FUDiabetes

A1c vs. CGM vs. Blood Glucose reading - confused!

,

I’m experiencing a fascinating metric situation:

My CGM (Libre) has me at between 105-110 average glucose over the past year.

Yet, when I compare my CGM readings 15 minutes after my blood glucose level at the time of blood draw, they are about 20% lower than the lab’s glucose levels.

Meanwhile, the past two A1c tests have returned results of 4.1 and 4.2.

I don’t have any of the common conditions that would lead to an exceedingly low A1c. I’ve done some research and there is supposedly a possibility that I have a weird type of hemoglobin that can screw up A1c results. The question is - by how much? And are there any other possible factors to consider?

Any info/insights greatly appreciated!
Cheers,
Jon

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Jon, fantastic A1c and average glucose level, even if they are 10 or 20 points lower than your meter. As far as conditions, you could just have a normal but lower than average lifespan in your red blood cells. The test is predicated on everyone’s red blood cells lasting a similar length of time.

Here is a decent read on the subject: https://www.mlo-online.com/disease/diabetes/article/21085264/the-impact-of-red-blood-cell-lifespan-on-hba1c-measurement

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@JonDeutsch, your CGM typically skews your readings low. It seems to us that this has become more true in the past 2 years. I think the reason is clear: the CGM has some imprecision built in, and the manufacturer would like to skew the error off in the direction of the CGM being lower than reality, rather than the opposite. The Dexcom G5 uses several strategies to make this happen–we have not converted to the G6 yet by I am pretty sure that will be true with any CGM.

@Chris, kudos on the blood cell lifespan possibility: I had read about it a good bit, but it did not come to me at all when I was reading the question :slight_smile:

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From my most recent a1c lab report

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Per Sam’s post: anemia?

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Doctors don’t see any signs of anemia. But not sure what they’re looking at to make that assessment.

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I don’t know either… I also didn’t know there were two different kinds at would have the opposite effect on ones A1C

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They’d look at your MCV for signs of macrocytic anemia. When your red blood cells are unusually large (> 100 fL), the cell life is significantly shorter than the normal 120 days.

Glycation is based on 120 day life, hence shorter cell lifespans will result in lower A1C’s.

Macrocytic anemia is commonly caused by B-12 or folate deficiency (related to poor diet). Also can be caused by several diseases or alcoholism, as well as being drug-induced.

Are you taking an immunosuppressant like Azithioprine?

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Thanks for the info. I’m not on any prescription other than lisinopril. I’m on a low carb diet. I believe I eat pretty healthily!

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Is MCV a lab they can order?

It’s usually part of the CBC suite of tests.

You said you’re not anemic … .

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As far as I am aware, I am not anemic. And none of my doctors seem to think I am.

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Does it follow then that when MCV shows red blood cells are unusually small, the cell life is significantly longer than 120 days, and hence, could falsely INCREASE the a1c result?

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A1C aside, just addressing this comment.

If your BG is rising at the time of your BG test, your CGM reading will be lower. (It has not caught up to your blood yet)

If your BG is dropping at the time of your BG test, your CGM reading will be higher, for the same reason.

If your BG is flat, and your CGM is lower, then it’s just a difference between CGM readings and blood readings.

In general, don’t use the A1C to compare yourself to a clinical standard or compare it to your CGM or avg BG. Use it to compare yourself to yourself over months and years.

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That seems backwards. JonDeutsch got a lower A1C than he expected from his CGM readings, but anemia shows a higher A1C than the true value according to my reading of the statement in Sam’s lab report.

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Indeed it does - microcytic anemia

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Thanks. I specifically scanned my CGM at time of blood draw (105mg/dl), and 15 minutes after the blood draw (107mg/dl). Blood draw showed glucose at 130mg/dl.

I recognize they measure different things, which helps explain the delta a bit. But when the A1c comes back at 4.2… it makes you wonder which measure to trust the most!

I acknowledge that every measure has a margin of error of around 20% – including A1c, blood draws, and CGMs. If we extend 20% margins of errors over the worst possible spectrum, we get:

  • CGM: 107 + 20% = 128.4. Essentially the same as the blood draw.
  • Blood draw: 130 - 20% = 104. Essentially the same as the CGM scan (average BGL over 90 days on CGM is around 105).
  • A1c: 4.2 is “equiv” to BGL 74, which is 40% shy of the acceptable margin of error above, over 3 months, given my CGM reporting trends.

So… would any physiological issue create a 40% nosedive in A1c? Do we know how “off” these conditions can make A1c reports?

And, given all of this wonderful data at my disposal :slight_smile:, I need to figure out who to believe.

The conservative side of me tends to believe that the blood draw numbers are more accurate than CGM scans, which would lead me to the conclusion that my CGM is reporting back 20% lower than reality. But, honestly, the moderate side of me doesn’t believe this at all. The moderate in me believes that my CGM, being the one that’s in-between the two extremes, is probably the most accurate. And the progressive in me sees the CGM as the most modern assessment device we have now (and is the one that’s tracking me consistently for over 18 months), and is actually the one to trust!

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Thanks!! I’ve noticed this in myself but this is the first I’ve actually read about it!! My discrepancy is usually around 10%.

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Interesting! I found out I was severely deficient in ferritin (literally so low it was below their measuring capability), and even though my RBC was normal, my MCV had dropped a little, my RCW was higher, and both my MCH and MHCH were a bit low. My A1c had also gone up, which I expected so didn’t think much of it at the time, but I’ve been supplementing with iron, and my ferritin is now in the a bit deficient but not as horrifically range and my CBC stats are all WNL if not all quite to my personal normal yet. I expected my A1c to be about the same, based on my Dexcom, but it went down significantly. However, presumably the current value is probably the better one? since my blood cells are heading more toward normal, but it is more discrepant with my Dex. Hmm.

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Two different kinds of anemia— opposite effects on a1c per the lab

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