@Yerachmiel here is something I picked up on a post in another D forum about CGM, FS, & Calibrations
CGM Calibration Facts & rationale to avoid pitfalls & rabbit holes
NOTE: The term “rabbit hole” comes from the 1865 piece “Alice’s Adventures in Wonderland” by Lewis Carrol 1865 and has come to mean a bizarre, confusing, or nonsensical situation or environment, from which it is difficult to extricate oneself.
Test by finger stick (FS) anytime CGM doesn’t seem to match the way you feel.
Learn & verify acceptable deviation according to FDA approvals, ±20% above 100mg/dL and ±20mg/dL below 100. Examples, CGM =200, then FS can be between 160 & 240 and still be fine, or CGM = 70, then FS should be between 50 & 90 with level CGM.
Wait 60 minutes between tests if testing for CGM for validation.
Remember, CGM measurements are interstitial fluid and finger sticks are whole blood.
Whole blood & interstitial fluid are physiologically different & 15-20 minutes apart in the movement of glucose.
Call tech support if 2 tests are >20% off, 1 hour apart. You may need a new sensor or have other technical issues.
Only enter calibration when tech support instructs. This reduces the number of times you will get into the calibrate, calibrate, calibrate rabbit hole.
If you are calling tech support for a goofy sensor, request replacement, regardless.
Follow tech support about changing a sensor.
Avoid the calibration rabbit hole of the system requesting multiple calibrations for hours and sometimes days.
Not everyone (some doctors) who prescribed insulin fully understands CIQ & how it uses CGM.
Why FSs & CGMs don’t play nice together.
First, FS & CGM are only expected to be within 20% of each other according to published standards accepted in the USA by the FDA. If your CGM says 200 then ±20% is ±40 mg/dL meaning a finger stick is fine if it is between 160 & 240. At the same time if CGM says 100 the ±20% would be 80 to 120. BELOW 80 it is ±20 points (mg/dL) without the percentage according to US FDA standards.
NOTE: Some individuals expect much narrower values between FS & CGM, even to the point of expecting them to match spot on.
Second, as food is eaten, the food enters the blood stream in various places along the digestive tract. Some enters quickly from the mouth, like granular sugar placed under the tongue. Other foods swallowed quickly may not enter the blood stream until arriving in the stomach. The final absorption occurs in the small intestine.
After food is absorbed into the blood stream, it is ‘checked’ by the liver and then dispatched thru the body in the blood stream.
After glucose is in the blood stream, the glucose moves in the liquid part of the blood (sometimes referred to correctly as plasma or serum - like serum or plasma glucose for the doc’s office or hospital blood test) into the interstitial fluid that moves around all body cells outside the blood vessels. It is this interstitial fluid that CGM sensor wires are in to read the glucose and report it to our measuring devices.
The time for all of the glucose movement to take place is usually 15-30 minutes or more, depending on multiple variables including the amount of carbohydrates, proteins, & fats in the consumed food which changes the way the food is processed in the body.
It is this 15–30-minute interval and body fluid differences that give significant part to the variation in CGM and FS readings. This is also the reason most comparisons are done when glucose values are demonstrated level by the CGM before doing comparison finger sticks.
Wish they would play nice together……
Just for note, I have been using G6 since it came out. I have been using Dexcom for 20+ years. With G6, I have not needed to do a finger stick for more than 18 months.
I can report while hospitalized in Jan 2020, I had chemistries drawn every morning for 5 days straight. The difference between the hospital lab and the G6 was less half of the allowed difference between a finger poke and the CGM allowance. It was always 14mg/dL. Even the hospital laboratory supervising pathologist got in on the ‘game’ before the end. Are you expecting something outside of parameters or is there a need to RTM instead of SOTP. Thoughts?