Dexcom vs Labcorp Results for A1C

Hi everyone -

My endo sent me to Labcorp to get my A1C test and I was absolutely shocked to get a result of 7.6!

My Dexcom shows a 90-day reading of 6.9 (before that it was a 6.7 for the previous 90-day period). I’ve been using my Dexcom CGM for years now, and calibrating it every so often with blood stick glucose monitor with One Touch and Freestyle and the results on the glucose monitor are aligned with what I’m seeing on my Dexcom.

I can’t believe that the Labcorp results could possibly be correct - the difference between a 6.9 and a 7.6 is huge!

Has anyone else experienced this with a difference between Labcorp vs Dexcom results? What could possibly be the reason for the difference?

Thanks for any inputs, comments!

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Yes, it is good that your endo sent you to Labcorp because endo offices often have non-certified equipment they use to show A1C results, and they can be +/- .5% from a lab that uses certified equipment. That, however, is not your issue. You say that your Dexcom shows a 90-day reading of 6.9 but you don’t say that your Dexcom is showing GMI vs the lab which shows A1C. These are two different tests that are obtained using interstitial fluid vs using a blood sample to read A1C. These values tend to track reasonably parallel results, even though they are not obtained in the same fashion.

In most cases, they are within +/- 0.1%, and in 50% of people within .5%. You, showing a 0.7% difference although, rare, is not way out of bounds. You may want to have another A1C done at Labcorp or probably cheaper at UltaLabs in a couple of weeks and make sure there is no glitch in your original result. A1C tests are $21 at UltaLabs and since you had one recently, you can check what you paid.


Congrats on your use of more than one data mechanism to analyze and assess your overall glucose control. I agree with @CJ114’s point that A1c and GMI are two different types of number. I find that my GMI runs about 0.5% absolute difference higher than my A1c.

The A1c has been around for many years and I find that it is held in higher regard, especially among doctors, than it deserves. It doesn’t reflect one vitally important aspect of overall control, variability.

Good variability can, however, be integrated in one measure, time in range (TIR), provided it is adjusted. That adjustment is that the range needs to be changed from the overly generous 70-180 mg/dL down to something like 70-140 mg/dL. This improvement of the target inherently builds in a large improvement of variability.

I no longer use my A1c number to assess my quality of glucose control but let my TIR number assume that role. I’ve even convinced my endocrinologist of the value of that perspective.

To make this scenario work, following a narrowing of the glucose range, is to check your fingestick meter a few times against a lab glucose value. I do three consecutive finger sticks and use the average to evaluate accuracy.

Once you can depend on the reasonable accuracy of your finerstick meter, you may increase your confidence on the CGM data.

Again, I salute your interest in comparing your CGM data with other measures like A1c. It puts you far ahead of the average diabetic. I find that paying attention to the data invariably leads to marked improvement in that data. I encourage you to shoot for better. Over time it will produce remarkable results!


Hi @Terry4 and @CJ114 - Thanks so much, I have a ton of respect for the both of you - have read numerous comments of yours on multiple posts in the past, so really appreciate you both responding.

I wasn’t clear on the difference between GMI and A1C previously but your comments have helped to explain it - thank you. They’ve never been so markedly different before for me. The thing that’s most confusing to me though is how knowing this actually translates into daily management - if it’s true that my A1C is really a 7.6, but I’m relying on my Dexcom and finger pricks to make treatment decisions each day/meal…I’m asking myself, does that mean that I “really” should be taking a whole lot more insulin on a daily basis or changing my diet. I’m sure I’ll go down a rabbit hole on this now : ) You’ve both given me a lot to think about so I really appreciate it.

Bottom line short term is that my endo is going flip out when they see that A1C lol and I will get a lecture : ) I thought all this time I was doing pretty ok. My TIR for past 90 days in at 77%. Onward…I’ll.just keep on truckin’.

Thanks again to the both of you!

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There’s a lot to consider from your perspective but I encourage you to look at it from a relatively simplified context. There are really only two insulin levers that exert any control: timing and dosage size. Master those two and you’ve solved the challenge, at least until the next meal!

I suggest investing in getting better at insulin timing as the most likely factor that will produce tangible results. Delivering a better timed pre-bolus produces amazing control for the effort expended.

In addition, it doesn’t cost you anything to look at your TIR results in the 70-140 lens; give it a try and see if it doesn’t do something for you.


@Terry4 Thanks so much, Terry! Yes! Love the idea of better insulin timing - I had totally forgotten about that with my current work schedule, but I do recall a few years ago doing that diligently with intention, and it did help quite a bit (and I felt better too) Will also look at TIR as well. : ) thanks again!


Interesting discussion. Before CGMs, my Endo tracked A1c carefully. She now has switched to TIR as her key metric. Of course, when she gets a new T1D patient, I suspect she relies on A1c to get a quick assessment of the situation.

My Primary Care Physician still likes to add in A1c when he scripts a general blood panel of tests. I have tried to explain CGM and TIR, but have not had much success. No matter, since I rely on my Endo for T1D care.

My personal view is Endos still relying on A1c instead of TIR for patients with CGMs are just not keeping up to date. Of course, there is nothing wrong with having A1c tests from time to time to double-check the CGM accuracy. For example, a patient could have a BG meter with the strip code set wrong, then use those wrong readings to calibrate their CGM. If they did that regularly, the A1c test might flag the problem.

But in Jolene7’s situation, and assuming she is properly calibrating her CGM, the recent A1c test may not be helpful. Her LabCorp A1c was .7 above her GMI of 6.9, or 10% higher. As pointed out, GMI is arrived at way differently, but many have found it tracks reasonable well with A1c.

I agree with the suggestion that she get some more A1c tests before reacting to the one LabCorp test. That test might have been an anomaly.

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Not only that but a new T1D would not yet have a CGM. It is still less that half of the diabetic population that even have CGM’s so for now, doctors will continue to do A1C tests.

I find the Dexcom completely unreliable. I only use it because I have hypoglycemia unawareness.



Just to circle back - Took @CJ114 suggestion and went immediately and paid out of pocket for A1C test (they use Quest). Result: 7.2

So, this result was definitely better than LabCorp result of 7.6 - it was a wee bit closer to Dexcom 6.9 average for 90 days…but still not thrilled…interesting to see differences for sure!

Thanks again everyone - appreciate all the suggestions and comments

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Good for you for double checking your A1c. Trust your gut instinct that you can do better. It’s not easy but you are worth the effort! Good luck to you.

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Thank you, @Terry4 - you’re a true inspiration to so many of us : ) - really appreciate the encouragement!


Can’t speak to Dexcom and Labcorp but many years ago UHC only contracted with Labcorp in the state I was living in. There were so many problems that a large group of Endos worked with Quest in that state to give diabetics a special rate. UHC dropped Labcorp the next year

Good point. TIR vitally important
Thank goodness my endo looks at boo

Just popping in to say that I have the opposite result - my A1C is typically 0.9% lower than my GMI from Dexcom G6. This has been true for as long as I can remember, through cross country moves (and thus different testing facilities). I always assumed that this had to do with my biology - as the A1C depends on the turnover rate of your red blood cells, I suspect that my RBCs turn over more rapidly than the 3 month average that is always cited. Perhaps your RBCs turn over a bit slower than average.

For what it’s worth, most of the historical data is broken down by A1C (because that’s what they had), and so I think that A1C is relevant when using those studies to assess risk of complications. At the same time, I pay attention to my Dexcom data because I do think that this will be the standard of care moving forward. And of course I want to keep my numbers down and fairly stable on a daily basis.

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