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Does A CGM Like Dexcom Or Libre Really Read Low All The Time?

Does A CGM Like Dexcom Or Libre Really Read Low All The Time?

That certainly is the perception with those of us who have used or use a CGM (continuous glucose monitor) but do they really "read low".

First we need to understand exactly what they are "reading".

A Lab test, so when you go for A1C, 8 hour Fasting Test, A Random Test or an Oral Glucose Tolerance Test (OGTT) they are drawing blood directly from your veins.

Your veins contain the "most accurate" level of blood sugar "at the time of the test" because veins are "closest to the sugar source"… read more

A DiabetesTeam Member said:

So the CGM is ALWAYS going to be "behind" our capillary blood sugar level whether our sugar levels are Rising or Falling.

Now, unless you trying to "dial your own insulin" based on a given reading, while this (delay) may seem pesky, it is really of no consequence - does it really matter if you are taking a reading that is telling you "where your sugar level was 15 or 20 minutes ago"?

Not really.

But do they still read low?

Probably - because not all blood sugar will get into the fluid around the cells in our arm - organs and the brain use blood sugar so the cell fluids may not always reflect the "full dose", but are they "close enough" that it really doesn't make a difference?

Some are aware that was involved in program with Abbot (who makes the Libre system) for a couple of weeks where we looked at questions such as that.

So for two weeks I wore the Libre while continuing to "stick test" just like normal.

In the 2 weeks I did about 30 sticks tests, the Libre did the equivalent of 1345 "stick tests" in the same period. Even if you pick the couple absolute "best times" each day to do your stick tests you can't come close the precision of the monitoring you get with the CGM.

What did "MY" results reveal.

The Libre pegged my "average blood glucose" at 5.6 mmols/101 points for the entire period.

My Accu-Chek Guide, that did 1315 "less tests" put my average blood glucose level at 5.9 mmols/106 points.

So the difference between the two was 0.3 mmols or a measly 5 points on average - that is within the "error rate" of the most accurate meters on the market - effectively a "rounding error".

While the "perception is" that CGM's read "quite a bit lower", it really is just a perception - ON AVERAGE they are every bit as accurate and reliable as capillary stick test.

posted 18 days ago
A DiabetesTeam Member said:

I know my Libre and my finger sticks are normally off somewhat. When the censor reads low, the finger poke is a little higher. When my sensor reads high, my finger poke is generally a little lower. After reading the posts here, it makes more sense I would get the different readings when I am too high or too low. Because at that point i am taking some sort of action to increase or decrease quickly. When I am at mid-range with my censor, my fingers sticks are only a point or two different.
I really appreciate having the censor. Helps me make better choices in the moment. I believe I have more control over my readings than I have in years since i started wearing the Libre.

posted 3 days ago
A DiabetesTeam Member said:

Also,a little more of my own personal experience about cgm's reading low.

A few weeks ago I went for my a1c draw and my annual cbc. I fasted for 14 hrs and was well hydrated. Fbg lab result was 7.2. The Libre and my Freestyle Lite meter both read 8.3 when I tested just before the draw. This has happened many times before with the cgm reading higher.
My a1c was 6.5, so happy with that 😄.
Take care everybody!

posted 12 days ago
A DiabetesTeam Member said:

Interesting,because my Libre and my Freestyle Lite meter read almost the same usually within .2 of each other.

edited, originally posted 12 days ago
A DiabetesTeam Member said:

That is impressive. My brother says that his reads about a point lower when he compares it to his metre. I have not seen him since Covid started and have not seen his LIbre but I think he is calculating in his mind oh by it says I am 5 so I am really 6....He does like his though as at work it is very helpful when it is difficult to take so many testing breaks. Thanks for the information my A1C is no where near that but I am working on it.

posted 17 days ago
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