Dexcom G4 accuracy issues

Hello Dexcom Users/Parents,
My 12 yo has been using the Dexcom G4 since February and it has been pretty accurate. In the last two weeks, we have had random readings that are really off though. Sometimes as much as 100 pts off, which is scary. This has happened with two different sensors (we change them out every 7 days). We are back to losing sleep at night, not being able to trust the monitor. I called Dexcom & after a series of questions, the rep recommended changing out that sensor & overnighted us a new one. I expressed my reservations, as this happened with two different sensors in a row. I asked about the transmitter, which is just out of warranty, and she thought that was a good idea and I should inquire with my DME about getting a new one. Don't know yet if my insurance will pay for it or if it will solve the problem. Any suggestions? Thanks so much!

I have had the same issues with it in the 3 months I’ve had it…it’s fairly accurate most of the time, but is often 50-100 points off at least once per day. It’s a crap shot as to guessing which time it might be inaccurate. Dexcom is always happy to send me a replacement, but geez, I don’t want to need a replacement–I just want them to work! All the times I’ve called Dexcom, they claim the issue wouldn’t be with the transmitter, but with the sensor itself.

Sometimes a CGM can be no more than a random number generator...they will never take the place of meter that measures Whole Blood. I hate to sound like the brochure but your CGM was not intended to replace your finger stick tests. Your CGM can lag behind, it can flat-line, and the sensor can move, get Jard around, causing a rapid change in calibration, I usually start seeing trend alarms and double up or down arrows when this happens.

Although CGM accuracy is better than 5 years ago I would not trust my life with it....

Karen, my mother still worry's about me and my diabetes, I'm 60 years old and she is 85. Get some sleep it's not going away... you should gain better control through watching trends and making small adjustments to the daily insulin dose.

A CGM is a good parachute but on rare occasions they also fail to open....;-)

Sounds like the way mine was behaving. A new transmitter after 10 months gave me back the readings I love!

Denise, That's got to be frustrating! Thanks for sharing about that. Our Dexcom has been off sometimes as well but not in the range that you mention -- until recently. Of course during rapid increases/decreases, the Dexcom and the meter read VERY differently. Interesting that Dexcom wouldn't even mention the transmitter. It's under warranty for the first six months. Perhaps the transmitter is faulty! I was surprised that the Dexcom rep just thought that a new sensor would solve it -- and that she didn't even bring up the transmitter. Good luck with your issue.

John, I laughed when I read your comment about your 85 yo mother still worrying about you. Definitely understand that. I don't know if it's his Celiac but our son is very labile and we have checked frequently for years -- and been glad we did as we have caught some unexpected lows. The CGM brought us some relief! ...Until two weeks ago. I have read people's comments saying that they treat their BG based on the CGM alone but we definitely don't do that. You are right, the trending from the CGM is most helpful. Truly, it has been quite off over the past couple of weeks so we'll hope for the best with a new transmitter.

Jrtpup, that is really helpful to hear. I feel encouraged! Thanks, I will get back to the process of getting that transmitter... I hope my insurance pays for it!

Has he started taking somthing that could be reacting with the sensor????..just a thought....

Good thought. The rep did ask if my son is taking Tylenol, I think. But no, no new medications. I talked with the CDE and she said that the transmitter usually needs to be replaced 1x per year -- if you use it often (we always use it!). So, I'm hoping that will fix it!

Karen, is it just totally bogus no matter where his BG is, or is it mostly at higher values, still being reasonably accurate lower -- say <110? The technology is already spec'd to be less accurate at higher BG's than lower. This is because the useful span of accuracy for the enzymes used on the sensor is quite a bit narrower (about 100 mg/dl) than we deal with (under normal circumstances), 200 or so. The system was deliberately designed to nail that delta to the hypo end of things, rather than the hyper end. So, it's most accurate in the low range -- 40-100 -- than up high. When BG starts to get about 170 or thereabouts, the inaccuracy increases.

One of the upshots of this is it's better to calibrate lower than higher. If you calibrate high, it'll can make things less accurate down in the hypo range.

If you're getting reasonable accurate down low (+/- 10), then don't sweat the high end, which can easily and routinely be off by as much as 40 or 50.

And always rely on actual fingersticks to make treatment decisions. The CGM is a canary in a coalmine, not an oxygen bottle :-)

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Careful... the important drug is acetaminophen, the main ingredient in Tylenol.

There are many, many other drugs that contain acetaminophen, but of course don't say "Tylenol" anywhere on the label at all (that's a brand name).

For example, vicodin is a combination of hydrocodone and acetaminophen, but many people don't think it has "Tylenol" in it (which, strictly speaking, it doesn't).

Make sure anything he takes doesn't have acetaminophen. Also make sure the school nurse knows that any product containing acetaminophen can mess his CGM up for as long as 8 hours.

Won't damage it... just won't work right until the acetaminphen clears his system.

Leading up to nine months, I started getting more ??? readings and some inaccurate nose dives when I compressed it against my mattress when sleeping. Around 9 months the warning came up to get a new transmitter. The new transmitter cured most of the ??? complaints. My insurance paid for the new transmitter with no questions.

On the whole, I've found the G4 to be accurate and reliable but from time to time it is way off. I fingerstick test a lot and don't rely on the G4 for most treatment decisions unless I know it's been correlating well with my fingersticks.

I don't see how the CDE can claim the transmitter lasts a year, when the G4 just came out last Sept. Many TU members have reported problems starting around 9-10 months. The Dexcom Seven Plus transmitter definitely lasted > 1 year, and had a 1 year warranty. The G4 only has 6 month warranty, and not surprised that it would last 6-10 months of 'real' use.

I've had my G4 about 9.5 months, and just starting to get more ???, and more inaccurate high readings. When I can keep my BG relatively flat/slow rise or fall, and under 160, then it's still very accurate. But I haven't been able to get > 10 days for the last 3 sensors - too many ??? after 7 days.

I ordered my new G4 transmitter, and no problem with insurance covering it, due to the 6 month warranty that has expired for me. I'll start using the new transmitter at my next sensor change.

Yes, the rep may have said Acetaminophen. Good information to file away -- Thanks!

Interesting to hear about your experience, Terry. Glad it has worked well and your insurance gave you no trouble about getting a new transmitter. We had an inaccurate nose dive once a few hours after we changed the sensor while my son slept. Completely baffling! Hmm, I wonder if he was sleeping on it, too.

Very helpful to know that most G4's transmitters are lasting 9-10 months! Perhaps the CDE was categorizing the G4 with the Seven. My son's monitor has definitely been less accurate with higher BGs as well. So from the feedback I"m getting, it shouldn't be difficult to get a replacement from insurance. Great info -- Thanks!

Today his monitor said he was 80 & he was actually 48. He had been slowly dropping though and should have taken action earlier... Really interesting info about accuracy! We've tried to avoid calibration when diagonal or up/down arrows are present. We'll keep that 170 threshold in mind.

With the new one they overnighted, I think I'd try going an extra week and seeing what happens. My G4 seems to be more accurate the 2nd. The transmitters are a disposable item that dies after 6 months, so if your insurance says 'no' argue with them about it.

Also, don't over calibrate. Don't calibrate when the BG is rising or falling quickly. Do it when asked and maybe a couple times a day when the CGM is off. Don't calibrate when your BG is over 200 since it generally isn't accurate in the high range. What BG meter are you using to calibrate it? Some are less accurate. At start up, I generally take 3 BG readings and input the 2 that are closest together. Then when inputting other BG tests, if they aren't close to the CGMs current readings, I'll test twice to make sure the test isn't off.

BG monitors can be off too. How close is test/retest with the BG monitor?

80 and 48 aren't that unreasonable when dropping. 10-20 is from the lack time, and then strip accuracy often isn't very good.

I found the same issue with the g4.I have been wearing it for 2 weeks and see how bad it lags.When I bolus it also seems to sense the insulin before the food and it will often go off for a low when in essence my bloodsugar is normal.I don’t correct or eat anything if it tells me I am low or high without doing a fingerstick