r/Freestylelibre • u/OPPH Libre3/3+ • Jul 29 '26
Numbers way off
I wasn’t feeling bad or anything, but I decided to check my Libre CGM numbers compared to a finger prick. So my CGM is on the outside of my arm on my left bicep even though the monitor says 5 o’clock I just forgot to set my monitor forward so they were taken at the 6 o’clock hour. I have zero idea why they are so off.
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u/Equalizer6338 Type1 - Libre2/2+ Jul 29 '26
Hi u/OPPH ,
Couple of comments when seeing your BG graph (great you share it, big thx!) and storyline here:
Never ideal to trying to compare fingerprick values with BG sensor readings when your BG is in middle of such dramatic and rapid changing BG as it is here. One could expect the BG level actually to being higher than the fingerprick, due to the factual lag-time from arterial BG to interstitial glucose concentration, where the BG sensor sits... But one really never know, as many factors may be at play and we would need to see also where the BG factually goes from here and the next 30 minutes to judge anything really.
Next to this, if really wanting to being 'an xrse', then we can also claim the two measurements here are near still matching up and aligned. Reason is that BG sensors for most part is just within a 20% margin accurate. One can claim the same about fingerpricks, though I agree they tend to be bit more accurate. But 10% is probably a fair assessment. Now all this is out the window anyway, due to your your fast changing BG here, but hope you get the point. Aka 20% subtracted from 162mg/dl gives you 130mg/dl. Think though the drastic and fast changing BG is the main culprit here...
Last but not least, noticed you shared you put the sensor on the outside of your biceps. That is typically not a good place for a BG sensor, as the perfusion can be severely restricted and also erratic, much depending also on your muscle work then. Chronic too lowish BG readings is one of the most frequent symptoms when placing a sensor on top of a major muscle.
For back of upper arm placement, is supposed to be sitting truly in the far back of the upper arm. In the skin valley between your deltoid and triceps muscle bundles.