day and night [2]. The sensor records a value every minute or every few minutes, which means hundreds of measurements over the course of a single day. Instead of finding out a single number at a given moment (with the glucometer), you receive a continuous stream of values. This stream shows you both where your glucose is now and where it is heading [4].

This way of tracking greatly reduces the need to measure your glucose by pricking your finger, but it does not remove that need completely: the finger prick remains useful in certain situations, for example when you want to confirm a value. The sensors can be set to trigger alarms when glucose drops or rises too much, which helps especially in detecting hypoglycemia, including at night [5]. The purpose of this monitoring is to give you a complete picture of your glucose, so that you can make better decisions every day.

The thresholds at which the alarms sound are set together with your medical team. When the low-glucose alarm goes off, confirm the value and, if it is below 70 mg/dL (3.9 mmol/L), take 15 g of carbohydrate, then check your glucose again after 15 minutes. If the alarms keep going off day after day, or if your glucose does not come back up after you correct it, tell your medical team. If vomiting, difficulty breathing or drowsiness appear, go to the emergency room straight away.

The main difference comes down to how the measurement is made. A glucometer shows you the glucose from a drop of blood obtained by pricking your finger, at a single moment. Each time you want a new value, you have to prick yourself again. A glucose sensor measures the glucose in the interstitial fluid automatically and continuously, hundreds of times a day. In addition, it shows you whether glucose is rising, falling, or staying stable [2].

A useful comparison is the one between a photograph and a movie. The glucometer gives you a single image, while the sensor shows you the entire evolution of your glucose throughout the day and night. Both methods are correct and useful, and the sensor does not completely eliminate the glucometer. In certain situations it is good to confirm the result with a finger prick, for example when your symptoms do not match the value displayed by the sensor. That is why it is useful to always have a glucometer at hand, as the figure below shows.