CGM vs A1c Accuracy

Hello all - I have the newest CGM and Medtronic pump as I recently “upgraded.” I had never had a A1c above 7 for the first eleven years of my diagnosis. However over the last year and a half for WHATEVER the reason my A1cs have been consistently in the 7s. Its a bummer for me and I’ve been working pretty hard to bring it back down to the 6s.

One of the strategies I’ve tried is utilizing the CGM all the time to attempt to have finer control. This is something I ruled out before as quality of life preference since my A1cs were “good enough” for me being in the 6s. Now I for SURE thought my A1c would recently be back under 6s. According to my CARELINK information my “average glucose” for both my finger sticks AND the thousands and thousands of CGM readings was 130. I originally thought ok my finger stick averages may be 130 however it must be high in between or high at other times…however the CGM data over the last three months seems to confirm that my overall average glucose is around 130…

My A1c was still 7.3 this most recent time though which DOESN’t line up with the “A1c conversion charts” you see everywhere. What gives! I’m frustrated cause know I don’t know what else I can do besides maybe meet with a dietition for review and keep trying…

In diabetes,

Christopher L Wilson

So, obviously, the A1C is going to be more accurate than the meter and CGM readings. If you take into account that meters and CGM’s can be up to 20% off, the difference between your readings and A1c falls slightly outside of that, but not by much. 104-156 would be 130 +/- 20%. It really does suck to think that you are having really good control based on the readings and then get the labs to show you that it’s not going as well as you thought. I had this happen during my last pregnancy, and it really was a huge blow to my confidence and self-esteem when it came to controlling my diabetes. One thing to keep in mind is that you just have to try your best each day and rather than using the data to see what your A1c might be, focus on the patterns that you see happening and make changes on the patterns. Another thing you can do is to have your meter tested to make sure it’s giving correct results.

So, obviously, the A1C is going to be more accurate than the meter and CGM readings.

Well, actually not really at all obvious to me. There are assumptions at the core of the A1C test which, if wrong, can alter the test results. I tend to view the A1C as just one more tidbit of background info. To get a full picture you need to try to integrate all the pieces available. I admit I’m not quite sure how to do this at this point with the info given so far.

My hba1c were actually coming less than the 5.0 range during pregnancy, which suggests frequent hypos. my endocrinologist said the test result can be a little off because of harmonal changes. you could probably look into that.

hi @thewilsons99, i agree with @powvens that, given the accuracy of the blood sugar meter (+/- 20%), the hba1c your test results indicate - 7.3 - is inside the potential error for your meter.

the other place for error (as @zjohnnyr mentioned) is the hba1c test itself. my lab indicates reference range range for my a1c test is 4.8 to 5.6 which means to me that I should add between 0.2 to 0.4 to my results for a better picture of average blood glucose - because this lab a1c results tend to run low.

if your lab reference range for example is 6.4-7.4 then your a1c results from that lab will be a bit higher than reality.

when you consider that we are talking about the total sugar in about 4 “skittles” then you may come to the same conclusion that I came to: that it’s a miracle we can control as tightly as we do without a deadly hypo. cheers, good luck. -Joe


You have been in this “T1 game” long enough to know that frustration is ongoing and almost expected based on the number of variables. For me, frustration shows its head when my before breakfast meter test is the “perfect 106” and I eat my controlled breakfast, bolus as usual and two hours later have a BG reading of 225 [when I expect to see a 125. WHY???

That said, I would expect that your CGM and meter values and meter values would correlate because you use your meter to calibrate the CGM. I do NOT use CGM, initially for the reasons you held off but now because my Social Security check and Medicare the cost is beyond my means. But why your HB A1c and CARELINK averages should now not coincide as they did in the past - I have seen my A1c drift apart from what I could formerly estimate from CARELINK 84 day average. For at least a dozen years my A1c ranged from 5.9 to 6.1 but now with my averages slightly lower [still testing the same seven times each day, my A1c now ranger 6.5 to 6.8. After two hypos where EMS had to revive me [BG = 10] the Endo ordered me to keep my A1c no lower than 6.5. In old-time T1 I need CGM but can not get that.

Brianna, @powvens, mentions checking your meter. If your meter is not accurate, your CGM will be off by that same factor. When I get “funny readings”, I recheck and use two different meters both drawing from the same finger stick.

I’m wondering if the calibration methods for A1c are shifting back to where they were 30 or 40 years ago when the scale was based on a reading was 6.5. I was part of the early 1970s developing the glysolated hemoglobin [now called HB A1c] blood test and at that time the scientists had set 6.5 as the goal for PWD.

Keep in mind that test results, although very important for us, are just numbers. More important is your overall health assessment and how you feel. When you compare average test scores on your CARELINK, how do your standard deviation readings compare? Mine have been dropping which indicate that my BG readings are showing considerably less swing.

Your a1c is just one more number in a pile which are susceptible to all sorts of influences (and inaccuracies). One thing to remember is that the a1c is very much influenced by the last three weeks of glucose levels prior to the test, so even though it is an average of three months, it can be on the ‘high side’ if your readings for the last few weeks were less good than the previous two months.

Also, it sounds like you’re doing a lot of good things right now – try not to get down because of one number. An a1c of 7.3 where you’re not having a lot of peaks and troughs (highs and lows) is probably better for your overall health than a 6.5 with lots of lows and highs. I started the pump a year ago, with an a1c of 6.5. That 6.5 came with A LOT of lows (and some pretty big rebound highs). I’m now at 7.0, but my overall control is much better and my team is really happy with where things are at.

My doctor has told me to rely more on average glucose sensor readings than A1C…he says that some people are high glycosolators and their A1C’s will not line up with their sensor readings.