Libre CGM vs A1c

I have been using a Freestyle Libre for 3 months. It has worked great for me. The numbers seem to line up well with my test strips. My control has gotten better, and I was excited to get my A1C taken, thinking it would be my lowest one yet.

The Libre says my Average Glucose is 95. So i expected to have a 4.9-5.2 A1C. It came back as a 6.0 A1C .

I guess my question is, How accurate do you think an A1c is?


@Dean, this is a pretty common phenomenon with the Dexcom too, to the degree that, recently, Dexcom took out the “equivalent A1c” number.

I think there are several reasons why— @Chris would be better at explaining it than me. One of them is the possibility of a bias in your CGM, of course. There are also other reasons that are linked to the fact that the A1c measures the result of a biological reaction which is not a mathematical average.

Measuring an A1c is a precise lab function: I think the result is +/- 0.1%. But the glycation of your hemoglobin, the average lifetime of your red blood cells etc. are not processes that precisely map to an average glucose level.

On the other hand, I think (my opinion) that comparing A1c from quarter to quarter would be a fairly repeatable process. So we may not know exactly how an A1c maps to average glucose levels, but when it goes down or up we know what it likely means. Maybe others have a different point of view?



As @Michel mentions, an office based A1c measurement is a very good measurement of your glycated hemoglobin.

Your A1c is excellent, and nothing that in my opinion should concern you. You are well below the area where your risk of complications is increasing rapidly. [see graph below]

When a physician tells you that your A1c is a measure of your average glucose reading they are over-simplifying the situation. I think they use this simplification as a way to communicate with patients in a way that leads to action i.e. my A1c was 9, I have to get my average glucose levels down to improve my A1c.

I think that anytime your blood glucose is above a personal set point the glucose molecule is attaching and your hemoglobin gets glycated. So if you have an average of 95, but had many peaks with high levels, i.e. like my son, who has many peaks into the low 300’s, then during these times your hemoglobin is getting glycated at a fast rate. So this may part of the issue.

Additionally, each person gets rids of red blood cells at a different rate, so if your body is slow to get rid of red blood cells, then you will have more glycated hemoglobin staying around longer. Also in the A1c test, the last week will have more impact than the first week. So if you had stellar control three months ago, but it slipped lately, the A1c will overemphasize the near rather than the far term. There are also, many other variable I haven’t discussed.

My point, is your A1c is excellent. If there is more you can do, great, go for it. If you are happy with where you are in your lifestyle, I personally wouldn’t worry and wouldn’t look back.


The average BG doesn’t truly correlate to an A1C value in a precise and linear sense… in a general sense it does…

E.g. It is true that with an average BG of 100 your A1C will be lower than it would be with an average BG of 150, but assigning a precise predicted value in either case is not really the way it works…

The charts and estimates you see are just estimates, for some people they happen to be more accurate in some cases than in others

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Oh great, so you want to take my long and convoluted thread, and condense it I see… /sarc


You had a thread? Post a link

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Well stated @Chris and @sam

Another explanation is to look at this overly-simplified example:

  • One person has BG’s of 60 and 180, which averages to 120
  • Another person has BG’s of 120 and 120, that also averages to 120

Despite the same average BG, the person who had the higher BG at 180 would have a higher A1C.


And if eric and I both had exactly the same blood sugar all the time, we would still likely each have slightly different A1C… just like if we ate the exact same and exercised the exact same we still probably wouldn’t have exactly the same cholesterol levels…


Thanks everyone for the reply’s. I’m not upset with a 6.0 a1c, just not as happy as I would have been with a 5.0!

I think what happened is as you mentioned, I had great control for months then lost it in the last week before my test. Nothing drastic, but was averaging more like a 6.0 then a 5.0.

My doctor keeps suggesting I look at things less academically, because the treatment is still the same (insulin). But I find it interesting to learn more, and am thankful to have this great resource of people to discuss ideas with.

I like the Chart, but wish it went to 5.0. A1C, Also is Relative Risk a % or does say 20 mean 20 times more likely.

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At the time the research was done (late 80’s early 90’s) an A1c of 5 was uncommon, so it is no suprise that an A1c of 6 is the lowest they report. Subsequent research has shown that an A1c in the 4’s and 5’s give a person approximately the same risk of developing the complications as a person without diabetes. I say approximately, because research has shown that all hyperglycemia does damage, and no matter what you do a T1D will have a higher lifetime exposure to hyperglycemia than a pancreas normal person will.

Yes in reporting relative risk, it is (x times more likely), so a person who maintains an A1c of 12 will have a 20 times risk of developing Retinopathy.

With all that said, there are many many people that live a full and long life even with an elevated A1c, as evidenced by the folks that have lived with Type 1 for a very very long time.