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Message: <blockquote data-quote="Deleted member 527103" data-source="post: 2695263">There are a number of limitations of all CGMs but doctors, DSNs and manufacturers either don't know or don't publicise them. Therefore, it is down to users to spread the word which you will find repeated again and again on forums such as this one.Off the top of my head the common limitations are - insertion trauma. Inserting a foreign object into your body can affect the readings near the Libre filament. Our body can take up to 48 hours to get used to this foreign object resulting in apparently random numbers for the first day or two. These affects vary from person to person but those of us experiencing it often insert our sensor a day or two before activating. - compression lows. If pressure is applied to a sensor, the flow of interstitial fluid (what is used to measure BG) is restricted. Therefore, the sensor may report a false low. This is most common when sleeping and we turn over onto the arm with the sensor. You can spot these on the graphs as they are typically very short dips and returns to the previous reading, likely to be when you turn over again. - reading delays. CGMs do not get their readings from blood, they use interstitial fluid. However, changes in BG are reflected in interstitial fluid readings about 15 minutes later. CGMs such as Libre use an algorithm to convert from interstitial fluid readings to BG. This algorithm takes the delay into consideration by extrapolating the current trend and effectively "predicting the current reading. This approach works most of the time but can exhibit some weird behaviours when our BG trend changes direction. For example, it can predict higher (or lower) than our BG actually goes so you may see a high (or low) reading which is later overwritten when the interstitial fluid "catches up". But the most common unwanted behaviour is a delay in showing a recovery from hypo. - limited calibration - the relationship between BG and interstitial fluid is not linear. Therefore CGM manufacturers have chosen to use a conversion algorithm that is correct at "normal" BG. As a result, high readings (typically above 10 mmol/l) and low (typically below 4 mmol/l) can be inaccurate.  - meter accuracy. In the UK, all glucose meters (both finger prick and CGMs) are required to be with 15% of "true" BG value 95% of the time. If we have a finger prick meter which is, say 10% under reading and a CGM that is 10% over reading, the difference between the two can be significant. I believe this is why people often report that "Libre is always high". It is likely to be that it is higher than their finger prick meter which they have relied on for some time.</blockquote>

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