This is not strictly true because the Libre has an algorithm built into the system which extrapolates the previous readings to predict what the reading would be in about 15 mins in order to try to make up the lag between interstitial fluid which the Libre reads and blood. When levels are changing at a uniform rate, then the algorithm works reasonable well and there should be no signficant lag but when the rate of change is changing or levels are changing direction for instance after a hypo treatment, then this is where the extrapolation of previous reading to predict the lag has an adverse effect, because it continues to predict you dropping for a further 15 mins after treating a hypo or when levels are rising sharply, reach a peak and then start coming down. This is often why Libre over exaggerates highs and lows.
Generally I find most Libre sensors read about 1mmol below my Caresens meter when levels are stable and mid range but I do those comparison checks when my levels have been mid range and stable for about half an hour with no significant insulin or glucose active in my system. A couple of checks in the first few days of a sensor's life under these circumstances are usually for me to be confident of using the Libre to bolus from provided it is within 2mmols. If it is further out than 2 mmols, I will do another couple of checks and if it continues to be more than 2 wide of the mark, it gets reported to Abbott customer support who will usually replace the sensor. If it is consistently within 2mmols ideally always lower or always higher then it is fine as far as I am concerned.
Also worth bearing in mind that the first 24 hours can be less accurate for some people due to what
@helli calls "insertion trauma", where the tissue that the filament has been inserted into has a slight reaction to the foreign body suddenly appearing in it, and this can cause the first day's readings to be less reliable. For this reason, many of us, apply each Libre sensor a day before the old one expires to let it bed in but not activate it until the next day when the old one ends. This gives the tissue time to react and then settle down before the sensor starts sampling the interstitial fluid and giving readings. Libre allow a 60 minute warm up period for this purpose I believe but giving it longer by applying a new sensor as much as a day in advance can be more beneficial. These are things that we have gradually learned by sharing our experiences here on the forum and are not necessarily things that Abbott would recommend.
