Disaster with changing from Libre to Dexcom G^

I’ve had a Freestyle Libre for over two years. Ever since the beginning, at least half of my sensors have given me readings twenty to thirty points or more lower than my ReliOn Micro glucometer. And lately they seemed to have gotten worse. I trust my ReliOn far more than the Freestyle, as I’ve taken it with me and compared it whenever I’ve had blood draws, and it has averaged reading only two percent higher than the blood draw result.

After much consideration I decided to switch to the Dexcom G6, and started the first one today. My first reading was 93. I thought “Good. That will allow me to have my beer before dinner,” as I expected I still had enough insulin left from lunch to cover it. But the number started dropping. 83. 57. 40. No symptoms. Okay, bring me some pretzels with the beer. The reading continued at 40 for over an hour. Then I decided to go ahead and eat dinner without pre-bolusing.

When I finished dinner and the Dexcom still read 40, I decided that it had surely had enough time to catch up with an actual blood reading, so I used my ReliOn to compare. Oops. 290. Then I realized that my last Freestyle sensor hadn’t quite expired yet. I hadn’t compared them up until then because I was thinking it expired an hour or two earlier. 261. Okay, so I injected 14 units of Humalog. My Dexcom is still giving me readings in the 40s and 50s, three hours after I cracked open that beer. The Freestyle sensor has expired. Insulin works so slow in me that I won’t bother to take another ReliOn reading for another two or three hours. Because of another medical problem I’ve had lately that has given me higher than normal readings, my guess is that I may have under-estimated the insulin I needed rather than over-estimated with that 14 units, so I doubt that waiting will present problems.

I’m obviously new to the Dexcom. I’d appreciate it if some of you who are experienced with it will give me some hints of what might be giving these crazy readings. Something I did wrong? I have a receiver only, no smart device to go with it.

Did you start the Dexcom sensor immediately after inserting it? If you insert the sensor and then wait anywhere from 12 to 24 hours to start using it that usually gets rid of wonky first day numbers.

I always test once or twice on a new sensor for at least the first few hours before I start relying on it’s readings. If my sensor is inaccurate I have always calibrated it to match a finger stick reading. This doesn’t seem to affect the sensors function or accuracy later on in the sensor life as long as BG was really stable before calibrating. Never calibrate if the dexcom arrow is anything but flat for at least 15 minutes prior to the calibration.

A wildly low BG right after starting up a new sensor is the number one issue I encounter with the G6 but if you wait it out and don’t rely on it’s readings for a little while it will usually correct itself but if not calibrating is definitely the way to go.


I agree with Firenza. The only difference is I don’t let it sit before I start it as I usually never have enough warning when one is going as I restart mine and when it finally gives up, it gives up pretty quickly.

But a brand new sensor always starts really low on me, I literally do a blood test the minute it starts and I calibrate it right away. Sometimes I will be calibrating it after about another 4-6 hours too and then maybe once the next day. Then it usually stays within 5 points of accuracy for me.

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Found my error. The gal who sold me the Dexcom told me that I wouldn’t need to compare it’s readings with blood tests. That led me to believe that I didn’t need to calibrate. For me, with as highly inaccurate readings as I got with the Freestyle, that probably isn’t the case here. Because I have an appointment with my endo in a week, I was in a hurry to get started with the Dexcom so I’d have some experience with it and be able to be in a position to ask any questions, so didn’t take the time to read all the directions before jumping in. Will have to spend some time today reading and get the thing calibrated.

@Uff_Da I had a freestyle Libre before my Dexcom and like you it was always off consistently by about 20-30 points. I just learned to add the points off for dosing. I still use a freestyle on my diabetic dog.

The Dexcom is great because you can calibrate it and it becomes very accurate, usually after the first 24 hours by 5 points for me. I calibrate mine a few times when it’s new until I have the accuracy I want. Just don’t calibrate too many times in a short period of time because you will confuse it.

Once you start using it to it’s full potential you will find it works much better than a Libre. Not only because of the accuracy with the calibration, but the alerts. I started with alerts set at the ranges I wanted to stay within, but as time went on I started narrowing my alerts so I had time to respond before I went out of my range. I have rather tight settings now. My A1C dropped over a point because of it. I really have become very attached to having alerts for lows and highs. At first when you are not used to them they can be annoying. So set the alerts to what you are happy with them at

And for right now we can still restart our G6’s. They keep trying to change that ability but it is a great way to build a back up supply and if you pay for them a way to save money too. The benefit I like is that a restarted sensor doesn’t need the calibrations a new one does at least not as much. I still have to calibrate it at least once. It’s also a lot nicer to just keep one on than switching it out every 10 days.

They tell you you don’t have to calibrate it, but their own booklet says “if in doubt get your meter out” . I think most people do calibrate it. But don’t trust the first 6-12 hours without checking it.

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I’ve learned to change my sensor early in the evening so it’s no hardship to ignore (or hide) my phone with all its low alerts! :laughing: I try to insert the new sensor at least four hours prior to the old one expiring.

One difference to some others is that I NEVER calibrate. I wait it out, and within 12 hours (usually much less) it has righted itself and is giving me accurate readings.

I loved the Libre but I love the Dexcom, too. I hope it helps you as well.

There seems to be no middle ground for me, they’re either fabulous or absolute excrement. I’ve been stuck on a bad batch myself. I think Dexcom is having some quality control issues since ramping up production. They’re good about replacing sensors, though. Eventually you’ll stockpile enough replacement sensors to cover yourself from the bad batches. I’ve only had one sensor I didn’t like before now, but I’ve gone through five sensors in just over a week.

The one thing I have noticed recently is that I occasionally DO get accurate readings right off the bat with a new sensor, even when I haven’t been able to do the pre-soak thing. I’m not 100% sure what makes the difference, but it happened with my most recent sensor change, and the one thing I’m aware of is that my BG in the period leading up to the change, through the warm-up, and for a couple of hours after was very stable, running between 90-120 give or take. I’m wondering if that might play a role. Because one thing about that is that it’s an awfully hard thing to arrange or plan for in normal Type 1 life. How often do you get six consecutive hours where you’re not coming down off a post-prandial, or starting one, or a doing a correction, or taking some exercise, or whatever anywhere along that time line? How about never? But it might be that you get the least wonky results by presenting the system the fewest challenges you can manage in that initial period.

If so, it seems like Dexcom might say that somewhere. “For best results, try to maintain stable blood sugars at the beginning of a new session while the sensor is settling in,” or whatever. But AFAICT they don’t even acknowledge the existence of the New Session Wonkiness problem, so that makes it hard for them to recommend ways to minimize it.

You can calibrate libre with third party apps like tomato and xdrip but you need to use that software and not the freestyle app. I assume you have a pump so you will need miao miao etc. Through daily calibration of one fingerstuck you save 3x over dexcom with same accuracy. Ialso have the g6 now mothballed.

No, I don’t have a pump. And I really prefer not to use third party apps or any more computer craziness than necessary. Now that I’m using Dexcom, I’m using the receiver only, no smart phone. I’m 78 years old and all this computer technology is getting to be too much for me. But I sure like not having to poke my finger six or eight times a day.

Dexcom replaced my first wonky sensor. Thankfully, my second sensor on Dexcom worked beautifully the whole time. Comparisons with blood tests were often only a point or two apart. I started the third Dexcom sensor a couple of hours ago and so far it is reading low - 56 compared to a blood test of 85. Hopefully it is just first day wonkiness and won’t give me a false alarm all night again. That was intolerable.

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Yeah, occasionally a sensor will just be broken right off the bat, or within the first couple of hours. Shame it was your first one that did it, as otherwise you’d have realized it was just bad. Stick your finger whenever your symptoms disagree with the sensor reading.

When it’s bad – usually either unbelievably low, or else any value but not moving – call support and tell them you have a bad sensor. Depending on what rep you get, they may encourage you to calibrate and wait a few hours. Those reps are reading from their screen; the more experienced will agree right off that it’s bad. If necessary, just keep repeating “this sensor is bad” and eventually they will send a replacement.


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It does not sound like you “soaked that sensor”. You may want to try that for your next sensor. Put the new sensor on 12-24 hours before your current one expires. That lets the wire get used to your interstitial fluid. When the current one expires, move the transmitter from the current sensor to the one that was applied (soaked) 12-24 hours earlier. This procedure solves the problem you mentioned for many of us.

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