Comparing CGM to finger stick

Good morning!

I was diagnosed in June at the age of 57. In October this year, I got a CGM - best thing ever in my opinion and makes me much more proactive in managing my diabetes.

Question for the long time folks on the board - how often to you compare you CGM to a finger stick? This past sensor seemed to be off more than usual so I seemed to be checking every other day. Honestly, some of the numbers were different (114 vs 142 this morning) but not enough to make me make any dosing changes. I like to stay in the 120-130 range since I tend to go low QUICKLY.

Any insight is greatly appreciated!

Happy Holidays,

Mary-Frances
“MF”

Here’s a link to Dexcom’s support page on sensor accuracy relative to fingersticks:

https://www.dexcom.com/faqs/is-my-dexcom-sensor-accurate

Hi Mary-Frances @MFBarry , this is often a question asked by recent beginners with a CGM. The answer for me to your question, and I’ll add more below, is that I very rarely do a fingerstick to “check out” my continuous sensor - a Dexcom G6. The times I do a fingerstick are when the CGM appears obviously ridiculous and it DOES NOT agree with how I’m feeling. I learned ages ago, decades before digital BG Meters were in dreams, to manage my diabetes through personal observation.

My reasoning on not using a BGM to validate my CGM is that the CGM when properly installed and operating [after the first few hours] is more accurate than any meter even when the fingerstick is “perfect”. A way I assure myself that the CGM is accurate is when I have a lab blood draw - write down my CGM reading and then compare my reading with the lab report sent to me the next day; for instance, on Friday my CGM read 69 at blood draw and the report I received Saturday morning also read 69. Bingo!

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I may compare readings when I start a new sensor. I might read the monitor a few times. IF it is consistent, I might recalibrate my CGM. Or not, if there’s agreement.
I find the CGM may tend to screw up more toward the end of the ten days. Sometimes I will turn the receiver off for a few hours so it can “pull itself together”.
I can usually feel if it’s giving me a reading that’s waaay off. (Hell no, that’s not right) Then I will double check.

@Dennis. I don’t know how often you have your lab drawn? But mine is every 6 months at best. I like to make comparisons if necessary. But I don’t obsess about it. Close is close enough.

It depends on what CGM you’re using.

The Medtronic Guardian sensor requires BG calibration at least every 12 hours, plus every time you want to use a BG value to adjust your bolus. I was testing more often than I had before starting on a CGM!

My Dexcom is far more reliable and technically doesn’t require calibration at all. Still, I calibrate first thing in the morning, just to make sure it’s on track. And, as others have said, if the reading doesn’t match how I’m feeling. I’ve actually been told by my doctor and by Dexcom support that it’s better not to calibrate too often. Just let the CGM figure things out on its own. Unless I’ve inserted it into a bad spot (like a capillary), it’s usually spot on without my having to do anything.

It can vary from person to person, too. So, as with most things diabetic: Start with the general guidelines from your medical team. And then see how it goes. Over time, you’ll get a feel for how these things work for your individual body, and you’ll learn what works and what doesn’t and how much flexibility there is.

Yes. I agree that you can get a feel for when a CGM is wrong or right. But if a doctor or educator (who presumably don’t have T1D) tell me not to check the CGM too often, my mind goes to “you don’t really know what this is like”

My advice is, check when you want. Check before you travel. Have snacks. Eventually you will get more comfortable.
Always have snacks.

I’m a bit of a weirdo. I love my cgm but I still finger stick super often too. Although I stick less then I used to (I used to be between 8-10 sticks per day). I would say I test 5-6 times per day but maybe more if if go low (like to know how low since my cgm will lag behind and what works for me at 50 is going to be different than what works for me at 65-70 personally and for me an over treated low can literally cause roller coasters of highs and lows) and I also like to verify with a finger stick before giving a correction bolus. I’m getting better at trusting my cgm more for meal time boluses but I have had too many instances of my sugars being off from the dex in the past where for my own peace of mind it’s easier for me to just do a finger stick here and there.

@Dennis
Thanks Dennis, this is perfect! I waited a few months after diagnosis before getting the CGM so I would understand what my body was telling me (fuzzy when low, jittery when high, etc.) I was only comparing when my CGM seemed different to how I was feeling but then I got a bad sensor (Dexcom client service was great to deal with!). I also am starting to notice that my readings seem a bit off around day 8 but still within range.

On a side note, I lurk quite a bit on this board and your knowledge has been incredibly helpful to me. Thanks for posting and sharing your experiences.

Happy holidays!

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@sgwilson Steven, I do the same thing - I check once it finishes its warm up and then again once over the course of the first day. I try not to compare after that for the sake of my sanity (and finger tips.) I like “the turn it off for a few hours” trick. I am going to borrow that one (and give credit to you of course!)

Thanks for your reply!

MF

@WearsHats Paul-Gabriel, I also was told by Dexcom support (and I saw a post about it on one of these threads) that calibrating too often on the G6 did cause a few glitches. I didn’t know about hitting a bad spot - I thought I installed improperly in the past.

Thanks for your response!

MF

@MFBarry there are 2 general guidelines I use with Dexcom. First, day 1 my sensor always reads low. (Lower than finger stick). I test by finger stick the first day as if I wasn’t wearing a CGM. And the second thing is that ALL CGM read interstitial fluid and so I don’t take a big difference between a finger stick and CGM as an issue if I know my blood sugar is changing. My favorite time to check CGM against a finger stick is first thing in the morning or if I’ve been sitting for an hour right before a meal. I generally finger stick until I can trust the CGM (I usually stop completely at day 3) Also I will finger stick if I feel low and CGM says otherwise but again if I’m dropping fast I don’t calibrate or take the CGM as my actual blood glucose. Cheers good luck :four_leaf_clover:

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As Joe said, the CGM is designed to read interstitial fluid. That’s the liquid between your cells, which carries nutrition, waste, hormones, and other things from cell to cell, to and from your capillaries, to your lymphatic system, etc.

Reading that fluid is less accurate than a finger stick. It also takes a little longer for changes to appear. When you eat sugar, it goes straight from your stomach into your bloodstream, and is then carried everywhere. But it takes a while for it to diffuse out from there into the interstitial fluid. And it takes a while for excess sugar to diffuse out. The moment you take a BG reading, that’s almost always going to be more accurate than the CGM. But the BG reading only gives you a single data point, whereas the CGM can show you the curve in the long run.

Thing is, the CGM is not designed to directly read values from your blood. If you hit a blood vessel when you insert the CGM, it can really mess things up. Sometimes it’s fine. The blood clots up and gets cleared away and the CGM can get back to work. Sometimes it’s not and you need to replace the sensor entirely. Depends on whether it’s just a small bit of blood or whether you’ve ended up putting the sensor strip inside a blood vessel. Sometimes you just need to leave the sensor alone for 6 hours or so and then calibrate. Dexcom can sometimes adjust itself overnight.

But, yeah. Bottom line is that if your sensor is reading blood instead of interstitial fluid, it won’t work right.

@MFBarry I think all of us have gone thru similar thoughts and I’m sure you’ll get a whole range of answers. I recall the old adage, “A person with one watch knows what time it is…a person with two is never sure.” It throws some “cold water of reality” on the issue. Personally, I may confirm a reading or two until the sensor has “burned in” and established a “level of trust”; my experience is the first 12-24 hours can be a bit wonky. After that I don’t bother with finger sticks unless the G6 is jumping around or just doesn’t “fit” the way I feel. Also, keep in mind meter’s are “allowed” to vary by as much as 15% from controlled lab tests, yet still be considered “accurate,” and the G6 can also vary (I think 8-9%?), so if one’s high and the other is low, the readings can be in serious disagreement and yet be considered “acceptable or within the margin of error.”

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It looks like most are getting 10 days from G6, I normally, almost always, do not. That is weird, because, on G5 for part of that duration, the insurance companies withdrew support, as Medicare had not yet stepped up (smartened up) so, being self pay (around $5800/yr) I would restart my G5 after its 7 day life, and get at least 5 more days, with good tracking. So, it was not broke, but they fixed it, I now average 5 days before I call them, and, of course Byram keeps sending my normal supply, I have G6 sensors coming out my ears. Oh well

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Hi Mary-Francis,

We are by no means long timers, but I thought I would add we finger stick at the start of every new sensor. My son is 13 and diagnosed in April 2021, and we use Dexcom G6. We usually wait a couple of hours after sensor warm up, or the next morning if the sensor change is at night and calibrate if it’s off by more than ten. Sometimes we don’t finger stick again through the life of the sensor (10 days) unless we see crazy numbers or if the line on cgm graph shows a lot of ups and downs. In my opinion it’s good to finger stick before your treat, especially if your symptoms don’t match the cgm…example; last night we started a new sensor and about three hours later the cgm alarmed showing bg 60 and we grabbed the juice box. My son started drinking the juice while we finger sticked, only to find the result showed 99…bg went up to 180-something afterwards, so he didn’t really need the juice. The technology isn’t perfect, but I’m definitely thankful my son doesn’t feel as much like a pin cushion as he did when this first began!!

@WarriorMom13 Happy New Year!

I also find that the lows on my G6 are not as low as a finger stick so I usually tread carefully with treating lows - especially during the night (compression issues sometimes).

I agree that the technology isn’t perfect but in the 6 months since diagnosis, it has been a life changer. Thanks for your reply and insight.

Take care,

MF

Hi Dennis,
I like what you are saying. The only caveat is that there are diabetics who do not sense how they are feeling. I think it is called hypounaware. It can develop for some after many years of diabetes, according to what I have read.

Samuel, you are quite correct in what you said; I found myself in that situation when I was about 50 years into a life of diabetes. What I did was admit that I was slipping and worked again at re-training my body, using BG Meters as a guide, so that now almost two decades later I’m pretty good at recognizing or sensing my BGL.