Meter vs CGM

I was put on a CGM yesterday by my CDE that will last for 7 days so that they can have more information on my blood sugars than what the snap shots of my pump tell them. Now that I've had it for 24 hours, it's making me wonder what's truly accurate for a blood sugar reading.

The CGM is giving me readings that have been way off from what my meter says. And they have been both above and below what my meter says. For example, the CGM alarm just went off saying I was low (69). I just checked with my meter and that said 114. I wouldn't normally treat a 114 at all, but which one is correct?

Also, this morning, the CGM was at 128, and my meter said 158. How can it be that far off? In looking at the CGM, I thought I was doing great! NOT!

And yesterday, the CGM gave me a reading in the 150's and my meter said 120's.

Which one is correct? Which should I trust? I've never had a CGM before and I really like that I can see where I am at all times, but now I'm just really confused!

A CGM will tend to drift over time so you enter your meter readings to "recalibrate" it. Have you been entering your meter readings?

I have been entering them in. My CDE told me to calibrate 3-4 times a day and not to over calibrate.

That is correct. You would expect some variation since both the meter and CGM really only have +/- 20% accuracy. And if your blood sugar is changing quickly that will affect things. The CGM measures interstitial fluid which has a delay of like 15 minutes from the actual blood sugar values.

I understand that, but I don't understand why it's telling me I'm low when I'm really not, or wasn't in the last 15 minutes or so. Right now it's telling me I'm below 40 and I just checked with my meter and my meter said 95.. It's making me freak out when I don't need to!

Which CGM? Typically, when it's a trial CGM it's blind and we're unable to see results. Did you get a trial Dexcom? If so, when starting out on any CGM it takes a few days for it to work. I had a trial Medtronic CGM but couldn't see any numbers, the sensor was downloaded by my Endo team afterwards. I use a Dexcom CGM daily and it took a good few days to get numbers accurate. In fact, I just got a new Dexcom and it's off, it will take a few days to stabilize. Again, I'm not sure which CGM you're using but you should calibrate only when the CGM tells you to and don't calibrate when rising or falling.

It's a Dexcom. I started seeing results after a couple hours. Sometimes it's accurate, but like just now it told me I was 39 and I check on my meter and it was 94. I entered the 94 into the Dexcom, only because it doesn't make sense to me that it's off by that much. I guess I'm just worried that my endo will adjust my pump settings based on what the Dexcom says, and if that's the case, I won't be getting enough insulin.

OK. Well, like I said it does take a few days. My Dexcom really is accurate, just a few points off most of the time. But when I change my sensor it can take a while to get stabilized. Also, sure you know, but please don't dose from the CGM, always use meter to treat.

Thank you for your help! Now that I've entered in that last blood sugar, it seems to be accurate to where I'm at right now. I really liked how I could see what was going on, so I was thinking about getting one permanently, but after seeing that, I thought there would be no point. But maybe newer ones do a better job and maybe it is worth it!

you're welcome! that's great. i love my Dexcom...would do this over the pump, if i had to make a choice. it will start to get more accurate. good luck!

CGM technology is not perfect but after the first day or two, it correlates much better with fingerstick BGs than you've experienced. As Sarah said, don't calibrate when the CGM indicates rising or falling with its arrows. And as Brian noted, the CGM lags fingerstick BGs by about 15 minutes.

Only make treatment decisions (add insulin or food) on fingerstick data. Since home glucose monitors are only required to be about +/- 20% accurate, I do two fingersticks when calibrating and then enter the average of those.

Using a CGM takes some experience. And it usually gets better with time. I often get more accurate numbers in the second week of my Dex G4 system. What system are you using?

It's really interesting getting feedback from everyone about CGMs. I'm brand new to it and I didn't have time to research it at all, so I was basing my knowledge off of what the CDE told me in a quick 10 minute run down. I have this one for 7 days total, so I'll see how I feel about it after then and make a decision about getting one.

The CGM that I'm using just says Dexcom on it and it's oval shaped, so I'm not sure how old it is or anything like that. I know it's not the newest one because I know those are in color!

My experience with the previous generation of Dexcom was that trend data was awesome, but the accuracy is not all that great.

CGM: 400, Meter: 200 (1.5 hours after a meal)
CGM: 50, Meter: 115 (3 hours after a meal)
CGM: 225, Meter: 120


After using it for 7 months, I gave up on it.
I was tempted to try the new generation, but not enough to bother with it.

The CGM you're wearing is Dexcom's 7+ model. I used that system for three years. The Dexcom G4 system is the latest. The G4 is more accurate than the 7+ and has greater range, that is the signal can transmit greater distances from you to the receiver. I had a good experience with the 7+ and like the G4 even more.

It takes some commitment to make any CGM system work for you. You have to carefully prepare and place the sensor and then feed it good calibration data. I will never willingly go back to living without one. Some people don't like all the data and the distraction that alarms sometimes give. My experience with false alarms is that they usually are limited to the first and maybe the second day. I'm hooked on knowing where my BG is and where I'm headed.

cjmnews - I'm sorry your experience with the Dex CGM was so poor. I had a much better time with it as you can read above. The CGM technology will certainly improve as time goes by. Until there's a cure, I know I'll always want to wear one. To each, his own!