Libre sensors used in the US can only be used for ten days (this will change soon) and the reader will only give readings twelve hours after activation is initiated. This is in contrast to the rest of the world where the sensor lasts fourteen days and will give readings after a 60 minutes warm up. The reason given for the longer warm up time in the US is that the sensor is less accurate during the first twelve hours. However, rather than recommending that US users insert the sensor and then wait twelve hours before initiating activation, the FDA required that Abbott prevents any reading to be made in the first twelve hours. Since the reader can only access one sensor at a time this means that in the US users will have a twelve hour gap every ten days when they cannot use Libre. In Europe this “down time” is just one hour every fourteen days.
European users have tested the relative accuracy of Libre with blood capillary tests after activation. Some people find that Libre gives consistent values within 8 hours of activation, and others find that it takes longer. A common strategy is to apply a new sensor a day before the existing one will finish, and then when one sensor finishes, activate the new one. This way European users only have a one hour gap in Libre measurements while still being reassured that the sensor has acclimatised to their skin.
Note that the rules of “only one sensor active at a time” and “activation times” are mandated by the official reader. The software available for suitable NFC phones does not impose these strictures. You can use both the reader and a suitable phone as long as you activate the sensor first with the reader. If you activate the sensor with a phone then for the remainder of the lifetime of the sensor you will only be able to use the phone to read it.
Libre User 