BG and interstitial glucose are not likely to be the same when taken at the same time. Interstitial will lag behind BG by 10 to 15 mins typically so if BG is rising it will read higher and if it is falling interstitial will read higher. This is however not a linear correlation and other factors such as activity level and local inflammation at the sensor site for the first few hours after application can all have effects. I find that when blood glucose is abnormally high or low Libre tends to exaggerate the effect in its readings and in these circumstances it is wise to do a BG test. I find the great value in Libre is in showing the direction of travel and the convenience of being able to scan 30 times a day or so to recognise trends and micromanage bolus injections to keep within normal parameters ( in my case I target staying within a range of 4 to 10 with a target average around 7). If I can achieve that this results in an HbA1c of about 6. Without Libre the best I could achieve was 8. It has been a game changer for me but it is not a total or direct replacement for finger prick testing: they measure different things and are not interchangeable.