If the CGM doesn't match the meter, is it a good time to calibrate?

I've been having more luck with the CGM lately. My current sensor has been in for 6 (!) days and is still going strong. But I do have one question. (I use a Medtronic CGM/Pump, but I don't know that it matters for this discussion).

When is a good time to calibrate? I know not to calibrate when I see the rapid-change arrows on the CGM, and not to "overcalibrate" (whatever that means), but what about times I test my BG and it doesn't match the CGM? Say, the CGM reads 115 and the meter reads 95. Or the CGM is at 150 and my BG is over 200 (and I never got the "High" alert!!).

In these cases, I feel like I should calibrate, to "correct" the CGM, but that doesn't always seem to work. Should I calibrate with that value, or am I better off waiting until my next "scheduled" calibration?

And, on a related note... how soon after one calibration is too soon for the next?

Thanks in advance for the help.

It's not really likely your CGM is going to match your meter. The CGM is more a tool to see trends. Try not to get in the mindset the CGM needs to match your meter. As long as your CGM is showing a steady level...not a rapid increase or decrease it means you BS is steady. I would advise calibrating before eating, and before bed as long as its stable. I dont try to calibrate to fix my cgm to my meter. Im more concerned about trends and dropping fast, regardless of what the CGM says my number is.

My CGM matches my meter all the time. If it doesn't, I'll fiddle with it to make them match before I calibrate if it's at all possible. It's worth postponing breakfast to get better data later.

Sometimes mine does...sometimes its off by about 20...sometimes its way off. I'm using mine more to see my trends than whether its matching exactly. If it does great, lol Im just wanting to make sure Im not dropping super fast, especially when I can't feel it at all. NOT a good feeling thinking you are ok, do a check and you are 42, hahaha.

BTW, I had to admit to my CDE today, I um started the CGM without going to be bored out of my head for 2 hours. I tried to tell her in a nice way, I read the info, figured it out no big deal lol. She remembered I was a nurse and left me alone, hahaha. She's like some people dont know but you can get...and Im like WHAT extended use out of my sensor, hahah yeah already knew that one but thanks. Oh they hate me at the Endo office. LOL

The CGM monitors from a different sample to begin with, so it will almost always vary - at least a little. It’s more for trending and averages than it is for minute by minute BG readings.

It is my impression that it is in no way meant to replace finger sticks, only to be used alongside to give better overall data and fill in the blanks between finger sticks. If the meter is significantly different from the CGM, it could just be displaying data that is 15 minutes different. If it happens often though I would be concerned a bit though.

Hmmm, mine seems to be different than BG when it's "moving", or going up or down due to "inputs" (food, exercise, stress?), in which case the CGM lags behind the meter but generally is going the same direction and will get where it's going one of these days. I try not to calibrate when it's moving or when I have IOB, although I just did so it may be a while before it gets straightened out? I try to calibrate when my meter and CGM are close. If they aren't close and I have a spare hour, I'll do what I can to get it close. I feel like I have a pretty decent feel for it and, while it's not always totally accurate, I can work to make it more usefu. A lot of times, I'll try to avoid "abuse" (junk food...) to "protect" the CGM so that when I lapse, a more moderate amount of craziness will show up properly on the gizmo?

Sounds like a good approach to it. Fingers (and toes!) are always the fastest to update and tend to be the most accurate because the blood vessels are so close to the surface. Where most people use their CGM it usually lags a good 10-15 minutes behind but it shows the overall direction your BG is heading as you’ve mentioned. Definitely useful for trends, and to see how steady your baseline averages are.

I consider recalibrating whenever my CGM and meter differ by 20+- As noted by the others, the purpose of the CGM is to give trend data rather than point-in-time data for treatment purposes, but a deviation 20 or more seems to me worth considering a recalibration at least.

As for how soon after one calibration should another be made, good question. I usually don't recalibrate for a couple of hours (since I've taken a meter reading and have presumably taken whatever action was needed to cover). I'll have to dig out my manual and see what it says.

One other thing I try to check is the time left for a scheduled calibration (on the sensor data screen). Nothing more embarrassing than having your meter start beeping up a storm when you are in a position where you can't test for whatever reason!

I hope this helps as I am still learning ...my Canadian contact shared this : quote " http://www.medtronicdiabetes.ca/en/using_your_cgm.html ....
The way I like to word it is choose times for calibration when your blood sugars are likely to be changing the LEAST...for example first thing when you wake up in the morning, 15 min prior to a meal, before bed (but not after a bedtime snack)."
end quote . She also mentioned to drink enough H2O during the day ...interstional fluid outcome may differ , if dehydrated . The link suggests to calibrate 3-4 times daily . I don't seem to get extended use with the Veo pump /sensor ...6 days is it !

I am definitely aware of the Conventional Wisdom around CGMs... calibrate when BG is steady (but I'm in the middle of a meal and my pump keeps telling me to METER BG NOW!!) ... use it for trends ... don't make clinical decisions based on it... and so on and so forth. I know that if I do two fingersticks within seconds of each other, the results will vary slightly, and the variation on the CGM applies more -- though the CGM variation is more time-sensitive (meaning it's giving a ballpark reading a moment in time... it's not giving a "within 20% reading" or a steady measurement of 120 would appear to be bouncing between 108 and 132 constantly.

But sometimes I appear to be running a steady 120 and then my meter tells me I'm at 150. And that ain't right -- the discrepancy isn't due to a time lag.

Acid, I wonder what you mean by "fiddle with it to make them match"....are you telling me you only calibrate a CGM displaying 80 with a meter reading of 80? I'm not sure what good that does (though I really respect your opinions from everything you've shared on Tu!). I must be misunderstanding.

Rich, I always use fingers to test BG. I've never done any of that "alternate site testing" (forearm) that seems to be all the rage these days. That actually seems more painful to me!

Niko, I agree with you on all points. But sometimes when I am being ordered to recalibrate (now I try to "beat the timer" but it doesn't always happen) it's not a good time to do so, so I try to make up for it with another calibration soon after. It doesn't always work. If the CGM is wildly off, sometimes I'll trick it and "Start New Sensor" with a fresh calibration, forcing all previous ones to be discarded.

Nel, 15 minutes PRIOR TO a meal? That might be the best time, but it isn't particularly economical, considering I'll also test AT mealtime in order to bolus. I don't like using a fifteen-minute-old reading for the purpose of a possibly large bolus.

Good advice about drinking lots of water, though. I had never heard of that before, but it makes sense!

I believe that calibrating when you BG is stable is the foremost point.

Beyond that. I like to calibrate when my BG is in my targe range. I think this makes my CGM more accurate when it is in my target range and this is where I will hopefully spend most of my time. I think this helps keep my CGM a little more accurate at picking up hypos too. My CGM is often off by a good amount when I am running high and it doesnt really bother me (high is high right). I always do a BG and I can see from the trending weather the bolus(es) I take are heading me in the right direction. If it is common for your CGM to not read as high as your BG when you are high, then I would suggest lowering your upper limit.

I also will calibrate the CGM quite a bit when my BG is 95 and my CGM reads 115. It will nudge the CGM in the right direction (assuming the CGM does not catch up in the 15 minute lag). Like I mentioned to you before, this could be due to the aging of the CGM and the ISIG values slightly decreasing for the same BG reading. I think of this more like fine tuning to get all the horsepower you can out of your muscle car.

I have used the newest Dexcom CGM for almost two years. In my initial training I was told to accept a 20% difference in the two readings.
I calibrate three times per day, before breakfast, dinner, and going to bed. This usually keeps my glucose in this range.
My Endo. keeps telling me that neither the blood test nor the CGM reading are the true glucose number.
Why bother?? I have an excellent reason. Prior to starting the CGM, in my attempts to keep a healthy sugar level, I experienced low sugar reactions frequently during sleep. My wife became adapt at properly treating this hypo. event many years ago, but she really hated this and would be upset for some time.
Since the CGM I have not had even one hypoglycemic occurance. I honestly feel that the CGM is the best advancment in diabetes care since synthetic insulin, and improved models will be coming.
This 2012 year is my 60th with T1 Diabetes. I have been very fortunate.

All good comments. As this dexcom sensor does not have absolute oxygen sensor as part of sensor, the readings can drift. Therefore occasional testing and entering BG help control drift. Dexcom has set it up so that at minimum, it wants 2 readings a day - morning and night.

When I walk a lot, I do not see DEX follow that very well - yes it catches up but seems stalled for a while.

Water and walking help keep circulation/blood chemistry in range caveman machine/dexcom is rated for ( albeit FDA has allowed vendors to put out absolutely useless spec sheet with all the real meter ranges - hemocrit, oxygen sensitivity, non glucose D4 response and water saturation - all now deleted.) Right - all meters are the same -good luck there.

I have a one touch which really is a good meter that I was always having to drink extra water or get readings 40 to 100 points off.

It is unlikely that your CGM can tell the difference between 115 and 90, so trying to adjust to make it match is a little fruitless. For that matter, your regular meter may not be able to tell the difference either. Just be happy that your BG is somewhere around 90-115 and go live life.

I agree with everything you say, Christy.
Been using Dexcom for 2.5 years and at the beginning I was quite frustrated with the difference in my numbers. My Endo. ended this frustration after explaining that neither the blood meter nor the CGM is right on the mark. The sole purpose of the CGM is to show trends, and alert you when high or low.
Before starting with the CGM I had all to frequent hypo. reactions. This, of coarse, was due to the fact that I tried hard to keep glucose levels at a decent range.
Due to the CGM I have not had a serious hypo. event since I began with it.
To sum all this up: No more hypoglycemia and
I'm alerted when trends occur
I am satified with these terrific assets, but will be happy when improvements arrive.

I have found that if my meter and CGM are way off like 90 on meter and 150 on CGM it's either a new sensor or an old sensor. If it's new, then I will calibrate to "teach" the CGM more values - if that doesn't work sometimes turning off the sensor for 3-4 hours lets it reset; however, if it's a sensor on it's 5th or 6th day the calibrating doesn't really seem to help as much, yesterday I had a perfect flat line but I know there were some peaks and valleys in there during the day. I try to really spread the calibrations throughout the day too, too close together and it seems to get confused. I don't see why you can't bolus and calibrate at the same time, even if you wait 10-15 minutes before eating, at least your insulin gets a head start and you will have less of a peak after meals.