CGM unreliable readings?!

I have only been wearing my CGM for about 3 days and I like it so far but I am concerned about the differences in the readings between the CGM and my test strip meter. My meter last night read 90 and my CGM read 123. I think this is a big difference. Has anyone else experienced this?
Also, can you shower with the CGM on?

One thing to remember is that there is a lag between the sensor and a fingerstick. The sensor reads interstitial (sp?) fluid and not whole blood like a BG meter. It takes about 15 minutes, give or take, for the interstitial fluid glucose levels to read what your meter said. That being said, it “should” be a little closer than that. I think Minimed (that’s what I use) says it should be within 20% of a given meter reading. Which would put the sensor for you at 108, not 123. But it’s still not THAT far off. If you haven’t eaten in a couple hours or bolused for a couple hours, then I would do a sensor update just to bring it a little closer. Always remember that sensors do not replace fingersticks. They are there for trend identification and to warn you if you are quickly going up or down so you can do something about it before trouble strikes. I usually do sensor updates about 4 times a day. You should be doing a minimum of 2 (if you’re using Minimed), but no more than 5 or 6. Too many sensor calibrations will make it REALLY unreliable.

For your other question, I shower with my CGM on all the time. Its waterproof so its no big deal. Just make sure its taped down pretty good since hot running water can lift the tape from time to time. Just never get into a hot tub with it. Being submerged in 103+ temperatures (which most hot tubs are) will cook the circuitry in the transmitter.

Hope that helps some! :slight_smile:

Hi Eden, that range is pretty close to what I get. I have found that the CGM is best for tracking changes of blood sugar, not for really accurate readings. If it is high or low according to the CGM I check with the meter. Any reading below 100 via the CGM,I check with the meter. If you have a fairly stable flat line on the graph, you are probably OK. With time you will recognize the trends, but I do not trust 100% the CGM for a good glucose reading. It is great for following trend levels, not actual blood glucose.
As far as showering,as long as the sensor and transmitter are connected, it is waterproof. I have a clear"bioclusive" dressing over the whole thing on my thigh. Otherwise it kind of of flops around and once fell out and was lost in the driveway for a day.

Eden, read all you can on this site regarding CGM calibration. The process you’ll need to follow to get accurate CGM readings is critical. I was a Medtronic CGM user for two years before I dropped it due to inaccuracies. In my case the CGM was off by an order of magnitude. Sometimes over 100 points (CGM vs. BG meter reading). Anyway, in my case it didn’t work. Keep trying & study this site for info. There is a lot for you to learn before you’ll get to a point where you can decide if it really works for you.

Just another one joining the band wagon. Keep in mind each as a ± 20% range. If you CGM and BG match, you are ready for the lottery.

Which CGM are you using?


Justin’s comment is excellent and very complete.

As he mentioned, on calibrations: Do it a couple of times a day, as recommended by the manufacturer, when you haven’t eaten in a couple hours or bolused for a couple hours, to bring the readings a little closer, within less than 20% difference.

Well, when you figure the meter is off by plus or minus 20%, and the CGM is off, not only because of the lag, but also because it’s an imprecise machine, then the difference you got is not extreme. When it’s off by 100 mg/dl, that’s another story.
And remember what the manufacturers say – if you get a high or low reading on the CGM, then you should do a fingerstick before attempting to correct. I wouldn’t correct for either 90 or 123 – both of them would tell me I’m in a pretty good range. But if the sensor said 180, then I would do a fingerstick and correct or not depending on that.
And, as others have said, you’re really looking for trends, not exact numbers. If I see the double arrows going down (I have an MM), I am surely going to watch it, whereas before, I just wouldn’t have known.
The other thing I have found for myself – don’t know if it applies to anyone else – is that when my CGM reads low, I’m usually 20-30 mg/dl higher than that, and when it reads high, I’m usually about the same amount lower. So I usually depend more on the arrows than on the actual number.
Good luck with it – as you get used to it (which took me weeks), you’ll figure out its idiosyncrasies, and it will become more useful to you!

I use a navigator and my teadings are consistently closer than 20%. I know that it is impossible to get one anymore but the key for me is calibration. Check bg as recommended to calibrate and you shpuld be close. I use my calibrated reading to treat although I know it is not recomended. I find it is very close to meter after calibration, unless I have recently eaten something with simple catbs or sugar. Keep calibrating and ot should get closer.

Hi Natalie. Please don’t take this as being arrogant or whatever, but what BG meter are you using that you are figuring that it can be off by 20%? All meters nowadays are 97%+ accurate, at least to my knowledge. Or did I misinterpret what you said?

Excellent point Paula. Timing of the calibration is essential. I learned that one the hard way 2 1/2 years ago when I started on the pump and CGM with Minimed. Once I figured that out, my sensor has much less than a 20% variance from my meter. Well, usually anyway.

No, you didn’t misinterpret me. I just wonder where you got the info about 97% from, because I’ve seen the 20% off figure lots of places, but never seen a 97% on figure. And I’ve read posts in various places about pressuring the FDA into requiring the meter companies to make their meters more accurate.

I have an Accuchek Aviva (actually 3 of them!) and it’s good enough for me – when it says I’m low (which is where the accuracy really counts) I can feel that I actually AM low. If I had hypoglycemia unawareness, I’d be a lot more worried!

I looked into this more too. You’re right. The FDA hasn’t changed their accuracy guidelines since about’86 or so. Go figure. The weird part, then, is that I compare my bg readings to the lab when I have my blood drawn with my 4 meters (I know, anal retentiveness at its best!), and my bg readings are always within +/- 5 of the lab report. They’re all different brands, Accu-Check Compact, Freestyle, One-Touch, and a Contour USB. So that’s where my accuracy comment came from mostly. That and I honestly thought the meter companies had increased their overall accuracy. So, basically I was wrong (my apologies), and I my meters must be either super lucky or super accurate. I’ll take either one though, because I am hypo and hyper unaware.

The other MAJOR POINT to remember, which is supported by most CDEs and common sense, is to never dose insulin by CGM. ALWAYS DOSE INSULIN FROM FINGER STICK!