Underwhelmed with first 24 hours on the Dexcom

I finally decided to jump into the CGM pool, and I chose the Dexcom. My first 24 hours have been less than satisfying for a few reasons.

  1. The first sensor failed after 4 hours. I called Dexcom, and the phone rep was quite nice and told me a new sensor would arrive the middle of next week.

  2. I have been checking my blood glucose fanatically (and entering the readings into the receiver), yet I’m finding that the Dexcom is often varying by 30-50 points from my true blood glucose.

  3. The Dexcom has alerted me to rapid glucose rises when my blood glucose has actually remained stable as confirmed by my glucometer.

So far, I do not understand the value the Dexcom brings to the table if it isn’t going to give me a reasonable approximation of my blood sugar (yes, I understand that it’s measuring interstitial fluid, etc., etc.) and it has difficulty reporting trends in my blood glucose.

Is my experience atypical? Does the transmitter take a while to calibrate the data it collects from the transmitter with my data I enter from my glucometer? I appreciate any comments or feedback from the group.

Thanks in advance.

Jaybear, sorry about your rough start. When I got my 7+ the big surprise was that I could not trust my bg meter. For the first 7+ calibration my bg meter showed 96 and 136. I was totally floored. I learned to wash my fingers and don’t squeeze. This helped with the bg meter accuracy. When I calibrate and 7+ and bg meter results differ by more than 10 points I test again. I believe the 2 out of 3 value. Many times this boils down to the 7+ and a bg meter result, not the 2 bg meter results. Another factor is the lag between finger prick result and 7+. It happened that I felt low, the 7+ showed 80 and my bg was 50. Of course, I was disappointed. I did not give the 7+ credit for showing 50 ten minutes later. Now I pay attention to the trend arrows. It took me about 4 weeks to internalize the delay. Fast moving bg also interferes with calibration. I only calibrate when my bg is stable.

“Does the transmitter take a while to calibrate the data it collects from the transmitter with my data I enter from my glucometer?”

The 7+ measurements line up with finger stick results from 10 minutes ago. When you enter data, the data is for now. The 7+ cannot reconcile data for different times. It seems to wait 10 minutes to be able to compare apples to apples.

Thanks, Helmut. From my limited use, I agree.

As the Dexcom collects more and more pairs of data points (i.e., interstitial readings and glucometer readings) over days and weeks and months, does the accuracy seem to improve? If this is as good as it gets, I’m thinking about returning this thing.

I don’t think that it is matter of collecting more pairs of data points. Calibrating less seems to work better even that this is counter-intuitive. Somehow the dex gets confused by too much data. It is best to calibrate only when asked. If you think the values are too high or too low just take a mental note of it unless it is extreme. My first 10 sensors were very good. I had one spooky experience when I tested with bg strips 4 times during a 12 hour period and all 4 samples matched exactly to the point. I know that this should never happen. But it did. My last sensor was the other extreme. I eventually called DexCom and DexCom told me to pull it. I will get a replacement. I urge you not to give up too easily. In the early days of my D all I had was urine strips. The switch from bg strips to CGM is as big as the switch from urine strips to bg strips. I would not switch back ever. This is not say that for some reason the 7+ will not work for you. Some people get better results with the Nav than the dex. It is all about body chemistry. Ride out the 30 day money back guarantee time and decide then. Good luck.

JayBear, most of us have experienced two areas of trouble when we get our Dexcom machines:

(1) The first day of any Sensor is always flakier than later days. You’ll have to live with this, although MOST of your problems will probably go away if you pay attention to #2.

(2) Frontal Abs, the site which the FDA actually approved, works very badly for many Dexcom users. I’m one of them, I never got a good result from the location specified in the manual. Note that because this is the FDA-approved labeling and instructions for use, Dexcom can’t even document that most people get better results elsewhere. Support cannot bring up this subject on their own, either.

If your male, then try your two “love handles” first. Straight down from the armpit, about an inch above your belt line. if that works for you, GREAT, and you should be able to also do future sites a couple inches towards your front OR back with similar high-quality results. Don’t go too near ribs, and don’t go anywhere near your backbone on the back side.

Next, try upper butt cheeks. Again, not a bony area, one with actual flesh. (But be sure not to go too far down, less than 2-3" above seat cushions will affect results. Badly.) Most slim women prefer these sites, because USA fashions work really badly with the above-the-belt-line, on-the-side sites which usually work #1 for guys.

(3) Never, EVER enter a fingerstick during a period of rising bG. Not even a “slow” rise, where the graph goes up the the arrow shows level. Wait for treated high glucose to level off at the top of the curve. Sharp downward times shouldn’t be used either, but “slow” during "slow downward periods are OK.

(4) You need to learn good insertion techniques. Unfortunately, this is a really slow process, (because “practice” Sensors don’t exist, and the real ones cost $$$$ each.) I’ve made other posts about this before, please search.

Thanks, Helmut and Rick. I think I’m starting to understand this a bit better. Today has indeed been a bit better.

For example, this afternoon the Dex read 171. I checked my BG with my OmniPod PDM, which read 192. What was valuable was not the precision of the Dexcom reading, but the quality of the reading, i.e., my blood sugar was slightly too high. It’s nice to get that type of information quickly with a button push on the Dex, but the limitation is that I still need the BG reading for a more precise correction bolus.

I’m also looking at the reading on the Dex as more of a time machine – i.e., an approximation of what my blood sugar was 20 minutes ago, rather than a current BG reading.

I think the other frustrating aspect of this is that there is a lot of extremely useful cumulative wisdom among the successful Dex users that just doesn’t appear in Dexcom’s materials. Thanks for the tips.

I don’t actually have my Dex yet (it should be arriving next week) but when I did a trial run with the Dex rep at the beginning of the month he informed me that there were some bad sensors out that Dex is aware of that shut off after 2-8 hours. I expect this is what happened with your first sensor. They are aware of the problem and willing to send replacements but they haven’t been able to nail it down enough to do a general recall on the bad sensors. The rep also confirmed what everyone is saying about the first day frequently being off and the slight lag between the Dex reading and the finger sticks. With all that said I found that in my 7 day trial (which I was able to stretch to 9 days before my return meeting) I usually fell within 15 to 20 points of the DexCom using my Omnipod PDM glucose meter. Again, remember that normal glucose meters are allowed to varry up to 20% (a glucose reading 100 could be anywhere from 80-120) so the Dex not being exactly on target with another already flawed system isn’t really a surprise but you should be able to get a fairly good idea with it. Good luck!

My first sensor went something like that also, but after that my dexcom and finger sticks are almost always within 10 points of each other. The one thing I didnt know because of course I didnt read the entire book was not to take tylenol. That caused some trouble in the beginning.

I had a lot of the same experiences as you in the beginning. There is a lot of learning that needs to happen and unfortunately, it just takes time and experience.

One additional thing that I don’t see mentioned in comments below is that it sounds like you’re totally over-calibrating. Calibrating more than a few times a day is bad and can cause sensor failures and a whole range of other problems (including readings that are just off). Believe me, I KNOW that doesn’t make sense, but in time you’ll learn that it’s true. I don’t completely understand this, but Dex has explained it to me several times and my own experience has agreed with what they’ve told me. In time, you’ll start to learn what the readings look like when it really needs to be calibrated and when it doesn’t.

Also, I’ve found that while Dex claims that it’s safe to calibrate when you’re rising or falling, things tend to work better if I calibrate while I’m level.

Hope this helps. Best of luck. Hang in there.

The first 24 hours on my sensor tends to be pretty erratic - but then, by the second day, it gets pretty dialed in. So maybe… patience, grasshopper? :wink:

There’s a learning curve with this thing, for sure. I hated mine at first, and now I feel weird if I don’t have it on. Hang in there!

  • T

The big rule with any CGMS, DO NOT expect your glucometer and CGM to match. If you expect this, you will almost always be disappointed! You have to take into account the lag factor and the fact that glucometers can be up to 20% off from your real BG. Helmut is right about the trend arrows, pay attention to those and you will start to be able to really see the value of the CGMS. I am catching 99% of my lows now which is amazing since I have hypo unawareness. It just takes time to figure out how it works for you and what your normal variation is.

You will have to experiment and find what spots on you work best. For me, the abs are best then the upper butt. I can use the outside area of my thigh (lots of meat there for me) but the sensor is never as accurate when I wear it there.

Bad Sensors: Yes, and I think that Dexcom has phone-called ALL customers who ordered Sensors during the period of bad manufacturing lots. (I got a “cold call” on the phone from them 3 days ago.)

After a few days, you’ll catch on to the fact that the trend is really of more value than the numbers. Even though the data is 10-30 minutes “old”, the shape of the graph is a good indication of where your bG is going in the near future, and you can adjust (in advance) for dosage problems and suprises which are barely getting started, and haven’t completely happened yet.

None of us had such a graph before getting CGMS devices. Even if you DID take enough bG readings, you’d only a set of numbers. (And at a mere 20x per day, I never did enough tests to get a quality graph anyway.) Until you’re poking at least every 10 minutes, AND you graph them on paper, or on the screen, you don’t get a good feel for the 2nd-order shape. (The high school calculus “derivative” of the curve.)

Alarms are great when you’re sleeping or too busy to poke, but the shape of the graph is a totally new tool for us PWD’s. You get to see “roller coaster” events starting to happen, and you can deal with them before they reach extreme values of bG.

Dexcom has a tendency (seen by really finicky people who study it closely), to be more prompt during falling bG, and much “later” when trying to catch up with increasing bG. Basically, falling Glucose levels happen locally at body cells and ISF first, while bG falling in your big volume of body blood happens as an almost secondary effect.

So, during falling bG, Dexcom’s delay across the membrane and into Glucose Oxidase is partly a “tie” for the delay of seeing the bad news in fingerstick bG. So this creates ‘Good News’: For some people, in many situations, Dexcom is sometimes just 0-5 minutes “late” compared to blood bG. Bad news: Both values are late, your cells were suffering from low glucose supply before either measurement fully indicates the problem. (I’m sure that you’ve often FELT signs of “going low” before finger stick shows the fall, it happens to all of us. This is why.)

But, on the “going up from uncovered food” direction, Dexcom is very late. Whether obtained from the stomach, or directly at the mouth, or the small intestine, or liver converting non-glucose compounds into glucose, the blood gets it first. The delay from blood to ISF is additive with the delay from ISF into the Dexcom’s glucose oxidase.

I don’t know why Minimed Sensors doesn’t show a similar “more prompt when falling” behavior, but if it’s there, it seems to be much less.

Sounds like I am overcalibrating. I thought more data would IMPROVE accuracy. I will try to resist entering each glucometer reading. So counterintuitive!

Thanks to everyone else for the feedback. Y’all are great!

We have been having problems but Dexcom is working with us. Dex works about 50 percent of the time though. We received a new Receiver. Kept documenting and downloading. The tech could tell by the vertical yellow lines that she is going out of range, and we always keep Receiver on/near her. So they just sent us another transmitter. Keep documenting problem; download your Receiver every three days and call Dexcom technical support. The Dexcom techs will figure out any problems. Now about the 30 to 50 points from true blood glucose… that can and will happen. That is because Dexcom should be within twenty percent of your blood glucose value. Coupled with the fact that Dexcom is 15 minutes behind your blood glucose. So when in ranges 80 to 140 points blood glucose, Dex WILL read within 20 to 30 points of blood glucose. But if your blood glucose is 270, 20 percent off would mean Dex is approximately 60 points off. I have noticed Dexcom often reads 30 points higher on her lows, but that happened with our Minilink as well. Maybe she drops quickly. Once you get used to interpreting the trends, Dexcom can be a great help. P.S. AND once you get your system up and working properly. Keep documenting. Keep downloading. Keep calling Dexcom. They will fix it. May take time but they will do it.

Well, ANOTHER sensor failure today. I called Dexcom, and they were quite nice about it and will be sending me yet another replacement. Hey, at least I got 5 days out of this one. Three sensor failures in the first week has not been encouraging, though.

The Dexcom customer support also told me that I should enter only about a half dozen calibration blood sugars per day. Any more than that, and I’m overloading the system. If this is the case, then Dexcom needs to revise the manual to reflect this.

One very cool thing about the Dex. I was able to see when my blood sugars start rising in the early AM, and I have reprogrammed my basal rates to deal with it. I have been waking up with lovely 95-120 blood sugars the last several days.

I am starting to appreciate the Dexcom, but I’m also still very frustrated with it – by the manual lacking the secret yet necessary tips to run this thing optimally, and the three sensor failures in my first week.

I used 12 sensors so far and the tape looked as good on day 7 as on day 1. No sign of peeling off. After I put a sensor on I press the tape down with the back of my finger nail. I pinch up my skin a little bit so that the sensor/tape does not sink in when I am doing that. I used this technique when putting my DMV sticker on the windshield of my car. I was reminded of doing this to my sensor tape when watching the “1 Happy Diabetic” video on YouTube.


Bill pointed out how important it is to press the tape down.

With my latest sensor I am on day 13 and the tape starts peeling off a little bit on one end. I still get good readings from the sensor and I am not close to pulling it.

Helmut’s Right!

After attaching the Sensor (with Skin-Prep fully dried before I stick it down), I press down all of the tape surrounding the Sensor mount into my skin, using the wide rounded end of a paper clip.

Using a paper clip, or back end of a finger nail, allows for really strong pressure in the single spot you’re working on-- while NOT smashing the already-inserted Sensor wire into a mess.

Usually for me, an edge of the adhesive pad starts peeling up somewhere around days 10-12. I stick it back down using Mastisol (instructions elsewhwhere, “search” is your friend). That one touch-up always lasts until the Sensor dies.

Jay, are you trying any “alternate” sites yet?

Don’t forget that the limitations of the manual, compared to the real-world experience of several hundred user-years, are mostly a result of the FDA Approval Process: The manual’s instructions are reviewed and approved along with the product, and Dexcom would have to undertake another study to support our “alternate site”, and/or “good for up to 22 days”, and/or “you can wipe this stuff on without leaving an empty dot where the wire goes through” experiences.

YMMV. But as you’ve just said, hunting around to find all of the ‘secret but necessary tips’ which have been posted here might be time well spent.


It would be nice to have “Rick’s 7+ User’s Manual”. The discussion thread format is not ideal.
“Rick’s 7+ User’s Manual” should be on a page that can edited indefinitely so that new insights can be integrated. I have no idea how to technically do that. Do you?