Control-IQ and Dexcom calibrations

This forum is new for me, so this may have been discussed and I haven’t been able to find it. If so… sorry! and please point me to the right place. I also apologize for the length of this post…

I graduated from my old Minimed/Medtronic pump (the last of about five) to a new Tandem t:slim (and who came up with that name?! :slight_smile: plus a Dexcom G6 in January/February. Since I’m on Medicare, they couldn’t ship me one of the new pumps with Control-IQ technology, just Basal-IQ. I thought I’d have to wait until Control-IQ was approved by Medicare before I could get it. But apparently not! I just finished the training videos for Control-IQ, and have gotten the link to download the software. Don’t tell the feds…

But I have a couple issues I’d like clarified. First: users and professionals have been telling me that the true value of the Dexcom CGM is to let me be aware of trends and patterns, not to provide precise numbers that match your blood glucose (I’m sure you’re all aware that CGMs don’t measure BG, right? And sometimes the two don’t agree well with one another.). But… both Basal-IQ, and even more so Control-IQ, DO use the CGM data as if it was accurate BG data. There are several numerical parameters in Control-IQ that control its behavior (when your glucose is due to go over 160 within the next 30 minutes, for example, the pump starts ramping up your basal rate). So it’s interpreting the Dexcom values as BG values to do its control thing.

When the Dexcom values are close to true BG values, no problem. I’ve had that be true for several of the sensors I’ve used since January. But not always! Today, for instance, my new sensor started out with values 30-40 below the true BG values. At one point, it shut off insulin delivery altogether because it thought I was at 105 and going down, whereas my glucose meter told me I was at 165 (and it turned out I was NOT going down! So the Basal-IQ was misbehaving.

I know I can correct such differences by doing a calibration, which I did today. But: I’m given to understand that one should avoid calibrating if possible, and that once you start calibrating a sensor you can count on having to keep doing it for the rest of that sensor session; and I have a tiny bit of experience that supports that. Sometimes it seems I can just wait through the first day and the sensor “fixes” itself, and runs close to BG from then on. But that would mean my pump doesn’t work properly during that first day of the sensor session, and that would mess up Control-IQ even worse. And maybe give me false alarms all day.

Any advice? And if any of this is inappropriate for some reason, let me know that, too.

I will say I love the t:slim, for several reasons. I’ll put up with having to do the frequent calibration route if I have to, but that was one reason I rejected the Minimed 670 route (only one of many reasons, though :slight_smile: . I think I’ll be installing the Control-IQ technology in my pump in about four days, as most users seem to like it and its features look very nice. But I can hang with Basal-IQ if I find that seems a better choice.

Just FYI: I was diagnosed with T1 back in 1960, so I’ve had some experience with various types of therapy. Dealing with T1 may be a bit of a hassle now, but I’ll tell you it’s WAY better than it was back when I had to boil my glass syringes! :slight_smile:

For the most part, I find the G6 to be spot on. I have no problem trusting the t:slim to auto-magically control my values (I keep mine in sleep mode constantly)… BUT, I don’t hesitate to do a fingerstick if I feel it’s not right. I always test and calibrate immediately after the warmup, when I get my first numbers. I also check if I feel “fuzzy”/hypo and the Dex doesn’t agree. If I were unaware of hypos, I’d probably be a little more scared and test more often. As it is, I probably test less than ten times a month now.

I don’t really abide by the calibration rules (only calibrate if it’s more than a 20%), because I expect better than 20% accuracy and I don’t mind calibrating it into submission. I do make sure the trend is flat, though. Most sensors only require the rare calibration, thankfully. In the six months I’ve been using the G6, though, I did have one sensor I just hated. It seems like there’s a little gamble in luck of the draw.

While I have experienced one or two G6 sensors that make me wanna tear my hair out most for me in the year I’ve been using the G6 are spot on, but I do not hesitate to calibrate if it is off by more than I’m comfortable with.

If your sensor is misbehaving though I would turn off the basal-IQ feature until it settles down. You can turn off basal-IQ and control-IQ and your pump will revert back to your set basal rates while still displaying CGM readings. For instance this afternoon I had a crazy fast drop on my G6 but when I decided to test to confirm my BG was in the 170’s. In that instance I would have turned off automation and waited until the CGM values were stable again before resuming. This is all theory for me at the moment though since I just started the process of getting a T:slim.

Many thanks! Everything you mention makes sense, and helps me confirm my feelings about how best to use this device. I won’t experiment with staying in “sleep” mode 24.7 just yet, but will keep it in mind. I appreciate your response.

This seems reasonable, though it seems to go against the common wisdom of “don’t calibrate if you don’t have to.” In the past, once I started calibrating, I found I had to keep doing it, as the Dexcom seemed to drift a bit. This time, that wasn’t the case: I calibrated a few hours after I started the sensor, and the calibration seems to be sticking quite well so far. Thanks again.

Duh! Of course! Why didn’t I think of that?! It won’t alarm or take bad actions if it’s not even on. I guess I didn’t realize I’d still be getting CGM readings (though I tend to view them on my iPhone, for maximum convenience and display goodness). I’ll remember that for my next goofy sensor. Thanks!

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Keep a record of CGM accuracy, for your own piece of mind. But, also so that you can get replacement sensors from Dexcom if there are inaccuracies. Sometimes people get a bad 3 month shipment of sensors. We do not have a well discussed, well established procedures for dealing with that. Its a fairly new thing. You will find disagreement within the community. We are still working that out, so your input and experience are appreciated.

There are a few strategies, some mentioned here. Keep us up to date on how you are doing. This can be a tricky thing.

My first shipment of G6 was terrible. That was a 90 days supply. The 3 shipments after that have been fine.

I’m a little concerned about the timing in changing tech. In some ways, it might be ideal because you have time at home to get to know the devices. But, on the other hand, there might be increased risks. I assume you have your old tech and you are comfortable falling back on that if you need to.

Thanks! I’m keeping a record of sorts already.

You say: “Keep us up to date.” (1) How do I go about doing that? and (B) Does this mean you’re a Dexcom rep? (Sorry: I’m new here). If so, very good of you to discuss this so openly with us end users, and kudos!

I am not a Dexcom rep. I am a person with diabetes. But, you can just keep posting on this forum thread and we will keep trying to help if issues pop up. Eventually, either we, or the Dexcom nurses line will need to talk to you about the 20/20 rule. We have posted it elsewhere. I’ll look for it.

OMG, we are just going on and on about it here…we should find a better write up for this. Its too much. This is the other guys trying to explain 20/20 to me. We could summarize this better.