A couple of weeks ago I posted about how to make your Dexcom sensor last longer than seven days. This week I want to talk about some reasons for Dexcom inaccuracies.
The dexcom is an INCREDIBLE tool for diabetes management, but that tool is only incredible when it’s accurate. I get so sad when I see people giving up on their CGM too quickly because they got off to a bad start. Tho most of the time it works great (I’d say 95% of the time) there are times when it’s off.
Here are 12 reasons for Dexcom inaccuracies:
1. Bad Robot Parts
Just chalk this one up to bad luck. You got a lemon. Just like any piece of technology or machinery there are instances where it’s just got bad robot parts. Whether it’s a faulty sensor, transmitter, or receiver, it’s just junk. That’s okay tho! Dexcom has an incredible customer service and tech department that work, literally 24/7, to help you in the instance that something like this happens to you. They know how important their product is to your diabetes management and will ship you a new [insert broken Dexcom part here] overnight or 2-day air depending on the urgency.
2. Bad Insertion
Some people find that their dex is more accurate on certain parts of their body than others. This could be for a few reasons including how close to a muscle the sensor is, the quality of interstitial fluid in that area, or the amount of scar tissue in that area. The longer you have the Dexcom the more you’ll learn which parts of your body work the best.
3. Sliding In and Out
Check your site. Is the bandage lifting up or coming loose? If it is, that’s probably why your readings are off. Every time that sensor slides in and out of your body, due to the adhesive coming loose, it has the potential to throw of your readings. Key work here being potential. Sometimes half my bandage is hanging off and half the sensor is out and I have great readings. Sometimes I’ll just notice a little lift and my readings are totally off. If this happens to you it’s an easy fix: tape that sucker down!
4. Too Close to a Muscle
Typically when the sensor is too close to a muscle or near the fascia layer surrounding a muscle it will give you erratic readings (usually a false low). For me, this is especially true when I sleep on it AKA compression low.
5. Compression Low
I’m a side sleeper, so when my dex is on the outside of my leg or arm you bet I’m being woken up in the middle of the night from a compression low. A compression low is when your Dexcom is compressed, usually pushing close to a muscle, and giving you a false reading of a low. If this happens, roll over and sleep on the other side and you dex will balance back out.
6. Dehydration
Dehydration affects the circulation in your interstitial fluid big time; which is the fluid that the sensor reads. So if you’re getting wonky readings, try hydrating yourself.
7. Exercising
Exercising can TEMPORARILY throw off your readings because the glucose in that particular area (where your dex is) might be burning off more glucose than the general circulation. Especially if your Dexcom is on your quad and it’s leg day, for example.
8. It’s too Close To Your Pump
The interstitial fluids in an area where insulin is being delivered will be different than the general circulation. This is why Dexcom recommends not putting your dex too close to your insulin pump, or injection site. However, I myself have never experienced this, and sometimes my two robots are just an inch away from each other. But, if this happens to you and you’re getting weird readings, just add some space next time.
9. Medication
Certain medications mess with your interstitial fluids, which is the fluid your dexcom reads.
Acetaminophen (Tylenol) or any drug containing paracetamol for example are notorious for causing false highs. If this happens to you, ignore your sensor readings for the next 8-10 hours and just use finger sticks (blood readings).
10. Roller Coaster BG
If I’m having a particularly bad BG day and my sugar is all over the place you can bet my Dexcom will be off for at least few hours. This is because interstitial fluids naturally lag 5 minutes behind blood sugar… you try keeping up with roller coaster moving that fast! If this is happening to you just know that your dex will catch up once your BG takes a chill pill.
11. Bad Calibration
There are a few rules to calibrating your dex if you want to get the most accurate readings.
- Make sure you have clean fingers! Giving your Dexcom a false BG reading because you had donut crumbs still on your finger will single-handedly destroy your readings for the next 12 hours.
- Don’t calibrate with straight up or straight down arrows. These arrows indicate rapidly changing Dexcom readings. Calibrating at these times will throw off the calibration algorithm built into the dex. If the Dex asks you to calibrate when you have an arrow straight up or down, wait until your BG levels out a bit and the arrows are gone. A diagonal pointing arrow is fine to calibrate with.
- Don’t calibrate when below 40 mg/dL or above 400 mg/dL. If the Dex asks you to calibrate at one of these times, wait until your BG get’s into range.
12. Over Calibrating
I learned this one the hard way. Calibrating more often than asked (by the blood drop) will not make your Dexcom more accurate; it will do the exact opposite and likely worsen the accuracy. The only time I calibrate, other than when asked to, is when my dex and blood sugar reading is more than 80 mg/dL off.
The Five Percent
Don’t let these 12 reasons make you think the dex isn’t a capable CGM, because it is. Many CMG users prefer the Dexcom over other CGM for it’s accuracy alone! In my experience, the Dexcom is extremely accurate. 95% of the time when I check my BG, the Dexcom is within a 20 mg/dL range. And if it isn’t, run down this list and you’re bound to fix it.
Question of the day
Have you noticed another reason for inaccuracies? Or do you have tips/tricks that help with accuracy? Let me know in the comments below!
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51 Comments
Great tips! Sometimes I put a new sensor on a few hours before actually starting it. Letting it “brew” for a bit seems to help with my accuracy. So this means I’m wearing two sensors at a time 😛
Robot Level: Expert! LOL
Ooo I’ll have to try that!
So, you put the second sensor on, but leave the transmitter on the first sensor, then transfer the sensor over after “brew”ing?
We struggle with it being WAY off the firs 12-24 hours of EACH new sensor application. I’m wondering if this will help. TIA.
I was trying to understand what you were saying but I think that you are not actually discontinuing the first sensor in the app and just switching it over to the new sensor once the new sensor has been inserted for two hours. That’s a good idea so long as the app isn’t actually doing anything to the new sensor other than timing 2 hours.
Yes I do the same thing. I will insert a new sensor 12-24 hours before the old sensor expires. When the old sensor expires, I start the new sensor after switching the transmitter to the new sensor.
What are some good areas to place a sensor for a 5year old. Her sensor every night goes off due to compression lows. She is a very petite lean little girl. So places seem to be limited. Otherwise the dexcom is great. I’m just wondering if there are places to stay away from or place that are good for kids. Right now we are just switching arms. Hence the lows we seem to get. Thank you for all your help and advice you give.
Arms are great spot! They are my favorite spot to put Dex. Does she sleep on her stomach? If so, then back of arms is great and maybe try lower back/upper butt area if there is enough meat there. I have also had really good luck with the tops and outer areas of the thigh.
Currently had a “LOW” reading in the middle of he night. Was pretty freaked out and then I did a finger poke and was reading around 70. Your article was actually helpful in calming me down because of all the possibilities for this false super low reading. Thank you!
Those compression lows can be scary! Especially when they wake you up in the middle of the night and you feel all discombobulated. It’s like being woken up by a fire alarm.
Just had one of those low alarms last night and it was crazy scary! I did a finger prick check and that was 70, so that was the only thing that put me at ease – I wish I’d found your article at the time. THANK YOU for writing this and in such a succinct style too. Why is this info not part of their app? Why not give users some tools to cope…
So scary! They get me every time. I agree that this info should be disclosed, especially with how easy and frequently it can happen. And thank you for the kind words <3
Great advice, thanks.
🙂
So THAT’S why my Dexcom keeps waking me up in the middle of the night with numbers that are 40 points lower than what my meter reads… I guess I’ll sleep on the other side. Thanks for the article!
Compression lows can be so annoying (and scary)! I always try to sleep on the other side too 🙂 <3
3am and my wife just woke me saying I was at 55 and falling rapidly (I share my sensor data with her; she’s a nurse). Felt fine, and a finger stick showed 81. Find this and realized I was tummy sleeping and laying on my Dex. Thanks for the tips, this helped set my mind at ease.
Oh my, thank you. This was very helpful. I wanted to do some independant research today before calling the diabetes clinic. My CGM is amazingly accurate during the day but every flipping night it wakes me saying that it’s shutting off insulin delivery. A few times, I have even received the “Call 911” alarm on my pump. I flop out of bed, test, and I’m fine. This week I’ve been suffering false lows all night and incredible highs within an hour of waking up (and I’m neither – my blood sugar is nice and steady). Well, the last few days I’ve awoken with a headache (probably due to lack of sleep LOL) and taken Tylenol.
SO – now I must note if I’m sleeping on my sensor at night and find something other than Tylenol for those morning headaches.
Seriously – this article was fantastic. Thank-you.
Ugh, Compression lows totally stink! The good news is… Dexcom’s new G6 is Tylenol friendly! I’m glad I was able to help!! 🙂
Be forewarned about what they mean with “Tylenol friendly.” We thought that as well only to earn (as we were weaning off of Rx meds) that it is a max of 1,000mg per day any only for short periods of time. In excess sent the numbers off the charts.
Thanks a lot for this post. It’s been a nerve-wracking first night ^^
Happy to help! 🙂
We use a tempurpedic topper on my dd mattress to alleviate the compression lows.
that’s awesome! great thinking!!
Have you ever noticed that stretching/yoga gives you sensor errors or crazy readings on your Dexcom?
yes! i have definitely had this happen a few times
Great article. I’m new to my Dexcom and the readings are all over the place. I keep calibrating it and instead of ignoring that the first 24 hours what do you recommend?
One cause of abnormal high readings I’ve discovered comes from eating certain foods or spices. Almost every time I eat something seasoned with ginger, my G6 will l go sky high yet finger sticks consistently show normal BG. This morning I made celery and cucumber juice with fresh ginger root and the G6 spiked to 157 when my BG was actually 96. Getting the G6 calibrated is always a nightmare!
I’ve also had this happen after eating fresh cherries or drinking cherry juice.
The only answer I’ve ever gotten from Dexcom is that the sensor measures chemical activity in the interstitial fluid and “the sensor must be bad,” so here comes a replacement!
Thank you! My son and I new to his Dexcom and I’ve been starting to wonder why I fought the insurance so hard for it. Some days it’s nothing but trouble. I love your “Roller Coaster Days” tip. Pair that with the over-calibrating one, and I wonder if we’ve found 80% of our problems. Mom AND his BG need to take a chill pill for the day and just let it sort itself out. =)
Can you elaborate on working out and false results?
I’m finding the Dexcom G5 to be a totally inaccurate item and it’s constantly 10-20 units higher than what I actually am when I calibrate. For example my meter read 87 when I was actually 64, which is dangerous. I’m about done with these stupid and useless monitors. I quit using an insulin pump for the same reason and I’ve been 10000% better off without a stupid pump.
My daughter was swimming and we lost signal for about an hour. When I pulled her out the Dexcom G5 was reading 3.0. I check to make sure it was on correctly and that the tape was secure. She said she didn’t feel low at all. We did a finger poke and got a sugar of 4.9. I let her have a bit of candy as she was going back in the water. Her skin around the CGM was quite cool to the touch. I am thinking that maybe some vasoconstriction was occurring and that that may be affecting the reading. Anyone else have this issue?
Very good explanations. Before reading your tips fell victim to compression lows while sleeping only to realize I can’t sleep on it. Also, with regards to swimming, I have an 18 foot round pool. I place my receive on the rail. I ALWAYS lose my signal even though I’ve not exceeded the 20 foot rule. I’m guessing that the water density is the cause. Since I live in Michigan, I just started swimming in early June. This is when I began getting false readings. In the last 2 months, I’ve had to calibrate my G6 3 times. Each false reading was within 50 mg. Twice showinglows, once high. I started using my G6 in January and never had an issue, till now. Could it be from the amount of time (ALOT) I’m spendingin the pool? Thanks
Every time I play soccer my readings spike. Is it a case of wrong readings because of all the movement or is the reading actually correct locally?
If the readings come back down after about 10 minutes or so then its a false spike. But if they stay high then it’s probably bc of the workout.
I just discovered your DEXCOM post with interest. I’m Type 1, use G-6, know it well and sometimes it’s close to the meter, however, more less than not. One major issue no one has been able to figure out until DEX tech stated, “When you rise from a sitting position, from bed, anywhere, a movement can slightly effect the “cannula” (size of human hair) and you will get some false readings? WOW! Never knew this? No one higher up DEX will communicate back? My issue, I rise, per se, 165. 20 min later, BSL starts to “rocket” down. 10 points, then 15, then 20? No reason? I get hypo feeling at 140 so stop it there with glucose tabs. Take one, helps, then down she goes. Take 2, helps, then down she goes? Finally, after 4 or 5 tabs, (16 to 20 grams carbs) DEX arrow levels off somewhere above 150 or 160 which is fine with me. This is a daily occurrence. ENDO has no idea, USC has no idea, UC Irvine, no answer. DEX says, canulla? Hard to believe or DEX would never be accurate? If you know anyone out there who has heard of this issue, I would love to know? Many thanks, Russ
That is odd. I get the occasional off reading that doesn’t fall in line with the rest, but that’s usually when my transmitter or sensor is about to die. And it’s definitely not a daily occurrence for me… maybe once every few weeks or so. I’ll keep my eyes and ears open tho to see if I hear of anyone else having this problem.
My 3 year old (with unaware ketotic hypoglycemia, not diabetics) had a great first 10 days with her g6 in her arm. No issues except some sensor errors around day 8 and 9. But we inserted a new one yesterday and right after the warmup, it kept giving “urgent low” readings and not even displaying a number, even though her glucometer said 98. Called tech support, they said change the sensor, so we did. Same issue again. After about 4 hours, it evened out, but tonight, we got another false low. Cgm said 68, meter said 98. I thought it was compression but it’s been an hour since I roller her off her dex and it’s still in the 70s. Not sure what is going on. How long does it take to regulate after a compression low? They said it could be too close to her muscle because she’s little and it’s in her arm but I’m at a loss as to what to do because we only really need the dex at night, but I’m not sure how to keep a toddler from sleeping in any position to try and avoid false lows.
Wow thank you for these posts. I really liked the idea of getting a bed matting that may help to cushion or lesson the compression while they sleep. I also found that we needed an updated phone for the G6 to work well without the receiver. Our iPhone 6 was not cutting it. It’s been a really rough start for us, but we are keeping at it and this really helped. Thanks again.
Sorry for the super late reply! My compression lows typically take 15 minutes to even out once dex isn’t compressed anymore. If it still hasn’t evened out after 30 min then I’ll calibrate it and that usually does the trick. Dexcom really needs to make a kids version; smaller needle, smaller size, smaller everything. And I agree, it’s impossible to keep a toddler (even an adult) from sleeping in any position to avoid false lows, they really need to figure out a better solution. I’ve found the best spot for me (in regards to avoiding compression lows) is on my legs, however I can only do this in the spring, summer, and early fall since skinny jeans compress dex so badly. Dex is great, but they have a lot of improvements they can make.
Does anyone else experience transmitter/sensor problems when doing cold winter sports? I’m still on the G5. I cross country ski every weekend and as soon as I strip down to only one layer of clothing (cross country skiing is high intensity aerobic exercise) my Dexcom stops reading my BG and shows ??? (meaning the transmitter does not understand the sensor readings for the moment.) Dexcom cannot give me any advice but I am starting to think it might be because my skin/tissue where the sensor is located is cold and numb (cold-induced vasoconstriction?) It’s also happened when I’ve been out cycling in colder temperatures. It only starts working again 30-60 minutes after I have gone back inside and my skin on my abdomen has warmed up again. Has anyone else experienced this problem?
Why does the Dexcom average the blood sugar posted on it and the blood test? Seems like it should calibrate to the blood test. It drives me crazy to wake me up with a low and my blood test is 80.
While these are decent tips, it doesn’t address a seriously important factor that contributes to dexcom being inaccurate: the calibration ‘algorithm’ is a crock. It literally just averages the difference between what it thinks it is and what you’ve entered. For example, if you’re trending diagonally up (eg 6.0), and you calibrate with a number lower than what it thinks (4.0), it’ll drop the level to half way between the two (5.0), but that would be wrong. You can’t have been 5.0 in the last 15-30 mins and trended up to 4.0
This is where the excuse of ‘over calibrating’ comes in. Over calibrating is not a real thing, the more you calibrate something, the more accurate it should become. If any sort of calibration makes something less accurate, it because the algorithm running the calibration is a giant pile of crap. Which in this case it absolutely is.
I’m not disagreeing with your points, but dexcom should also be held responsible for the half arsed engineering.
If I want to do a calibration, 2 times brings the cgm to the blood glucose number that I put in. However, when my actual blood sugar number is 50 off from the cgm, the cgm won’t take the actual number I put in. I then calibrate in steps by changing it 20 points at a time. But then I am going against the rule of not calibrating too often. A never ending battle sometimes.
I enter the calibration number twice and then the Dexcom accepts the finger prick reading. Not sure if that’s a good idea or not but if Dexcom is 5 minutes behind finger prick… it should accept the actual current reading for calibration.
I am on vacation in Florida and I live in Maine. Only since being in hot & muggy Florida have I been experiencing ridiculous false lows on Dexcom and I’m wondering I what’s up with that. I’m also on new meds and wonder what other meds affect readings besides Tylenol .. but I have a G6 so I guess Tylenol isn’t an issue?
Where can I put the G6 sensor. The literature says belly only but I am reading in here that people are saying arm, lower back and thigh.
Some of my favorite spots are belly, arms, thigh, and back (love handle area). I’ve also seen people put them on the calf and pecks. The reason Dexcom says belly only is because they have only studied the accuracy on the belly so legally they can’t recommend you put it anywhere else.
I’m thinking of getting the Tandem IQ pump but am concerned that my CGM readings are usually 20-25% off from a finger stick. Being as I’ve had Type 1 for 68 years I can live with them not being perfectly accurate. However, this can be an issue if a pump is now responding to the (inaccurate) CGM readings. Any ideas?
Honey, I am with you! Your numbers will never match your pump. I started out with dexcom and then my endocrinologist moved me over to medtronic (Btw..dont ever go there! The worst!) Moving back to dexcom and tandem IQ once military insurance will allow. Dexcom is easier to deal with and more accurate in my humble opinion. I am 65. It’s a never ending battle. Some days I just want to go back to dexcom and injections! Best of luck!
I cannot agree with the writer in regards to the accuracy of the Dexcom G6. It is wildly inaccurate at times – especially when your BG is rising or falling rapidly. – and it can be off 40% – 80%. Don’t get me wrong, I depend upon the G6 to manage my T1D and it is the best of some very mediocre CGM technologies that purport to read BG but actually never do but attempt to interpret interstitial fluid. One piece of advice: don’t trust the Dexcom G6 as a basis for driving. And don’t count on Dexcom support to fix inaccuracy issues; their solution is to send you a new sensor no matter what.
Honey, I am with you! Your numbers will never match your pump. I started out with dexcom and then my endocrinologist moved me over to medtronic (Btw..dont ever go there! The worst!) Moving back to dexcom and tandem IQ once military insurance will allow. Dexcom is easier to deal with and more accurate in my humble opinion. I am 65. It’s a never ending battle. Some days I just want to go back to dexcom and injections! Best of luck!
The Dexcom 6 sucks. For 1 week I monitored insulin intake over device reading. I can now predict w an absolute certainty it always takes 4 hrs btwn injection and Dexcom showing a change.
So if I want a donut with 30 carbs. If I give myself 6units I have to wait 4hrs to eat my donut so I don’t bottom out.
I can set my watch by this delay.