How long a glucose sensor lasts (CGM)

A glucose sensor works for a limited number of days, set by the manufacturer. Transcutaneous sensors, which have a thin filament under the skin, usually work for between 7 and 15 days [1]. The device counts the elapsed time and lets you know when it is nearing the end, so that you can prepare a new sensor. Each sensor needs a warm-up period at the beginning, during which it does not yet display the estimated glucose values [2]. With the implantable sensor, the warm-up lasts a whole day and is followed by calibrations with a glucometer [3].

Implantable sensors, placed under the skin by a doctor, work for up to a year, as the figure below shows [4].

When the sensor reaches the end of its lifespan, it stops transmitting values, and the app or reader shows that it has expired. As a rule, the system warns you in advance, a few hours or a day before, so that you are not caught by surprise [1].

From that moment on, you no longer receive new values. To continue monitoring, you need to apply a new sensor if it is transcutaneous, or go to the doctor to have the implantable one replaced. If you need to make a treatment decision during the interval without a sensor, you will have to check your glucose with a glucometer.

Officially, no. The sensor stops automatically once the approved period of use ends, because it is programmed by the manufacturer to do so [1]. There is no safe or accepted way to extend its use beyond the approved time. Any attempt to force it to keep going gives values you cannot trust, so you cannot base treatment decisions on them.

Toward the end of the lifespan, measurement accuracy declines anyway. This happens because the enzyme used by transcutaneous sensors degrades, and all sorts of proteins and cells build up on the working surface [5]. For your safety, use each sensor only within the working lifespan approved by the manufacturer.

Transcutaneous sensors measure the glucose concentration with the help of an enzyme, which gradually loses its activity. As the days pass, the signal therefore becomes less accurate. This is the main reason for the time limit on use imposed by the manufacturer [1].

In addition, the body reacts to the foreign object introduced under the skin. A number of proteins and cells gather around the filament and form a thin layer, which prevents glucose from reaching the sensor [5]. The adhesive and the battery also have a limited service life.

Yes. Implantable sensors work much longer than transcutaneous ones: up to a year, compared with generally up to two weeks [4]. Implantable sensors use a different measurement method that is more durable over time [6].

Because they sit under the skin, they cannot accidentally come off, and the detachable transmitter is attached with a silicone adhesive, which rarely irritates the skin and only briefly [7]. On the other hand, they require a minor procedure at the doctor's office, where the medical staff inserts the sensor and, at the end, removes it [6] [7].

It depends on how the system is built. In modern transcutaneous sensors, the sensor and the transmitter form a single piece. As a result, when the time of use ends, you have to throw away the whole unit as sharps waste, in a rigid container, following the manufacturer's instructions. Then you apply a new one. In older systems, where the sensor and the transmitter are separate pieces, only the sensor is replaced, while the reusable transmitter is kept and reattached to the new sensor.

The implantable sensor is removed and a new one is inserted, generally in the other arm. The transmitter worn on top of the skin is reusable. As a general rule, the part in contact with the body is the one that expires and is replaced. The reusable electronic components are kept whenever the system's construction allows it.