Bad sensor vs. bad location

I’m convinced that a sensor that refuses to be “trained” may be: 1) Just plain bad, or 2) Placed in a bad location. And there’s no way to know which is which.

Your thoughts?

I’ve only had one truly bad sensor that needed to be replaced. Other times it would give the 3 ??? and sort itself out. Never had a bad location yet.

Where is the sensor placed? Is there enough tissue under the sensor to bathe the wire in body fluids?

What a coincidence! I was about to start a thread about this topic. For almost a year I placed my sensors only left and right of my belly button because these sites are pressure free when I sleep and give me good readings during the night. However, I noticed that sensors placed to the left of my belly button are close to perfect and the ones placed to the right are many times flaky. I put a ‘right’ sensor in 2 days ago and it was bad from the start. 50 points off twice during the first day. I thought that I caused death-by-calibration because I calibrated when the readings were off that far. I restarted the sensor after a day to purge the bad history. This restart did not help. The 3 hour line looked jagged like on a sensor that is past it’s prime. After catching the sensor with another 50 off reading I pulled it. I looked at some text books to find out what is so different between left and right. As far as I can tell there are no organs there, just my intestines. Today I decided to give up on the right side. Too many sensors don’t work there. I hope that my left side will sustain the double-duty until the technology improves and this will be a non-issue.

This is very strange. Which side do you sleep on? Also, even with 50 point spread, what did your graph look like? IE when you were high was the graph up and when you were down, was the graph down?

As far as differences between the two sides, keep in mind the sensor is in fatty tissue outside the abdominal cavity. So the underlying organs do not have any impact on the “interstitial fluid” around the sensor wire.

I have seen this same 40-50 point spread variation in my data. The CGM gets closer to finger stick about day 4. Keep in mind, because of the technology, it is only a fluke if they are the same (+/- 1%). Most of the time they should be within 5-10% of each other. This means if you have a finger stick of 100 +/- 10% is 90-110. If the CGM reads 120 +/- 10% this is 108-132. They are overlapping at 108-110. OVERLAP is good. It took a clinical pathologist to explain the variation in the testing methodologies in “live” testing of glucose. SWAG was a term used by him about finger sticks and CGMs. He shared back to back tests in a reference lab, not just a hospital lab may have a 3% variation 100 would read 97-103.

Hope this saves a few sensors.

I make an effort to sleep on the side that the OmniPod is not on. This means that I switch the preferred side every 3 days. Sometimes I cannot resist the urge to switch sides. My guess is that for 80% of the night I use the preferred side.

The graph was not smooth. A lot of measurements were out of line. A lot of double arrows up and down. With a good sensor I never see double arrows. Even single arrows up and down are very rare.

My BG typically stays within 70-120 during the week. After breakfast the dex showed 150. I checked my BG and it was 97. I don’t remember the other off reading. Just looking at the graph I could tell that the numbers were suspect.

Ha…Same exact problem…If I put the sensor to the left of my belly button I get +/- 5% in readings almost perfect, to the left I get over 20% and alot of ??? errors. I have tried this 3 different times

I appreciate everyone’s responses. Didn’t want to respond with my theory right away to see what others thought.

In this particular case, I believe I was clearly to blame. My favorite location is my waist, on the side, between my hip bone and my lowest rib. I wasn’t careful and placed it where the needle was directly over the lowest rib. Although there was no discomfort and the needle didn’t appear bent by the rib, it most likely impaired the sensor. Today, on day 11, I got the first ??? and finally ripped it out.

I’ve only had 1 successful sensor in 3 months. There is no pattern as to why its unsuccessful. It’s just very frustrating!

I had just changed my Dexcom for the first time on Saturday and overnight I got the ???. The battery was also low at the time so I put it on the charger. Later it started giving readings again but when I woke up Sunday it had the ??? for about 3 hours. I called Tech Support and they said to just change the sensor and they would send me a replacement. The first sensor I had on for the 7 days was on my right side, the new one I had put on Saturday was on my left. When I had to remove it, I put the new one back on the right side.

After 3 months on the Dexcom no bad sensors yet (knock on wood) But I have had differences in number with fast drops or spikes, but i assume that is from the 5 minute lag and then trying to playing catch up.

Etta, you say “there’s no way to know which is which”.

Of course there is. If it is a bad sensor, then it would be bad no matter where you place it.

Only a good sensor can be affected by the location of insertion.

In my case, it was pretty obvious I put it in a bad spot. I don’t know how you can say for sure that a sensor was bad since it’s not always clear that you’ve chosen a bad insertion site.

My theory is that bad results are caused by movement between sensor and tissue. Some sites have a higher probability of holding a sensor steady. I don’t know of any sites that always deliver good results or always deliver bad results. It is always a gamble. The site seems to influence the odds.

How do you tell when the readings are bad whether the cause is a bad sensor or a bad insertion?

All I meant was,
If it is a bad transmitter, it will still be bad when you do a new sensor, and a new location.

If it is a good sensor and a bad location, the next location will work.

As everyone has mentioned so far I have noticed that it depends on the area and how much diabetic abuse it has sustained and if clothing/activities get in the way. I have also noticed that whenever I have my laptop on my lap or stomach it always gives me the ??? for a couple hours. They replaced the dexcom meter cause it kept giving me the ??? but the second meter does that as well and that’s when I figured out the lap top thing. Now that I keep the comp farther away from me I haven’t gotten the ??? crap anymore even with the same injection sites as before.

I’ve been on the Dexcom for a little over 2 weeks now and yesterday had my second sensor pop up the ???. Waited awhile and it started giving readings again but they were pretty erratic. Was ‘working’ until I got up this morning and it had the ??? again. So I decided to take it off and start a new sensor. By the time I got to work, I did my 2 BG readings and within a half hour it popped up the ???. The first two I had placed it on the left side of my stomach, but the replacement I did this morning was back on the right side. Called up Dexcom and they suggested that when I put the pod on, I should do it at bedtime but not start it until the next morning… giving the new pod time to get adjusted to my body. I left the one on that I put on today and after starting and ??? again it finally started giving readings again this afternoon. Just not confident in what it’s telling me now. It’s really quite frustrating.

So I’m not sure if it’s bad sensors, bad placement or a wonky system!

I had this frustration a year and a half ago when I started on Dexcom. My Dex Clinical Manager reminded me to keep the display definitions close and in mind all of the time. ??? means the receiver is getting signal and data from the transmitter but the value is too goofy to make sense. Action I was told to take - put receiver on the transmitter for 15 minutes to eliminate spurious RF (makes the transmitter the loudest signal) at the receiver. Most of the time this worked. The next bailout was to restart the sensor session ( STOP sensor, then START Sensor, wait 2 hours or a little more while the sensor “warms up”). It is interesting to see what is really happening behind the scene during the two hour start up. I now have a new receiver and an old receiver. When things get a little questionable with a sensor, I check the sensor with both receivers - calculating about 15 minutes out of sync with each other.

The lesson I learned is to put the “crazy” receiver in a filing cabinet for about 30 minutes to “clear” itself, then continue. Thirty minutes is enough for the computer in the receiver to clear itself and go on correctly.

Two important points:

  1. Do not load finger sticks more frequently than every ten minutes EXCEPT at start. Start up finger sticks can even be two tests from one poke.
  2. Do not load finger stick numbers when ??? is showing.