Glucose sensor warm-up period (CGM)

The warm-up period is the interval from the moment you apply the sensor to your skin until it starts displaying estimated glucose values [1]. During this time the sensor settles into the fluid between the cells, where it measures glucose concentration. In addition, the area under the skin where the sensor now sits gradually returns to normal after the initial microtrauma of inserting the sensor's filament [2]. The sensor is technically working, but it isn't showing you any value just yet.

You can think of this stage as the slow start-up of a new device. The sensor needs a few moments to synchronize with your body before it can give you data you can trust. It is a normal and mandatory part of every sensor application, and the app or receiver usually shows you a countdown until the end of this period.

When you insert the sensor's thin filament under the skin, the local tissue is mildly disturbed by this action. That is really just a natural response of the body to a foreign object. The fluid around the sensor needs to resettle, and the filament's sensitive surface needs to come into stable contact with this fluid [2]. Until this balance is established, the values measured by the sensor could be inaccurate, because the overall local signal does not yet faithfully reflect the glucose concentration.

The warm-up period protects you. The sensor is designed to show you no number at all rather than give you a wrong one that you might rely on for a treatment decision. It is a short pause that increases your confidence in all the values you will see afterward, throughout the entire period of use.

The duration of the warm-up period depends on the model of sensor you are using and is set by the manufacturer. In general, it falls somewhere between about 30 minutes and 2 hours. Some sensors start quickly, in just 30 minutes, while others need a longer interval before displaying their first value [1].

You can find the exact duration in your sensor's instructions or in the companion app, which usually shows you a countdown. It's useful to plan the application of a new sensor for a moment when you don't need values right away, such as before a quiet part of the day. This way you avoid being left without information from the sensor at an important moment, as the figure below shows.

During the warm-up period you don't need to do anything special with the sensor. You leave it on your skin, check that it's securely attached and that the adhesive edge is sticking down completely. In the first few minutes it's good to avoid heavy sweating, water and intense movement in the sensor's area, so that it stays stable and in place [3].

You can use this time to go on with your usual activities, while the sensor carries on preparing in the background. The main thing you need to do is keep a classic glucose meter on hand, in case you need to check your glucose before the end of the warm-up period.

During the warm-up period the sensor doesn't show you values, so you can check your glucose whenever you need to with a glucometer, from your finger. This method gives you an immediate, reliable value [3]. It's good to always keep such a device and compatible test strips, still within their expiry date, on hand, especially in the first hours after applying a new sensor.

It is recommended that you keep constant access to classic glucose measurement, precisely for moments like this, and for many other situations. Finger-prick testing remains the safe foundation whenever the sensor doesn't yet show data, or later on when its readings don't seem plausible. You use it whenever you need to know for certain where your glucose stands.

No. During the warm-up period you don't make treatment decisions based on the sensor, because it isn't giving you values yet. If you need to calculate an insulin dose or treat a glucose level that is too low or too high, use the value from the glucometer. This is the only reliable source of information during this special window of time [3].

Once the warm-up period has ended, the sensor starts displaying the glucose estimate. You can then go back to the decisions based on its data, as you normally do. This holds only for non-adjunctive sensors, that is, those approved for insulin dosing without a fingerstick confirmation. With an adjunctive sensor, the value is confirmed with the glucometer before a treatment decision. This caution prevents mistakes that could arise from a sensor-estimated value that hasn't yet been verified.

Yes, every new sensor has its own warm-up period, which begins the moment you activate it. This always happens because every time you insert a new sensor into an area of skin, that spot needs time to settle again [2]. There is no way to skip this stage.

You can turn this repetition into a predictable habit for yourself. When you know a sensor is nearing the end of its life, plan the replacement ahead, so the new sensor's warm-up period falls at a convenient moment. This way, you move smoothly from one sensor to the next, with as little interruption to your information as possible.

During the warm-up period, the sensor doesn't estimate glucose yet, so low or high alarms don't work during this interval [1]. You cannot rely on the sensor to warn you about hypoglycemia (glucose that is too low) while it is warming up. For this reason, your own direct attention and a classic glucose meter remain essential during this period.

The risk of hypoglycemia can be higher at certain times, such as after physical exercise or in the evening. If you know such a moment is coming, it's a good idea to check your glucose with the glucometer before and during the warm-up period. Once the sensor becomes active, the alarms start up again and begin warning you as usual, giving you back the safety net you're used to.