MedT CGMS users - question about restarting old sensors

I’m fairly new to the Medtronic system, and it finally looks like I have a sensor that will keep going for quite a while - I just hit day 3 and the ISIG values are still quite high (very happy about this).

I just did the “start new sensor” drill with the old sensor about an hour before the sensor end time (I didn’t disconnect the MiniLink before I did this), and within 20 minutes or so it prompted me to calibrate and has been giving me (very accurate) values ever since :). I was under the impression that when starting up an old sensor as a new one I would have the usual 2-hour startup time before seeing readings (not that I’m complaining!) - anyone have the same experience? I’m wondering if this is because the transmitter was never disconnected and so the pump was getting fairly constant ISIG readings… Any ideas?

The Transmitter is reset when it’s connected to the charger. Once it’s connected to a sensor it starts a 2-hour counter. When that 2-hour counter completes, the MedT pump detects it and asks for the first calibration. From that time forward the Transmitter just keeps sending Isig values to the pump until it’s timer hits 7-days (at which time you have to disconnect the Transmitter and plug it back into the charger for a reset). The 3-day timeout is in the pump. Once you tell the pump a new sensor is connected, it will prompt you for a calibration quickly (since the Transmitter has already gone past it’s 2-hour timer).
I hope that’s not too confusing, but it’s late and I need some sleep. :slight_smile:

Thanks Ken,
That makes perfect sense - I didn’t realize the transmitter had a different reset time than the pump!
Cool, that. It’s great not to wait 2 hours for startup every 3 days!

My sensors are the same way – the prompt for new calibration comes within 10 minutes when the sensor is restarted. My Minimed trainer said that they strongly discourage using a sensor for more than 3 days (at $38 per sensor, of course they do – the actual manufacturing cost is less than half that), but I have been told unofficially that they can go for 6 days. After that, there may be scar tissue issues. Also, I have noticed that the sensor is far more accurate longer it is in. But, accuracy is still an issue. Anyone else with the same issue? How bad is the inaccuracy you see? Thanks.

I too have noticed that the sensor is more accurate the longer it’s in - from what I’ve read this seems to be a pretty common thing. I was pretty frustrated with this system at first because I couldn’t seem to get sensors to last more than 3 days, and the values were also WAY off (30-50 points) unless my BG was stable for a long time - while I’d heard of people using the same sensor for a month with great accuracy!
I just assumed that my steadily falling ISIG was due to my fantastic immune system (more brawn than brains), but I discovered by chance that recharging and reconnecting the transmitter seems to result in a higher ISIG. It’s now day 9 on this sensor, and for the past 2-4 days I have seen a HUGE improvement in the accuracy - 8 times out of 10 the sensor reading is within 10 points of my BG when I test. I have also noticed an improvement in the lag time when i’m rising/falling rapidly.
As for the scarring after 6 days bit, not sure what I think about that. Seems like it might be a bit of an exaggeration, considering there are 7-day systems on the market and it’s in MedT’s best interest for us to be changing them often - but I’m sure scarring does occur at some point. Regardless, I plan to use each sensor as long as I possibly can so that I can stockpile them!
How long have you been able to keep your sensors going?

I have not tried to keep a sensor going longer than 6 days. I think I will give longer a try. As for the lags and the inaccuracies, when I download/upload all my information into the Carelink software and look at the daily results from the sensor and the OneTouch numbers, the trends generally track fairly well.

It is frustrating to me that the sensors are always behind the OneTouch, often by a half hour or more, when my BG is changing. It is also frustrating that the sensors do not seem to have the same sensitivity to range – when my BG is falling and the meter says 55, the sensor is still over 80. The over-riding reason for getting the sensors was to have an alarm system to let me know before I get too low, particularly at night, which is particularly important for me since I live alone most of the time (separate, with a young child), and they have not helped out in that regard. It is a useful tool, though, for getting better control.

Yeah, those lag times are a pain in the ■■■ - I guess it’s kind of hard to avoid since we’re measuring interstitial fluid vs. blood and that can really differ between people, but hopefully they can eventually work out some fixes for the software that would allow the system to “learn” how quickly an individual’s plasma levels rise or fall compared to their BG.
I have the same issues with range, so I usually just set my low alarm to around 100 if it’s reading high or lagging a lot. It alarms too much, but at least I know when I might be going low before it happens. I have found that it seems to have better range sensitivity if I give it a calibration right when I treat a high or low (only if the onset was slow, though) - so the calibration can stick before the sugar or insulin have time to work. Have you tried this at all?

My experience is similar. The CGMS doesn’t tend to go quite as high or quite as low as my glucometer. I’ve found that tightening up my alarms helps with that. If I actually want to do a bolus correction at 180, I’ll set my high alarm at 160. When it happens if it looks like it’s still trending up, I’ll take a finger test and correct, as needed. Similarly, if I tend to experiences lows at 65, I’ll set my low alarm at 85. Sometimes that can cause excessive alarms, but at other times it’s better safe than sorry.

I’ve had 2 sensors go for about 12 days. A few only 3 (that was early on, before I learned some of the tricks). But I generally get a solid 6 accurate days and then the accuracy decreases (especially after I recharge on day 7).

The system definitely needs to be better at “learning”. My Minimed trainer suggested that the system is supposed to learn our patterns from the calibrations, but there has not been much change in the variations over time.
I set my alarm at 80, because it should give me an indication before I get into too much trouble. If I set it higher, I’m afraid it will go off continuously.
I will definitely try your idea about doing a calibration when there is a low. Interesting idea.
Do you find that you are testing more or less frequently now that you have the CGM? I think I actually test much more often than I used to.
Also, do you have a problem with Minimed’s supply line? They obviously need to increase their production of the sensors. Every time I order more, they are out of them.

I’m on day 13 with my sensor now and I’m starting to see a little more inaccuracy (I think). I’m not giving up on it yet :slight_smile:
I’m definitely still testing a LOT, especially when the sensor’s not too accurate. But the past few days I’ve been testing about 9 times a day, definitely down from the 20+ i was doing before I got the sensors.
It’s been really tough for me to get supplies too, Jonathan. After I placed the order for my transmitter it took about 5 weeks to ship, and there’s been a 2-week backorder for the sensors each time I’ve ordered them. I’ve just been calling in a new order each time I get a box of sensors so I don’t run completely out, that seems to work well and I’m ending up with a little surplus now that I’m able to keep the sensors going longer :slight_smile:

I’m testing on average 12-15 times a day with the sensor. Right now, I’m waiting for the backorder to come in, so I am without the sensor, and am actually testing less, perhaps 8-10 times a day (excluding exercise periods, when I test 3-4 times an hour). Thankfully, test strips are covered by insurance, but I keep getting inquiries from my insurer about why I need to test 15 times a day, when their guidelines are testing 4 times a day (ha!).
Good idea on always putting in an order for more sensors. I’ll try that.
Do you get soreness or irritation for having the sensor in that long? I’m always concerned about scar tissue developing around infusion sites and sensor sites.

I have gone 10 days on one sensor. I do not know why it doesn’t take the 2 hours for calibration but, I like it. I have had pretty consistent success with using a sensor for 7 or 8 days. The deciding factor in whether I can “start a new sensor” without actually changing the sensor has been the movement in blood sugars the few hours before the time is up on the sensor.

I’m now on day 16 with my sensor and it’s more accurate than ever! My ISIG values are still really high, around 12-20. No irritation from the sensor itself - the tape gets a little itchy when I exercise but once I clean the area around the sensor and change the tape it’s fine. I don’t know for sure that I’m not getting scar tissue, but it sure doesn’t feel like it.
This is the first sensor I’ve tried on my upper butt cheek - I had terrible luck with sites on my abdomen and couldn’t get them to last past 4 days. Needless to say, I think i’ll stick with the butt sites from now on. :slight_smile:

I’m always looking for a new place for the sensors and the infusion sites – how do you twist around to get them in to the “upper butt cheek”? I used the abdomen for a while, but after I lost a huge amount of weight, there wasn’t any room for them. Now I use the upper thighs, which are getting a little pock-marked with the sites, plus, with my weight loss, there is not much room left under the skin for the canula on the infusion sets – thankfully, Minimed came out with the 13 mm canula, but that is still too long for me.

I envy the ISIG levels you’re still getting. My butt cheek sensor is only on day 9 and the ISIG’s have dropped to the 4 - 8 range. Sometimes it’s way off and I’m about to throw it, but then after a little new tape and/or massaging it starts tracking pretty well and I think I"ll let it go a little longer. I’d really like to consistently get 12 accurate days per sensor, then the cost wouldn’t seem so bad.

I just pulled out the sensor, on day 20. It wasn’t tracking so well and I wanted to make sure the skin was ok underneath, but I probably could have kept it going longer. The site seems fine, just a little red bump. The transmitter just finished charging and I am working up the courage to put in a new sensor.

Jonathan, to get the sensor in the upper butt-cheek area I twist around, pinch an inch or so of skin in front of a mirror, and use the insertion device at a 45 degree angle so that the sensor goes in towards the body. You’ll want to make sure it’s at least a couple inches below your waistband.

As I understand it, the ISIG value is close to what your glucose measurement is, according to Medtronic. For example, if your CGM graph says 130, your finger stick says 130, your ISIG value should be around 13.0, give or take the 20% leeway in sugars for lagtime. To avoid sensor errors or calibration errors, you can even look at the ISIG value before you enter your reading to determine if the sensor will accept the reading. Medtronic will give you the calibration calculation over the phone if you are really interested.

Medtronic requests that you do calibrations when your glucose is in your target range (which is difficult) but I have found that when I do that, the meter and the sensor agree almost to the number in accuracy for future readings, i.e. sensor says 170 and meter says 172. They also say that if you have to treat (low or high) do not calibrate, since there is a lag time for the sensor to recalibrate itself and by that time, what you have eaten or the bolus you have given will make the sensor reading less accurate. I do not know, since I am quite new to the whole system. This has helped me test less, and I test a LOT, because the readings actually are accurate, or if I do test, it matches the sensor value anyway and it makes it easy just to do use the easy bolus feature or the bolus wizard without much fuss.

Anyone with similiar experiences?

Hi folks…when do you re-charge the transmitter if you are tring to stretch the sensor out for a minimum of 6 days? I am going tomorrow to start a new sensor (not really) on my 71st hour. If I have to untape to charge the transmitter to I have to re-start the sensor too? Thank you.


Hi Staci -

I recharge the sensor for the twenty minute recommended time and reconnect the transmitter back to the sensor.

The sensor start screen will only allow a “start new sensor” option for me once I have disconnected, charged, and reconnected the transmitter. I do not know if there is a way around this?

Typically, to extend the life of the sensor and keep the transmitter at its strongest signal, I make sure to use a waterproof dressing (for me, as soon as my sensors get wet the signal transmission is not as strong, and the water seems to lessen the life of the sensor), After much trial and error with lots of dressings, I discovered that the Tegaderm HP so far has been the best option - it is waterproof, does not stick to the transmitter or sensor so there is no gunk to clean or the chance you’ll pull the sensor out by accident, lasts three days without a need to change it, and is clear so you can keep an eye on your site and make sure that there is nothing that needs attention. Tegaderm HP is sold by Minimed, Amazon, etc. To be extra cautious sometimes I will put a little gauze or cotton right over the transmitter before laying down the dressing - it does not interfere with transmission.

Hope this helps and that other people give you good advice!

Yes that is what I have done. I do charge the sensor first.