Dexcom Question---How often do you use your meter?

Hi all,

My son just started on the newest CGM this week. Somewhere along the way, I interpreted the CGM as a monitor I could count on most the time (in place of using a meter all day long.) We do calibrate the CGM twice per day (the readings have been spot on with our meter) and of course, if anything seemed "off" we would double check with the meter. However, I was shocked today our dr's office told us he has to use his meter just as much as he was before (8 times per day!) So what really is the point of wearing this thing??? It is working fine, and we haven't had a reading yet that wasn't within 10 points of the meter. So I guess, if he is feeling fine, why can't we count on the CGM when the new model has 87% accuracy. I feel like it is so redundant to check with a meter every few hours when he's wearing it. Also, he HATES wearing it, and the only way I have been able to bribe him to wear it (he's 11) is telling him he doesn't need to check himself with the meter but just here and there! But I guess I dreamed that up on my own. Ugghhhhh very frustrated to learn this or is my doctor's office exaggerating to cover their behinds in case something wasn't accurate?

You didn't dream it up on your own and I think your son's doctor's office is just covering their butts. But they may also be more used to the old CGM's which were not particularly accurate. It will take a while before the doctor's office can see the correlation between the twice daily calibration and the actual readings you get from the dexcom itself. I now use my meter twice a day to calibrate it and if it says I am low otherwise I rely on the CGM all the time. If you haven't had a reading outside of 10 points from your meter I would agree it is redundant to continually check with the meter and do fingersticks to prove it is correct.
I'm sorry he hates wearing it, I have found that the sensor placed on my arm is totally comfortable and I don't notice it at all. While this is not where the dexcom was "approved" for use by the FDA it certainly beats having it stuck on my stomach.
There is a really good Dexcom users forum here where you can post questions specifically about the G4 or CGM's in general.
All I can give you is my personal experience and I have been uniformly impressed with the G4 and how accurate it is. I'll let my endo's office catch up with the newer technology when they can, but since the G4 was only very recently approved by the FDA I know there are many doctors who are not familiar with it.

So actually, the official role of the CGMs is as a monitor and almost all medical professionals will tell you to only make treatment decisions based on meter readings. I think some CGM users may feel comfortable doing meal boluses based on CGMs, but it is still a risk. And taking corrections based on a CGM reading is even more problematic. Still, the CGM should let you avoid most postprandial meter readings and if you time things, you can make the preprandial readings count for calibration. I don't think that has to add up to 8.

I got the 7+ about 2 years ago and the g4 this fall. Since going on the cgm I test more often then before (usually about 8 - 12 times per day). I know that g4 is very accurate and most of my sensors are within 10 points of my actual blood sugar. But it does not change the fact that a cgm does NOT test blood. It's interstitial fluid. And while it gives a great trend, I am not comfortable relying solely on it when giving myself insulin. Anytime I bolus, I test first. No matter what. Pain in the neck? yep. But the "what if it's wrong this time?" scenarios are too great IMHO. I think your doctor gave good advice and don't think it's a CYA kind of line.

As to your son not liking it, have you tried different locations or even taking a break on weekends or one week a month? Sometimes that's what I need to feel less cyborg-like :)

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In my personal experience I would say I actually tested more when using cgm than I do without. This is because I was constantly seeing results on the cgm that didn’t make sense to me or fit in my patterns, so I was constantly testing with the meter to double check. This was with the seven… It was not very accurate or reliable for me, personally. I think I may have tested less if the cgm had worked better for me.

I agree with Kate and Sam unfortunately. I have a Minimed CGM (not the greatest, but better than nothing..) and it is 4:00 now and I have already tested 6 times. I also find that mine is accurate about 80% of the time. But I never count on it to bolus or correct---ever. I check, on GOOD DAY, seven times per day. Problem days? Don't even go there!

The MM CGM is about 20 minutes behind my actual reading. So if it alarms me for a high or low glucose reading, it is probably worse.

Tracey, I also hate my CGM and I am a great deal older than your son. However, it keeps me out of trouble---real lows and highs--and helps me organize my readings to more of a flatline. If I had been able to use one 50 years ago....

It is tough with a child. I get it, as it was me. He will still need to test a lot.

What I have personally found with the CGM is that I now test smarter rather than at set times or intervals. I have not found that it reduces my testing frequency because I probably still test at least 3 to 6 times a day on a stable day depending on how often I’m eating. If I’m having a lot of lows or highs then I may actually test more than I ever I did with fingersticks alone because the Dexcom revealed problems that I couldn’t see before.

My Dexcom is extremely accurate most of the time. If the readings indicate that I don’t need a correction and I’ve done a fingerstick somewhat recently I don’t see the need to retest to bolus for food only. I’ll set temp basals based on CGM trends. I do not do corrections based on a CGM reading. I will treat a low off of a CGM reading especially if I’m not thinking straight.

I have the Medtronic CGM and test a lot, maybe 8-12x / day, because I like having pretty tightly controlled BG and like being active. I also like trying to kick diabetes' ■■■ as much as possible and think that helps. Perhaps something along those lines might make it more marketable to an 11 year old than "the doctor says so"

I test on average 10 x per day, even while wearing the G4 24 x 7.

(Before the CGM, I tested 12-15 x per day).

I think it is somewhat dangerous to rely solely on the CGM when dosing insulin. I have occasionally found the G4 significantly off from the BG meter, most scarily on the low side. I have had instances where the CGM read 80, yet I was really in the 60's. 80 is a good number.... the 60's start to get close to danger. If I had not tested with the BG meter I would have been blissfully ignorant until something bad happened. THe G4 is certainly better than the 7+, but it is still not good enough, in both the opinion of the FDA and the manufacturer, to be a substitute for a BG meter.

Perhaps the G5 version will make it to that level of accuracy and precision.

Why should the G5 be more accurate? These devices are accurate for the environment they are operating in. The glucose level in the interstitional fluid is always different from the level in the capillary blood. The problem is that the difference between capillary and interstitional fluis will be equalized by diffusion processes. This will always take its time. Most likely 15 to 20 minutes depending on the blood circulation in that area. Even the blood in the finger capillary is some minutes behind.

Imagine you are flying an airplane in the simulator. In this simulator you will see the landscape as it was 15 minutes ago. Can you interpolate from that where the plane might be now? Can you land it safely and spot-on with that information at hand? I think it is not possible - even for a pro like you. In my opinion this is why cgms can not be used for dosage decisions. They are fantastic for trends but with rapid changes in blood glucose they will often be too late or too erratic.

I meant the new Dexcom is more accurate at reading interstitial fluid than the older Dexcom model was. I do realize that glucose meters and CGM's are measuring two different environments. What puzzles me is the Food and Drug Administration requires all blood glucose meters to be accurate within a 20 percent margin of error for readings of 75 mg/dl or higher when compared to plasma readings. So neither are 100% accurate. I just find it interesting that our physician want us to rely 100% on our meter, when it has an a margin of error of up to 20%. Our doctor is telling us to rely on the meter reading over the CGM reading, but how do we really know its the meter that is more accurate when meters also have variability. If I check my son's blood sugar using three different meters we have at home, we get three different readings. Sometimes they are very close (i.e. within 5 mg/dcL) but other times they can be 50 points off. So far (we are only in week two) the most variability we have had between the CGM and his meter is 15 points. Yesterday, the most variability we had was 5 points. Since there is a 15-20 minute delay to equalize glucose levels in the interstitial fluid, why is the CGM so spot on to our finger prick each and every time?

I feel like I have a pretty strong sense of when the CGM is "on", which seems to be most of the time, despite the lesser reputation of the Medtronic Paradigm I'm using, and when it's off or "challenged." I don't notice my meter reading "off" from how I feel hardly ever. I feel "slushy" when my BG gets up > 150 and I feel a buzz when my BG runs low, starting maybe 75ish or so and stronger at lower levels. I usually try to test my BG when I drop off blood for A1Cs and it's also usually pretty close, like +/- a few points which could just as easily be some sort of BG drift rather than a meter error.

That is an excellent point. Delay destabilizes control systems. (This can be shown mathematically as well as being intuitively clear as you described it).

Yet the artificial pancreas systems that are being tried in various studies are attempting to do just that. Some of the AP trials are using the Omnipod together with the Dexcom along with an external computer to model and calculate the insulin doses. On top of the blood-to-interstitial fluid delay, there is also the (subcutaneous) insulin action delay to consider as well (which is frequently more than 15 to 20 minutes). Without having seen the technical details, I am guessing they try to use a model of human glucose metabolism to predict what will happen in the future. But as you point out, this type of modeling has its limits in a dynamic environment.

When talking about the G5 being more accurate, I was referring to it being more accurate in measuring the interstitial glucose - reducing sources of noise (termperature, local oxygen level, etc) and measurement error. But you are right that the delay will always be a problem unless we get a CGM that measures blood glucose.

The meter and the cgms are regulated by the same requirements for accuracy. Most meters and cgms do exceed these requirements by a good margin. Chemical reactions do depend on temperature, lightning conditions, air pressure and so forth. Thus the requirements just reflect these possible influences. This is also valid for the sensor. Its results can be influenced by temperature, tension on the skin area of the sensor, medications and so forth. But we make the biggest mistakes with the judgement of carb content and insulin dosing. The 20% margin in our blood glucose is in comparison neglectible in my opinion. But a 15 to 20 minute lag of the cgm is not neglectible - that is the fine difference.

The cgm was spot on for you because the blood glucose was stable for a longer period of time. Thus interstitional and capillary blood had enough time to equalize their differences. I just think that you can not rely on your experience because the interstitional measurement has its limits you need to be aware of. The more volatile your blood glucose the less useful the cgm gets. A trend warning is still a very valuable information.

With a SD of 15 to 25 mg/dl your experience of excellent accuracy is no surprise to me. However typically the SD of T1s is higher.

I think they try to implement the software of the AP like a rule system with a learning component. Thus the system could take previous experiences into account too. If the AP system will be really aiming at the A1c of healthy humans and not lousy 7% then a glucose pump will be needed too in my opinion. The error of margin is so small that the lag in the unfolding of the insulin activity must be moderated with glucose to prevent lows. This would be comparable to the healthy regulation. In general it is "just" the current limited sensor technology that prevents the AP from becoming a reality. Sadly the "just" is not that easy to fix.

The G4 is more accurate because the math algorithms involved in transposing interstitial fluid glucose levels to blood glucose levels are forever being revised, updated and improved. The G5 version is already being tested, but from what I can see it is merely an improvement as far as where the results can be accessed, which in this case is via the cell phone.
And yes I do find the CGM exceptional for trends and if I am trending rapidly in either direction, it doesn't matter what the meter reads, I still have to do something about it.

It's not only the algorithms they change, but also the construction of the sensor itself, the enzymes it uses, etc.

It's interesting to look at one of their recent patents, where they show how using 2 electrodes, they can subtract out various interfering signals, the changing sensitivity of the sensor over time in your body, the glucose to oxygen ratio....and it looks like they are working on trying to correct for acetaminophen induced errors too.

Well, that's the sensor glucose. I was thinking it would be useful to get a cable to check my meter and see how that compares but, at this point, the A1C correlates ok w/ the results and I'm pretty comfortable most of the time and able to do what I want most of the time. If my BG were flying all over the place, I would be more concerned and maybe consider whether or not it would be worth it to try a Dexcom instead of the Paradigm but I figure if I am getting the results I'm getting with the Medtronic, and would be inconvenienced by the extra gizmo, I will just sit tight until the Enlite sensors are released in the USA and see how they work.

I often dose off my Dex G4 and I have many hours of the day where I don’t use a meter at all. That being said, I’ve used CGM’s for about 5 years (Medtronic, Dexcom 7+, Dexcom G4) and I’ve had enough CGM mistakes to know that I need to be very careful about trusting the CGM. At the same time, I’ve had enough crappy meter readings to not trust meters either.

If my sensor is reading about 90-120 and I feel good, I may go hours and hours without testing. I figure if it’s 20 points off either direction, thus 70-140, my BG is OK. I may take a small correction for the 120 without testing, but I won’t correct a 100 to try to get to 85 without testing. If I get a high reading on the Dex and I don’t have a reason for that high, I will always test before correcting. I’ve had the Dex say 180 and 2 arrows up and have my meter say 108. I’m also conservative about trusting the Cgm before driving. I can remember leaving work once with my CGM saying 100, driving home and a meter reading saying 45.

But I don’t trust meters either. If my CGM and meter are far apart when I test or calibrate, I always do a second meter reading before trusting the meter. I’ve had times when the meter says 160 on the first reading and then 110 on the second. Then I test a 3rd or 4th time before I do anything. So sometimes I think the Dex is more accurate than my meter.

I believe it takes a lot of judgement to know when to trust the CGM. I doubt an 11-year old is mature enough to make those judgement calls.