Some Dexcom disappointment

Here is a typical Dexcom experience for me. Sometimes the numbers it shows are basically worthless for me.

I put my actual BG values on the Dexcom trace to show what I am seeing.

Here are the BG numbers I had:
69 @ 4:36 pm
76 @ 4:47 pm
77 @ 5:08 pm
107 @ 5:29 pm

At 4:36 I was dropping and tested at 69 and took 23 grams. Came up nicely, tested several times.

Almost an hour later, at 5:29 pm, I was 107. But the Dexcom still had me in the 60’s. Totally clueless. 53 minutes later it is still showing me as flat in the low 60’s, absolutely no upward trend or any indication my BG is coming up. A 46 point difference 53 minutes after carbs!

Almost an hour later?! Hey Dex, that’s what I’m paying you for!

Anyway, just wanted to share this so people would understand my perspective on the thing. :face_with_raised_eyebrow:

Hey, Dex…C’mon man!


This is pretty typical for me… I’d been putting the sensor on my thigh where it doesn’t bother me at all and always getting these huge disparities, recently I switched to my stomach which has a little bit more padding than my thighs and have seen a little better tracking, but it annoys the hell out of me there


@Eric, the longer we use the CGM, the more I’m realizing this myself…I don’t trust our “low” and “severe low” numbers at all because…I would say 8 times out of 10, when the CGM reads low for us, he’s not low. OR he is low only for 1 dot (5 minutes), but the CGM won’t register him being up again for 30 minutes or longer thereby hurting his online numbers. Anytime he’s trending low on the cgm we do checks on the PDM to verify / correct…but often times the CGM numbers just aren’t accurate.

So, although I do look at the Dex website trends, A1C, etc., it’s not a fully trusted source for us because actual blood sticks show the numbers off a large majority of the times.

It’s because of this, also because the Endo doesn’t care what the CGM shows anyway, they base their “judgements” off of the numbers on the PDM (Glooka), that I no longer put a lot of stock in the percentages that come out of the data from the CGM.

But the CGM is still imperative for us, though…without it, we’d never have a clue of the “general area” that Liam was in at any given time. While the data is always “within 100” of his actual BG’s…which is great…because knowing he’s at least “between” 100 and 200 is great…but where in that 100 difference that he actually is, is sometimes the question for us,



  1. How old is this sensor?

  2. How often do you calibrate and how long since the last calibration?

  3. How old is your transmitter?

Situations like this let me know that I’ve worn my sensor way past time. That’s about the only time I get deviations like that.

Also, I calibrate four to five times per day, basically whenever I do a finger stick. XDrip+ automatically reads the Contour Next One and immediately gives me the option to use it for calibration, so it’s one click.

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I’m betting it has a lot to do with the part of your body and the “quickness” for the interstitial fluid to change as your BG changes. I see that certain places are more responsive than others. Dehydration can also complicate that.


Love the visualization of your graph!

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This sensor was 4 days old at the time of this trace.

I have been doing the correct calibrations, only when it asks for them. I had tried doing more calibrations before, but it didn’t make any difference. At the time of this, it had been calibrated within the 12 hours like it was supposed to be.

The transmitter has not even given me the initial expiration warning, so it has plenty of time left.

Just for the record - I break all the D rules in general, except I try very hard to follow the Dexcom rules. Because it sucks so bad for me, I have tried doing it exactly like I am told.

I was actually wearing this one on my abdomen - an approved location!

Maybe that’s the problem :wink:

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Me, neither: the device just doesn’t handle the recovery phase very well and I cross-check with a fingerstick to make subsequent treatment decisions.


Thisjasovcasionally happened to me. If it happens within the first seven days and is persistent, I’ve called Dexcom to get the sensor replaced. If it happens with an older sensor, I recalibrate multiple times a day (up to four) until it gets back on track.

Overall, for me the occasional erratic readings are far outweighed by the nemehirs. I’ve been without Dexcom sensors for a week (due to an annoying pickup location and working) and am really, really missing it!!!

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My experience has been that in the lower range, dexcom is most accurate with following the trend. When my BG is rising fast, over 150, dexcom is slow to keep up. If I correct with Afrezza, my BG trend slows down, but often overshoot the actual high BG. But I do look for the bend, rather than the number at that point.

I am amazed at my current sensor, and will have a new record time when it ends. Currently on day 26. Previous one from same Lot lasted 23 days, and broke my previous record. My average has been 15-20 days.

The difference is I used my inner thighs for both, AND they were both expired when I started them. (Sept 2017).

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My average when I’m eating low-carb is about 30 days. I’ve been eating like crap lately, so my last sensor only lasted 21 days. Hoping to get back to low-carb soon. I pay out of pocket, so an extra week added on to my sensor average is a big deal!

Why does what you eat affect how long your sensor lasts?

My theory is that my blood sugar is much more stable when I’m eating less carbs. I think the constant (relatively) level blood sugar uses up less sensor enzymes than constantly going high. (My understanding is that the sensors have an enzyme, and they die when it’s used up.)

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That’s interesting.

Dexcom has many patents. Understatement. However, it is my belief the following actually does describe their sensors:

Patent No: US 9247900 B2

In preferred embodiments, the working electrode comprises a wire formed from a conductive material, such as platinum, platinum-iridium, palladium, graphite, gold, carbon, conductive polymer, alloys, or the like. Although the electrodes can by formed by a variety of manufacturing techniques (bulk metal processing, deposition of metal onto a substrate, or the like), it can be advantageous to form the electrodes from plated wire (e.g., platinum on steel wire) or bulk metal (e.g., platinum wire). It is believed that electrodes formed from bulk metal wire provide superior performance (e.g., in contrast to deposited electrodes), including increased stability of assay, simplified manufacturability, resistance to contamination (e.g., which can be introduced in deposition processes), and improved surface reaction (e.g., due to purity of material) without peeling or delamination.

The working electrode 44 is configured to measure the concentration of an analyte. In an enzymatic electrochemical sensor for detecting glucose, for example, the working electrode measures the hydrogen peroxide produced by an enzyme catalyzed reaction of the analyte being detected and creates a measurable electronic current For example, in the detection of glucose wherein glucose oxidase produces hydrogen peroxide as a byproduct, hydrogen peroxide reacts with the surface of the working electrode producing two protons (2H+), two electrons (2e−) and one molecule of oxygen (O2), which produces the electronic current being detected.

In preferred embodiments, the working electrode 44 is covered with an insulating material 45, for example, a non-conductive polymer. Dip-coating, spray-coating, vapor-deposition, or other coating or deposition techniques can be used to deposit the insulating material on the working electrode. In one embodiment, the insulating material comprises parylene, which can be an advantageous polymer coating for its strength, lubricity, and electrical insulation properties. Generally, parylene is produced by vapor deposition and polymerization of para-xylylene (or its substituted derivatives). While not wishing to be bound by theory, it is believed that the lubricious (e.g., smooth) coating (e.g., parylene) on the sensors of the preferred embodiments contributes to minimal trauma and extended sensor life. FIG. 23 shows transcutaneous glucose sensor data and corresponding blood glucose values over approximately seven days in a human, wherein the transcutaneous glucose sensor data was formed with a parylene coating on at least a portion of the device. While parylene coatings are generally preferred, any suitable insulating material can be used, for example, fluorinated polymers, polyethyleneterephthalate, polyurethane, polyimide, other nonconducting polymers, or the like. Glass or ceramic materials can also be employed. Other materials suitable for use include surface energy modified coating systems such as are marketed under the trade names AMC18, AMC148, AMC141, and AMC321 by Advanced Materials Components Express of Bellafonte, Pa. In some alternative embodiments, however, the working electrode may not require a coating of insulator.

The reference electrode 46, which can function as a reference electrode alone, or as a dual reference and counter electrode, is formed from silver, silver/silver chloride, or the like. Preferably, the reference electrode 46 is juxtapositioned and/or twisted with or around the working electrode 44; however other configurations are also possible (e.g., an intradermal or on-skin reference electrode). In the illustrated embodiments, the reference electrode 46 is helically wound around the working electrode 44. The assembly of wires is then optionally coated or adhered together with an insulating material, similar to that described above, so as to provide an insulating attachment.



I used to think that the sensor declined because the enzyme got used up. I now think that the main issue is a process of biofouling and encapsulation that over time reduces the ability of interstitial fluid to reach the catalytic enzyme and associated sensor.

From here: What Are the Next Steps in Continuous Glucose Monitoring?

Improvement of the interaction of the sensor itself with the subcutaneous tissue by using appropriate coatings can be a big help when it comes to the duration of successful usage of an individual glucose sensor and good measurement quality.1 With most current CGM systems the so-called biofouling is associated with a profound reduction in sensor sensitivity toward glucose over time of usage and acts as the major source of unreliability of given CGM systems.

From here: Current and Emerging Technology for Continuous Glucose Monitoring (with a good references section if you want pointers to more background)

the current gold standard glucose biosensors are invasive, and CGM systems still present a number of limitations, such as biofouling, fibrous encapsulation of the implanted electrode, inflammation, and loss of host vasculature, which seriously affect the precision and accuracy of the BG results


In contrast, I’m about 36 hours into a sensor that expired in August (oops, turns out I have a bunch of them…), and it’s performing excellently/within 10 points at each check I’ve done. I never trust the severe LOW readings to be precise (usually those just indicate a major low (40-50s), but not a LOW), but they are functional enough for me to not be bothered much by that lack of precision. I really think that in addition to amount of body fat being an issue and possibly site location, there must be some physiological individual differences that affect why the sensor wires stay in good condition longer in some people and degrade much more quickly in others, and perhaps some of these (or others) affect accuracy as well. Could be tissue structure or healing differences, immune response differences, who knows what (except I bet Dexcom has some idea).