CGM vs Finger Stick

I double-check with a finger stick if my BG reading from my Libre3+ is too high or too low. But you have to remember the sensor is reading interstitial fluid, not your actual blood. This is going to make a time difference which you should take into consideration. I love my sensor and take it for what it says most of the time. Libre also recommends checking with a finger stick for the first few days after applying a new sensor, but I don't always do that. I think it's like a "back-up". Libre also recommends placing the sensor on the back side of your arm. I do that because they did the scientific work, not me!! Some people put them other places on their body but I am happy (at least on this subject!) following their rules.

I also use the G7 connected to my Tandem t:slim pump. The one thing I’ve learned, in regard to calibration, is to calibrate when the sensor has a steady horizontal arrow for 15 minutes or so. In theory the blood and interstitial fluid will be at the same value by then. I calibrate when the G7 is outside 20% of the finger stick. Ex: If finger stick is 100, the G7 is considered in calibration if it is between 80-120. I’ll calibrate if G7 reads 79 or lower or when 121 or higher. Seems like a lot, I know but that’s the way it is. That percentage is from Dexcom and not made up my me.
Doing calibrations on rising or falling arrows is an induced confusion factor for the sensor. Giving a new sensor time to acclimate to its new home and not forcing it to “fly right” out of the box, I’ve found to get accurate readings thereafter. But again, when in doubt, do a finger stick. IMHO

@matt2024 thank you! I am using a Dexcom G6. I wonder if your calibrating technique will work with the G6? ( Off the record I will try nearly anything once! I did NOT just say that!)
My frustration comes from the 24 - 72 hours after a sensor change and the G6 seems prone to alarm low but a finger stick is ok (sometimes more than a 100 point difference) like 37 vs 147!
Does the G7 actually show “under advisement”? WOW!
My other BIG frustration is that I’m also using an Omnipod 5 insulin pump. The Dexcom and Omnipod are supposed to be in communication. If the Dexcom is inaccurate it screws up the insulin delivery of the Omnipod pump!
Whatever, I am left with pressing on to the best of my ability. Thank you for sharing what is working for you!

@drlundeen thank you! I use a Dexcom G6. I’ve heard those percentages from tech support at Dexcom. It makes sense. I have issues with waiting 15 minutes for a horizontal line on the graph, particularly in dealing with a high or low blood sugar situation. (My graph has been looking like a mountain range lately with peaks and valleys!) I also get upset with waiting for the 5 minute updates. (Libre products update each minute). I have also noted that a number will appear in the app. Then it takes another 5 minutes before the dot appears on the graph. Then it takes another5 minutes before it is communicated to my Omnipod 5 pump. Then another 5 minutes before a dot appears on that graph. So that is a total of 15 - 20 minutes for one reading! I can’t imagine how long it would take to get 15 minutes worth of a horizontal line on a graph! Perhaps that is the difference between G6 and G7? I will run that by my provider! I just had an endocrinology follow up in April. The bottom line was that they were pleased with my A1C of 6.6 and commended my verifying CGM results when in doubt with a finger stick. I follow up again in July. Thank you for your input!

@ksdm thank you for your input! I like that Libre products update each minute! Dexcom is 5 minutes. It tries my patience.
I learned about that interstitial vs blood difference years ago when I got my first CGM sensor so I’m aware. That also tries my patience!
Regarding the placement of the sensor. My daughter told me that she was trained to place it on the back side of the upper arm, like you said. She is using a Dexcom G7. This was taught to her by CNP’s at Mayo Endocrinology who are also diabetics and use & prescribe this technology. After hearing that I tried it on my next sensor change and had a compression low and alarm that same night (sigh!).
I also use an Omnipod 5 insulin pump. For years I placed the pump pod on my lower abdomen (below the waist line right or left side of my body). I placed the sensor on the upper abdomen (above the waistline, same side right or left as the pump). I may go back to doing that if I have anymore false compression low alarms in the middle of the night interrupting my sleep! It can be challenging!

It sounds like your diabetes is a lot less predictable than mine. My last A1c was 5.5, so I'm very happy with that (I'm on Mounjaro and it has made a huge difference in my blood glucose). When I feel off - I check my CGM readout. And so far, my "feels" and the "results" are pretty much in line - if this was not the case, I would definitely be using the finger poke more often. My type 2 diabetes was caught very early - because, as a fat woman (without diabetes), the doctors were sure I had it and checked it every time I thought of going to see the doctor (for whatever reason). Diabetes runs in my family (both sides), so I figured sooner or later I'd develop it. My glucose kept going up bit by bit, just at the time that the acceptable glucose numbers were going down (a lot) and finally, the numbers met.

Sure thing @cehunt57 - I used the same process with the G6 too. The G7 seems to act different with calibration. The G6 would just take the calibration and go with it (unless too big of a change) where the G7 seems to say "calibration entered" and then it can be up to 10-15 minutes before it is accepted, if ok. I have the same trouble with the Tandem t-Slim x2 when the Dexcom is giving incorrect readings.

The biggest thing that helped me recently is the last software update for the Tandem allows me to set a temporary basal rate while Control IQ is active (previously had to stop Control IQ) https://www.tandemdiabetes.com/support-center/pumps-and-supplies/automated-insulin-delivery/article/temp-rate. This helps me much more than bolusing to drive down highs and the dreaded insulin stacking.