CGM vs Finger Stick

There are likely multiple reasons for this discrepancy. One such reason I have encountered in the past is referred to as a “compression low.” This occurred when I slept on my side on the sensor. Due to compression, the sensor had restricted access to interstitial fluid, leading it to erroneously interpret my glucose level as significantly low. As soon as I rose to perform a blood test, the CGM reading increased. I believe there are other factors that can influence CGM results, such as high vitamin C levels. It’s frustrating.

I use a DexcomG6 CGM. I get discrepancies too. A VERY common thing is in the 24 - 72 hours after placing a new sensor the data is inaccurately low (similar to what happened to you). The fix for that is to calibrate using a finger stick. (I get REALLY tired of doing that up to 3 out of 10 days!) My daughter was on a Libre and is currently on a Dexcom G7. I look forward to hearing her opinion on any differences between the two devices.
Regarding “compression lows”, I thought I had that problem solved by placing my sensor on my abdomen away from my waist line. (I don’t sleep on my stomach and my clothes don’t constrict in that area.) Yesterday my daughter told me that she was instructed that the sensor should be on the back of the upper arm. Oh wow there is always something new to learn!
When did you get the Libre 3 plus? What led you to choosing that CGM?

I also was told by the Dexcom people during a tech support session that Dexcom has issues with accuracy over 300 and under 40. They started quoting the acceptable differences for various ranges and I have forgotten the details. I think the bottom line is that “when in doubt use a finger stick” I had an endocrinology follow up appointment recently and my provider complimented me on doing this. Do you all continue to double check with meters?

I do double check my Dexcom G7 several times during the first day or two of a new session and on occasion during the 10 day cycle. One thing that I have learned is that if you are off by too much - don't try to calibrate the difference all at once. I usually halve the difference over a few hours and that seems to help dial it in without errors.

I almost never doublecheck my Dexcom G7; I rely more on how I feel - do I feel hypo? If it is showing alarmingly low (below 60) then I usually take a couple of Glucose Tablets to be safe; or I drink a sweet drink (like Koolaid Jammers - liquid sugar that won't go bad, like juice does); and check to see if the trend starts to go back up. I have heard there is a lag time between what the interstitial fluid shows for blood glucose and the finger stick, with the finger stick being more accurate/timely. I had not heard of the compression lows - great information to have. And overall, for health, I go by my A1c; which will give me a better overview of how I'm doing with my diabetes.

I have a DexCom G7. I agree that the first 24 hours after putting on a new sensor, I will get inaccurate readings. I now realize I should have been recalibrating during this time by doing a finger stick. My A1C is currently 7.3, but my DexCom G7 90 day is 6.8. One year ago my A1C was 6.3 and my DexCom G7 90 day was 6.5. I've been doing so well this past 18 months by losing 50 pounds + eating low carb/high protein foods + exercising. It's discouraging to see my A1C jump from 6.3 to 7.3 in 12 months! I've googled "Why is my A1C lab test higher than my DexCom 90 day reading? I found people posting the same problem and a Dr. Edelman responded to this question with "It has been known for quite some time that this lab test is commonly very inaccurate." Is this true?

I can see really different readings between a finger stick and the CGM (over 100 points difference) when the sensor is new (24 - 72 hours). That is when I’m doing finger sticks and calibrating (a lot). It gets tiresome and reminds me of “The Boy that Cried Wolf” fable. Today I had a low alarm on the CGM and figured that was what was going on. When I did the finger stick it was low also and the numbers were only 3 points apart! Sometimes you can do everything you’re supposed to do and things are still weird.
I’ve had diabetes 50ish years. When I was first diagnosed there were no finger sticks, CGM’s, insulin pens or pumps, nor A1C tests. There were just blood sugar tests at a lab, clinitest dip strips to crudely measure sugar in the urine plus vials of insulin and syringes to administer it.
So now I make use of all the resources I can to try and stay on track. I haven’t heard of Dr. Edelman or that “…..this test is commonly very inaccurate” (the A1C?) I used the A1C as one of many measurements for my Masters degree thesis research before it was readily available in clinics (45 years ago). Geez I’m old. My last A1C was 6.6! I get the 90 day Dexcom report from Clarity to review with my provider and we make adjustments.
My point is that you have been working hard on all the right things (weight loss, clean eating, exercise). Keep it up. Review things with your provider because you may have missed something that needs some fine tuning. Add treatment &/or support for stress because that alone can drive your numbers up. Don’t give up.

@matt2024 I’m not sure I understand your calibrating technique “one thing that I have learned is that if you are off by too much - don’t try to calibrate the difference all at once. I usually halve the difference over a few hours and that seems to help dial it in without errors”. My Dexcom app says to select calibrate, enter the blood sugar of a finger stick, confirm. Then after what seems like a long time the new number shows up on the home setting and a little dot appears in the correct spot on the graph. (Then eventually these updates get communicated to my Omnipod 5 insulin pump). Could you explain more about your calibrating strategy?

@thisfatoldlady it sounds like you have a pretty solid system for dealing with low blood sugars that you are comfortable with. I think I might be a bit more diligent about it because I use my Dexcom in conjunction with an insulin pump. My Dexcom communicates with the pump and affects its insulin delivery so I want to make sure it is accurate.
Also I do not always trust “how I feel”. For example 50ish years ago when I was first diagnosed the who knows how many months of high blood sugar had already done some serious damage unbeknownst to me. Years later I was diagnosed with hypoglycemic unawareness so I wouldn’t necessarily recognize the lows.
So now that I have a Dexcom CGM, when in doubt I back it up by verifying with a finger stick. Then I respond as I have been instructed by my provider to tend to highs or lows. The Dexcom 90 day report on Clarity and the A1C (as you said) give good overviews of what’s been going on for a 3 month period. How’s your A1C? Are you happy with it?

Hi @cehunt57! Sure thing. I sometimes find that my sensor is off by a good bit at start up or when it seems off from the way I feel. So, sometimes when you try to make too much of a change, the sensor can error. To avoid this, I make smaller changes. For example, if my sensor says I am 80 but a finger stick says 120, I will calibrate at 100:
120-80=40
40/2=20
80+20=100

I'll check again in a few hours and adjust as needed. Sometimes the small adjustment dials it and both are really close now. It's helped me a bunch. It seems the G7 takes a calibration "under advisement" until it registers and you can sneak up on it by making smaller changes.