Libre is the most common and cheapest CGM, especially if you have a compatible smart phone.
More recently, Dexcom One has been released and is gaining popularity. It works out about the same price as Libre but you have to commit to, I think, 3 months due to the design of the CGM - it has a separate transmitter which lasts for 3 months and a sensor which needs changing every 10 days. It too, has a phone app. In my option, the Dexcom app is more basic than the Libre.
There are more expensive options from Medtronic and Dexcom but, unless you need it to calculate insulin dose, I would not think they are worth it.
I would recommend trialling the Libre for 2 weeks by signing up to their free trial which is available on their website.
Whatever CGM you chose, please please familiarise yourself with the often repeated limitations. If you expect it to accurately report a 12.5mmol/l on the first day whilst your levels are rising very fast, you will be disappointed.
- Some of us find that our bodies do not like have an alien object inserted into our arm. It take a day or two to "bed in" a new sensor. As a result, the first 24 to 48 hours after inserting a sensor could be more random. Some of us insert a new sensor the day before activating it to overcome this.
- Compression lows. Take care where you place your sensor and try to avoid the part of your arm that you lie on. If you get a low alarm in the middle of the night, check it with a finger prick before treating.
- "Normal range". CGMs are designed to be most accurate at "normal" BG. This is around 4.0 to about 8.0 and, if you see a value outside of this range, it can exaggerate the high or low. Again, the advice is to check with a finger prick before treating.
- Extrapolation. CGMs read interstitial fluid which will react to changes in BG about 15 minutes slower than blood. This is where I know some CGMs differ. Libre handles this by extrapolating the current trend to "predict" the current reading. If your trend changes direction in the last 15 minutes (e.g. when treating a hypo), the prediction could overshoot. Libre will correct this when it has "caught up". But at the time, it will seem like you are going higher or lower and taking longer to recover. Again, another reason for double checking with a finger prick.
- Faulty sensors. Not every sensor will be checked in the factory so there are some faulty ones in circulation. If you read social media, it will seem as if all sensors are faulty but human nature is to complain when things go wrong and say nothing if things are ok. The most common "fault" is inaccuracy. Therefore, it is a good idea to check the accuracy against .. .yes, you have guess it ... a finger prick. I tend to check once a day when my levels are stable and in that "normal" range I mentioned above. Don't expect exactly the same numbers (meter standards allow 15% inaccuracy and both could be out by 15% in opposite directions) but it is useful to give yourself confidence your current sensor is in the right ballpark ... and staying there each day. If it is out by a lot or if you get a "sensor failure" reported, in the UK, Abbott are pretty good at replacing sensor either via an online form or by calling.
- Third party apps. Libre is "factory calibrated". Some of us find "factory man" does not represent them well. There are third party apps like Juggluco, Shuggah, xDrip+, Diabox, ... which allow calibration against a finger prick. These were very popular when Libre required scanning as they also converted the sensor into a rtCGM (real time CGM which did not require scanning). Some people still prefer them as they also allow things like integration with a smart watch.
