Glucose meter + CGM accuracy

I read this a couple of weeks ago and decided to switch to a different meter. I chose one of the top 6 meters, based on some non-accuracy factors. Since then, my Dexcom readings have been much more accurate! My 12-hour calibrations have been almost spot on since I switched. I wonder how many people have problems with Dexcom that are actually caused by inaccurate glucose meter readings used to calibrate the CGM sensor…


My choice of meter was actually decided for me by my pump company. The meter THEY supplied me with communicated wirelessly with my pump. It also is really small and has a rechargeable battery and USB connectivity with my computer. (Also, my pump readings are transferred to my computer using my meter)

I think most using pumps use the meter that comes with it because of the convenience. If medical device companies REALLY had our needs in mind, all the pumps and meters and CGMs would be able to talk to each other. Just because that’s the way it should be.

MY meter accuracy is usually most influenced by the cleanliness of my hands. My DECOM accuracy has very rarely been off. Most of the time it was, it had more to do with the sensor than the meter.

Years ago, when using a Dexcom G4, I found my readings erratic in that the CGM frequently did its own thing and I seemed to always be left wondering which device, CGM or meter, was my real reading. I was using a FreeStyle Lite then. I was meticulous about hand cleaning, not overcalibrating, etc. Frustrating beyond belief! I eventually started testing more and discontinued using the Dex. Fast forward about a year when I did a little science project and got my hands on five different meter brands, 50 strips for each, and set out to evaluate them over a three week period. I was comparing them to each other and to themselves for repeatability. Essentially, I threw time and money at the problem but I found MY answer. The FreeStyle, although highly rated and respected by many, was not cutting it (although it came in second place). The Contour Next seemed to work best for me with very high repeatibility and readings which I was able to prove matched laboratory results. So, I went back to a Dexcom, this time a G5, and the Contour Next One, and my Dex is spot on with meter readings. It’s a whole different game. Yes, I know there are other variables in play but you’d have a hard time convincing me the meter isn’t a big part of my CGM’s stable reporting.


I have always been one of those people that the Dexcom just wasn’t as spot on as I would like it to be. And I just saw my doctor this week and due to sensitivity and carb ratio numbers, I don’t have a lot of room to have numbers be off. He said I should call Dexcom to see if there might be something I’m doing wrong with insertion because the numbers shouldn’t be that far off. My problem is sometimes the numbers are right on and sometimes they are off by 30-40 points. So because I never know if this is the time it’s right on or is this an off time, I continue to test each time. I don’t mind, it’s what I have to do but I would love for it to work like everyone else.
A different meter would be a possibility but I am in a trial and have been using the one they supply test strips for. And than I realized that my strips were going to expire so I had to start using my meter. And I haven’t noticed any major difference. And of course I can only get best price using the one my insurance will cover. So using 2 different meters in the past month hasn’t really changed anything. So I guess I am just going to be one of those people. Or I guess maybe I should call and see what they have to say. Just hate calling and whining. I am a suck it up and deal with kinda person. Oh well, I am always so glad when others can find a solution! One day I may also find a solution! (Maybe a cure!)

@Sally7 - What is your Dexcom calibration procedure?

Sorry, Tim about how long it has taken for me to get back to you. But here’s my story and I think I have had this discussion before.
I followed the Dexcom guidelines as stated in manu. Insert, wait for warmup period, do two tests. Only calibrate when receiver asks for one or if way off. Don’t calibrate with arrows up or down, preferably when level.
Then there was the go to person here on this site that knew everything about Dexcom and she swore by the results they got for her daughter. Her recommendation was when starting up to do a test wait 15 minutes and do another one. And as I wasn’t getting super result, I tried it. Really didn’t help but didn’t hurt/change anything.
So when I asked for help awhile back, I was told to not use my arms (which I did as I am a major rotation person. Don’t like to use same area again and again-all those early years of site rotation beat into my head. So then tried just abdomen area for a month. No difference.
So my endo last week and we discussed the variances I can have sometimes. And my problem is, there are 3-4 tests that are close and than 1 that is off by 30 points.
30 points is a huge deal. I have a correction of 1-110 and carb ratio 1:13. So yes being off can have some very bad results. So while sometimes it’s right one if I try to dose from it and it’s that one that is off by 40 points, I’m in for a bad day.
I would love for it to work as well as it does for everyone else and while it sounds like I don’t like it, I love my CGM! I wouldn’t give it up. But my concern here is if the FDA now thinks we all can dose from it, the insurance companies won’t be far behind and will only allow 2 test strips per day. I am still testing around 8 times a day, more if driving/exercising/eating pizza etc.
I know I should probably call Dexcom but I have just come to realize I’m just one of those people. I don’t always fit the norm.

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As FYI, Dexcom G5 manual states:
"… enter two back-to-back fingerstick measurements …"
in terms of sensor start-up calibration.

IMHO, adding a 15 minute delay between the two fingersticks for sensor startup doesn’t make sense.

We only calibrate the G5 when straight and level (for at least 15 minutes) and between 70 and 160 BG. Even if the Dexcom is asking for a calibration, if the BG does not fit the mentioned profile then we ignore the request for calibration.

Yeah, I’ve seen the recommendation to wait 15 min, and it makes no sense to me mathematically, since if the point is to minimize error from the meter, you want to estimate the value at a single time point. That way the only potential source of variance between your two readings is meter error, and the Dex will estimate an average of them typically. If you want 15 minutes, you’ve now confused things further, since both meter error and change in blood sugar are both being estimated, so more potential error can get into the mix. If you were certain that your meter was like 99% accurate and always totally reliable, it might make sense to do the 15 min thing, since you’d be only estimating the change and not need to consider the meter error, but that’s not how home meters are.

@cardamom. 100% agree. I take as many finger sticks as it takes to come up with a good BG value. Then I input this value twice. No way that the rocket scientists at Dexcom can screw this up.

Thanks for the response! Like I said Rose was the go to person for a long time here and since she was getting such great results for her daughter, I tried her suggestions. I must say it didn’t really make any change and I was still and currently am still at the reality I can’t dose from my Dexcom.
I also only calibrate on a horizontal arrow and only between 80-200. If not in target I also wait. And as I said, I can have most numbers throughout the day within 10-15 points but there is always one that is off by 30+ points. So, that means for me I shouldn’t take the risk to dose from it. And I guess I could because I wear it all the time and it would catch the lows that would happen if it was off but than I get in that roller coaster ride and really not worth it. Just easier to test before dosing and realize I am one of those people that don’t fit the norm. I’m kinda use to it as most of us with diabetes are.
I was one who always ran low before my period started vs most run higher. I am one after all these years who is still very insulin sensitive etc. you know this disease doesn’t always follow he rules.
Thanks for the help and glad I am on target with my Dexcom protocols. I guess I should really call Dexcom & see what they think. Thanks again!

The Dexcom G6 is planned for late 2018 to be shipping to customers. Obviously depends on when the FDA decides to approve.
The G6 will have a number of changes including (but not limited to) changes to the sensor.

Perhaps you might get lucky and find your readings are more consistently accurate with the G6?

For me, the Bayer Contour Next consistently runs 10-12% higher than the FreeStyle Lite in side by side and I was getting hypo symptoms at higher BGs. My Endo told me order is AccuChek, FreeStyle then Contour Next, so I switched back to FreeStyle. An additional advantage with my insurance, I get the FreeStyle strips locally instead of mail order. I manually enter BGs into my 630G.

This brings up a question about comparing HbA1C comparisons between pumps/CGMs. Is any change/improvement due to the pump/CGM or just from shifting the reference? For example, shifting from FreeStyle to Contour with the same BG target would lower my HbA1C by about 0.3.


Gary Scheiner, who is highly respected in the 'betes world, has a comparison.

I am not speculating on your endo, since I don’t know him, but I do know that there are some endo’s who are influenced by their own personal familiarity with a product, or a relationship with a representative, or ease of dealing with insurance approvals. All kinds of human factors enter into their recommendations.

Gary has no dog in the fight, he is just reporting his results. So just something to consider.

I have been thinking about switching to the Contour Next One. A number of people on these forums have said they like it. In the mentioned article by Gary Scheiner the meter shows crazy accuracy.

I have no complaints about our One Touch Ultra but maybe the Contour Next One is an improvement.

Better is better.

Also the Contour Next One has Bluetooth so I am still hoping when the Tandem X2 gets its Bluetooth activated that maybe it will allow some integration with meters (which would clearly also need to have Bluetooth capabilities).

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I was calibrating in 1 setting with 2 sticks back to back within 1 minute of each other. Now I set a timer for 10 minutes after 1st to head back to do 2nd. Have not noticed any differences.
Note that if Dexcom does not readily adjust to read close (within their specs) to a daily cal, they recommend waiting 15 minutes to perform a follow up cal. And if still off wait and recal 15 minutes later again.
Is only unknown that the calibration is for to determine the BG offset OR is there a unknown scaling factor to solve as well (ie does a sensor measurement of X units increase/decrease convert to the same BG change in all people)?

Just a side note to the Contour meters. I loved it because if you didn’t get enough blood on the strip, you could add. Every other meter I have ever used, if you didn’t have enough blood on the strip you had to do another test. So that was another strip down the drain. I so hate wasting strips. And wow, on top if that it has always been on the higher end of most comparisons. Win, win here. For me, it is insurance companies and what they will pay for. One day they will figure out what works will for one person with diabetes, will not work well for the next. Big sigh😕

We read about that. Might be the deciding factor in switching to the Contour.

Yeah, I miss my little Contour. Hoping once I finish this trial I’m in , I can fight the fight with the insurance company to go back to the Contour!

When you are done with your trial, you can get the Contour strips for very cheap on Amazon. 22 cents if you buy a big batch.

I’m using the Freestyle lite now. I really like it - it has a backlit screen and a light for the test strip, it only needs a tiny drop of blood, and you can add blood for up to 60 seconds if you don’t get enough on the first poke. The only thing I dislike is that the elastic band that’s supposed to hold the lancing device in the kit is way too loose. The lancing device just falls out every time while I’m zipping up the kit. What an annoying design flaw!