Quick Dex G5 question

Hi there, I’m new to using the Dex G5 and Tandem X2 Slim pump combo. I have read the Forum Dexcom best practices. Whenever I enter a calibration number into the pump and the smart phone/iPhone, the Dexcom app and Tandem pump will average between the number from my meter and the current number on the Dexcom. So for instance if the meter says 100 and the Dex says 85, when I enter calibration number from the meter the pump and iPhone app will change to 92, not 100. If I enter the number again/a second time, they will go ahead and match the number of the meter. I’ve read people mentioning the same thing. I didn’t know you were only supposed to enter a value into one or the other so starting tonight I will only calibrate the pump and measure how long it takes for the iPhone to correct itself. But I’m wondering what is the recommended way to deal with the averaging issue? Let it run with the averaged number between the current Dex and the meter calibration, or should I always input the meter value a second time and force it to match the meter? Thank you for your help!

Welcome, @Jritchie70! You may want to introduce yourself in the welcome thread!

Only enter it once and do NOT force the Dexcom to match your hand meter. Your hand meter is only accurate to +/- 20% or so. Your Dexcom is averaging numbers over time, and is doing a better job than any single number out of your hand meter. So it is almost always better to let the Dex do the averaging.

It happens, in particular when the Dex sensor fiber has moved, that your Dexcom ends up way off, maybe 30 mg/dl or more off. In that case, you are better off hand metering twice and entering both calibrations over 5-10 minutes, once after the other, rather than double entering one value.

Do note, btw, that the Dex only averages when your hand meter value is ABOVE the Dex. If it is under, the Dex will go to your value or very close to it.

Also note that the number displayed by the Dex right after you enter your hand meter number is NOT the number that it will eventually display on your track, it is an intermediate value with no practical signficance.

No, that is not a correct statement.

The FDA requirement is that 99% of the BG meter values must be within 20% of the true value.

It depends on what meter someone is using.

Just because 99% must be within 20% doesn’t mean all meters and all readings are only accurate to within 20%. Those two statements aren’t even close.

@Jritchie70, what kind of BG meter are you using?

True. But when you look at actual tests, you find out that only 2 meters largely beat the metrics, and that one if them is not commonly used in the US. With its lifetime averaging, the Dexcom does a better job in most circumstances.

  • There are 7 BG meters that beat the Dexcom across all BG ranges.
  • 8 meters that beat it for ranges from 70-179.
  • And 8 that beat it for BG ranges >180

This is from the 2016 Journal of Diabetes Science and Technology.

It really depends on the meter. A lot of it also depends on the person, and how well CGM works for them because CGM accuracy is clearly not the same for everyone.

@Jritchie70, over time you will learn how comfortable you are with the Dexcom versus your BG meter. It varies, but eventually you will know which readings you want to trust.

A calibration entered for the G5 via any device (Dexcom Receiver, Dexcom app on Smartphone or Dexcom software running on an integrated Tandem X2 pump) will be sent to the G5 transmitter and then relayed back to any other connected devices. Within 10 minutes, you should be able to look at any other device and in the settings see the last calibration to prove to yourself that the calibration is in there. Entering it in multiple devices will essentially be the same as entering it in twice to the same device. You do not want to do this.

In terms of calibration - basically - relax about the numbers.

Use a good meter with clean hands. Enter the calibration. Let Dexcom do its thing. Do not obsess with what the Dexcom reports immediately upon entering the number. If the Dex has 100 and your meter has 85 then enter the 85 as a calibration and let it be.

The accuracy of the Dexcom G5 improves after the first 24 hours (as compared to the first 24 hours). At this point you should have three calibrations in the G5. The first calibration (two fingersticks) for the initial startup. You should then have a 12 hour followup calibration and another one 12 hours after that. (No need for exactly 12 hrs - plus or minus.) At this point, the Dexcom G5 should be having pretty good accuracy for the remainder of the seven day sensor run.

Calibrating with straight and level cgm graph will give better results then calibrating when the cgm shows a diagonal arrow. Calibrations simply should not be done when the cgm arrow is straight up or down.

For the vast majority of Dexcom users, these basic calibration techniques provide outstanding results. For a small number of users, (for one reason or another) they are not able to achieve the same level of accuracy with the Dexcom cgm.

We have switched to the Contour Next One meter as it is our opinion this is the most accurate meter we can use which will provide the best calibration data for our Dexcom G5 system.
According to FDA submissions for Contour Next Blood Glucose Monitoring System (BGMS) it is +/- 10% for more than 96% of tests. The FDA currently requires (at least) +/- 15% for 95% of tests. (Across the entire range the BGMS can measure.)
(Older meters would fall under less stringent requirements. These are the newer FDA requirements as of late 2016.)





Nice post, @Thomas!

This is what the measurements of 17 BG meters look like (from Ekhlaspour 2016 J.Diab.Tech, also quoted by @Thomas above):

which is why forcing the Dexcom to always follow the latest measurement of your hand BG meter is not the right procedure, and why Dexcom does not want you to do that.

I also agree with @Eric and @Thomas that some meters, such as the Contour Next, are much better than others.


Sage wisdom


I’m using a One Touch Ultra 2

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And One Touch is all we can use under my insurance coverage

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@Jritchie70, the Ultra2 is a mid-range meter, rated Very Good by Consumer Reports for instance, but not Excellent. In the OneTouch family, the most precise, I think, is the OneTouch Verio (although not as compact as the Ultra).

Here is the MARD (mean absolute relative difference) comparative table from the same Ekhlaspour 2016 J.Diab.Tech article I mentioned earlier:


I believe the OneTouch Verio and the OneTouchVerioIQ have the same sensing technology (but I am only surmising).

I wish I could use Verio but the strips are different and not covered under my HMO

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They won’t allow other OneTouch meters than the Ultra? Many insurance companies will allow any OneTouch meter, but only those.

That said, the Ultra is not a bad meter, comparatively. We have one and use it occasionally: compact, portable and easy to use.

I was using OneTouch with a Dexcom G5 for a while (both One Touch Ultra and Verio, the only strips covered by my insurance) and did have trouble with Dex accuracy. My average BG as implied by an A1c test was typically higher by about 10-15% than the average BG measured on the Dex G5, causing me to question all the Dex readings.

I eventually figured out that I can buy Contour Next test strips on Amazon for less money than my insurance copay for OneTouch strips. When I switched to Contour Next I became more satisfied with the Dexcom accuracy. I don’t have any hard numbers, not being a stats guy, but am convinced that (for me) the OneTouch strips were reading consistently low by around 10% compared to my “assumed actual” BG as measured on the Contour Next, and using those test strips was throwing off the Dex calibration. I’d recommend fine tuning your Dex calibration processes by trying a different meter/strips setup for those calibrations, starting with a new sensor session.

To make a long story short, my opinion is the Dex is a very useful tool for tracking trends and is especially useful for catching impending highs or lows before they occur. It has it’s limitations but with careful calibrations these limitations can be discovered and worked around.


I use a One Touch Ultra, and while I don’t think it’s that reliable at extreme blood sugars, I generally avoid calibrating then, so it works fine for me with the Dexcom. Generally I would avoid calibrating the Dex if very low or high, even if the arrow is horizontal, and waiting until you are back in range, since both the meter and the Dex will be more accurate then. There’s no need to urgently respond to calibration prompts, so while you don’t want to calibrate until it prompts you (unless it is very off), I’d suggest waiting until you’re in a range when they are going to work best (for me, 60-200 seems about right for reliable readings), as well as in a flat trend.

Also, my A1c readings have either been exactly the same as my 90-day estimate from my Dex or once .2 lower calibrating off of a One Touch Ultra, so it can work fine. People’s experiences vary across of these things though, so you’ll have to experiment to see what works for you.

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And meter accuracy also is not the same for everyone depending on how careful they are with hand washing, getting the BG sample, and applying the sample to the test strip. Excellent technique really helps BG accuracy, as well as CGM accuracy.

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