Past 3 months G6 sensor errors and poor accuracy

Since early June, my son has experienced an increase in sensor errors with the G6. We also started testing more with a meter and found the G6 isn’t as accurate as we first believed when he started early this year (even with many calibrations). Disappointing for sure. Anyone else experiencing this?

How much sensor error?

Sensor accuracy should always be verified.

If you show me the data, I can try to try and provide some guidance on how to respond.

For instance,

If you talk to the nurses at the Dexcom line, they will talk to you about how to calculate the 20/20 rule. But basically, you can always expect the manual reading and sensor value to differ. The sensor value lags 15 minutes behind the manual stick. That has always been true and is totally accepted as true by everyone.

If the sensor reading is more than 20 points off the manual stick, when BG is less than 80, you wanna keep an eye on that. That value has failed in what we hope to see in accuracy. But, during times when the number is decreasing rapidly, you will commonly see that. Once it stops dropping rapidly, it might taper off.

For instance, see the difference in my readings at noon. The sensor accuracy technically fails there. But, 40 minutes later its accuracy has improved a little.


Now, how we view sensor accuracy has changed over time.

The Doctor that I had ten years ago is the president of the American Diabetes Association in my state. She was among the hardest core on this stuff that I have ever seen. She would not accept any BG sensor readings that were not accompanied by recent manual readings. I was younger than and on a G4, and pretty happy with the accuracy of that device. So, I tended to believe she was a little ‘mean’ about this. Without lots of manual sticks, she would not accept my records and would throw them right in the trashcan, right in front of me and tell me to do it all over again. Pretty heartbreaking.

But, I tend to view her as practical, now. Much more so than a Doc I saw recently, who told me that G6 readings CANNOT be wrong. That perspective is flatly ridiculous to the point of bordering on insane,

Your best bet is to always take the sensor reading with a grain of salt. The accuracy level will depend on the person, might vary according to time, might not vary according to time, will almost certainly vary according to sensor model, etc.

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Thanks for your reply. All good information although disappointing. I guess in the first few months when my son started using the G6, it seemed quite accurate aside from the first 8-10 hours after a sensor change. However, it may be that he was not checking with the meter as often. In the past few months, we have noticed a pattern of the G6 being generally lower that the meter which is not so great if one were to take the G6 values as accurate. We were aware that the G6 may lag behind by 15 minutes or so, going up or down. Thanks again.

I did that same thing in the beginning. Woops.
My sensor almost always reads lower than my machine.
I use a Walmart ReliOn BG machine, which the guys here don’t love.
I use it because it is cheap.

Here is link to details from dexcom site on 20/20 rule.
Lower range 20 means 20 points, higher range 20 means within 20%.

If you’d like, you can calculate the 20 rule on your own. The Dexcom G6 reading must be within:

  • 20% of the meter value when the meter value is 80 mg/dL or higher
  • 20 mg/dL of the meter value when the meter value is under 80 mg/dL
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@Susan16. The issue might also be your meter depending brand\model. Some are more accurate than others which might be the difference you are noting.

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As @El_Ver mentioned you could be having meter inaccuracy. Have you tried testing the readings on your meter to a lab blood draw? I do this periodically with all of my meters by taking a finger stick reading right after I get my blood drawn and saving the reading somewhere so I can compare it to the results of the blood draw later.

Also being a teenager hormones could be playing a major role in BG stability and affecting the accuracy of the sensor. Another issue could be hydration or if your son has a very low BMI that can affect the G6 as well. You could try putting the sensor somewhere else on him, for instance if he is using his stomach have him try his arm.

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You know it is better to know now, than not to know about the accuracy. When the sensors are accurate, he will enjoy. All other times, it will be a secondary warning system at best. And thank you to others, we have multiple meters to ensure accuracy of meters. Changing up the site location is something he will experiment with ahead.

If the sensor fails the 20/20 test, Dexcom will replace it if you need to pull the sensor and replace it with a new one. But, always say that it was on the abdomen because they require that. They will only replace 3 sensors per month…or 3 per phone call - I forget.

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Good to know. Thanks.

Do you ever check your meter against the plasma glucose number for blood drawn at the lab? I do this as often as the opportunity presents itself. Unless you have confidence in the fingerstick number being close to your actual glucose level, your observations about any discrepancies between a meter and a sensor can mislead.

@Susan16 as I was reminded yesterday, hydration also plays a large part in Dexcom accuracy. Dehydrated = wonky numbers.

@Terry4, you got a Loop user asking questions on another channel. You wanna take that? Should I send him to you? I think he would rather talk to you than me. BradP is out of commission due to surgery, but he will chime in eventually I imagine.

Thanks Terry and everyone. Lots of good ideas. We have multiple meters to check. AIC only once a year and I will remember your idea to check all our meters then.

That’s the thing I don’t seem to get. If the sensor reading has any chance of being unreliable (even if it’s accurate say 90% of the time), how can you turn on autopilot and have control iq working - how is it possible that it’s been medically approved for use? It’s pretty crucial that you get accurate readings in order for control iq to make predictions and make decisions on your behalf isn’t it?

I‘m starting to think I may have developed a misconception that most if not all people on CGMS has auto mode turned on (ie control iq or equivalent). With the inaccuracies that has been reported I can’t see how it’s possible they can depend on control iq. If there is a chance of inaccuracy, isn’t it a risk that may send you into a hypo or hyper state?

Why is that? Is that the only approved site? All the YouTube tutorials I have seen on dexcom cgms, people put the sensor on the back of their arm🤔


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They didn’t do a study for the arms. So it was never approved by the FDA, however everyone does it.
I use the back of my arms because that’s the best place.
It doesn’t bend and it’s out of the way.
On my abdomen it will peel up and dislodge.
I’ve seen people use fore arms

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For the vast majority of people the Dexcom G6 is accurate enough for the automatic pump systems to use safely. If the CGM is not reliable for you then you shouldn’t use any automatic system that uses that CGM. I use the Dexcom G6 with Tandem X2 and Control-IQ. and I have no problems with the Dexcom hence no serious problems with the automation. However I do a finger stick test every morning and if I do not feel like my readings are matching my symptoms I will check my BG via finger stick without hesitation.

Since you are going to be trialing the G6 I would not let the pump adjust your insulin until I had worked out any kinks with the G6’s accuracy.

Do you get irritations?

For me its a bit weird, my abdomen is the best place to insert anything, however, it’s overused and blockages started to happen so I had to give it a rest. It had little to no irritation and doesn’t sting when i bolus.

Recently I had to insert my canuallas into other places like my thighs and buttocks and lower back to give my tummy a break and it wasn’t easy because it will irritate and turn red and become itchy - and most of the time it hurts, i was always consciously aware that something was there! I’ve since tried to put a piece of tegaderm on my skin and cannula on top and it seems to help relieve the pain somewhat but still not completely. I suspect the same thing will happen when i insert a sensor on the arm.