I'm sure everyone out there who is using a CGM or CGMS, I'm not sure which is correct, I usually just say CGM but anyways this is just a perfect example of when you should never rely on what your DexCom or ANY continuous glucose monitoring system shows. It is meant to show trends and does an EXCELLENT job at doing just that.
I thought I was feeling alright but felt just a little sleepy so I remembered that before I started using my Dex I always checked whenever I felt a little unusual including feeling a little sleepy in the middle of the day just like I was at this very moment. So I pulled out my OmniPod PDM to check and WOW!!! I'd better get something to eat!
I just thought I should share this for anyone who is thinking of starting or is questioning their own DexCom. I LOVE mine and use it every day watching my trends and learning even better ways to control my sugars. After nearly 30 years as a T1 I am by no means an expert although I feel as I'm sure many of you out there do that sometimes I know more than my Endo does.
I'm curious, please, how long after this 45 reading on your meter did the Dex reflect this drop? Your trend arrow on the Dex is horizontal, right? So how long before it became a downward trending arrow and how long before the number showed something closer to 45, if ever?
I'm pretty new to my Seven + (Oct '11), but I have noted numbers go awry a bit, and this is especially worrying on the low end, of course (as you discovered). Thing is, I can do two BG tests with my OneTouch Ultra and they can be quite different as well. Last week, my Dex asked me for two numbers as it always does when it is coming up to speed, and as I recall the two I fed it one right after the other were 124 and 147. Not too too dissimilar from your number differences, but because your actuals were putting you in the danger zone, of course that's more worrying. If I feel crappy, I always test.
How long had it been since you'd calibrated when you got the 92 but were actually at 45 (assuming your Pod was correct)?
I always go with what my body is telling me and test if I feel off from what the CGM is telling me. The CGM is an awesome tool for exactly what you said - trends.
I've been getting the same sort of results with my Dex. It has gotten to the point where I just can't rely on it. If the alarm goes off, chances are pretty good that I won't trust that it is worth checking. But my CDE says that people say the exact same thing about other CGMs, so switching would probably be pretty pointless. So now the plan is to put it in once a month to get a feel for what my post-prandials are doing.
I'm also a rookie with the Seven+. I started using it only 3 weeks ago but am very pleased with it. I'm a bit worried that the current sensor may be on it's last leg. I remember starting on a Friday and since this is Tuesday I think that makes this day 11? I may be wrong though. It may only be day 4. I'll need to check into those dates before I do anything drastic. I calibrated at 7 in the morning but as I had gotten an ERR1 the night before this could all just be a problem with the sensor.
I never worry about the difference in the numbers between my monitor and my dex because like you said you can get numbers that are quite different with the same meter. They are reference numbers and the trend arrows help to show which way I'm headed.
I did post another picture to my profile that showed the Dex and my meter showing the same numbers last week. When I posted that I thought "This will show all those naysayers".
The CGM is all about showing trends. I don't think it should never be used as a meter. That slow decline from 200 that you see in the picture was a sharp spike coming from 350 a couple of weeks ago. My CGM helped me figure out what I was doing wrong and correct it.
Very interesting. My doctor told me that the reason he didn't want me to get a CGM yet is that the interstitial fluid runs about twenty minutes behind our blood glucose. Throw in the range of error known for both devices, and you're left having to still make educated guesses based on all available information (meter, CGM, how you feel, what you know about what you've been up to, e.g. strenuous exercise, possible stacking, etc.) I disagree because I can see the value of the trend data, but still...it's a crap shoot.
He told me a year ago that someone was working on an implantable device that measures the actual blood glucose, but I haven't heard anything more about it.
I've heard great things about the Dexcom, but your advice is well-taken. I'd suspect, in this situation, you had a bad calibration and the Dex reflected values about 40 points higher than you actually were... so at the left side of the graph, you might have been more like 170 than 210.
Jean, do we have the same doctor? Mine said CGMs are useless regarding accuracy & more trouble than they're worth. Testing to verify, calibrating, changing sensors--no thanks. Not often I agree with my doc, but this time I did. I test a lot to know trends. Why have an expensive device that isn't accurate?
For me, this was one of the reasons I just didn't find much value in CGMs. I've tried 'em and, for me, they just weren't worth it. I test upwards of 14 times per day and find that keeps me honest and on track. When I had the CGM, it was just too tempting to glance at the receiver and bolus or correct based on whatever number I saw. I know, I know...that's NOT how a CGM is supposed to be used. In addition, I was really worried about developing scar tissue. I'm very insulin sensitive so, for me, a pump is basically a necessity. The thought of doing something that would render pumping impossible was too stressful. And then there was the cost.
That all said, I know many folks LOVE their CGMs, and I am sure there are people for whom CGMs are invaluable. I don't have a huge issue with hypo unawareness or low BG seizures. If I did, I'd probably still be using a CGM.
Different folks need different tools for managing their D.
This is good advice. DexCom is a fabulous tool. I've found there are nuances to using it that are learned over time. It's absolutely fabulous for trending. I've made the mistake of acting on the sensor results without confirming with a fingerstick far too often. Sometimes I just think the sensor is spot on, but it usually backfires. I hear people get a great life out of the sensors, but Caleb almost never goes beyond a week. Some people find accuracy gets better after a restart, but I get suspicious. Your pictures speaks a thousand words. Sometimes I find it's just on a lag, and that it would have gotten to that 45 you show on the PDM in about 10 of 15 minutes. But sometimes it's off and those 47 points are really quite meaningful!
I've been using Dexcom since September and it's saved me more than a few times. For me, it's the trend that's so important. When that line started trending down below 100, I would have tested regardless, just to get my bearings.
Those who are critical of the cgm accuracy should remember too, as Michael said above, that our meters are just as inaccurate...which really scares me because we're dosing insulin based on those numbers. I'm going on OmniPod and received a backup Freestyle meter with strips. I tested alongside two Accuchek meters that I use and the Freestyle was consistently 20 points higher than my Accuchek. Using an ISF of 40, that's half a unit of insulin right there. Makes you feel like it's all a crap shoot.
That's what makes me love my Dex even more though. I know if I'm going up or down and that's when I monitor more closely.
After 24 hours of watching my Dexcom Seven Plus and checking to confirm it's readings and realizing that YES this is day 12 for this sensor, I've realized that it's just the sensor. I usually do get fairly accurate readings from my CGM but this is the longest I've ever worn a sensor. It's been a busy week and I'd lost track. I'll change it tonight and I'm sure those numbers will go right back to their usual accurateness.
The trend data that I get from my CGM is great. It has helped me so much and I don't think I could ever give it up. I NEVER rely on it though. My body and the way I feel is the best tool that I have for detecting hypos.
I was warned by my endo, as well as people on this site in the Dexcom group (and I think even in the Dexcom manual) that Dexcom readings will be about 15 minutes behind a finger-stick reading, because CGMs measure your blood sugar via interstitial fluid as opposed to a blood sample. So, the fact that your meter read 45 when your dex read 92 is not surprising. Especially when you consider that meters can have a +/- 20% accuracy to begin with. I have had an occasional sensor that just seemed totally out of whack and never calibrated, and that can be very frustrating, since they're so expensive. What I've done to help prevent bad lows, is set my low warning on my Dexcom at around 70 or 80. When it alarms at 70 or 80 and my trending arrow is going down, I know that I'm probably lower that 70 or 80, given the time lag. Setting the low alarm at a higher number than I would normally treat, helps me catch lows before they get really bad.
I would definitely give up pump before Dex. I truly mean that despite the varying accuracy from sensor to sensor and from day to day with same sensor. Sometimes it reads within a point of fingerstick and then other times it is 20-30 points off. Under 20 points is very common. 40 points or more usually means it's time for a new sensor.
