CGM Accuracy and Exercise

Does anyone else experience significant discrepancies between CGM readings and true BG results during and immediately after exercise? I have always found CGM readings to be significantly lower during these episodes than what my finger sticks show and how I feel. This has been the case for years using Medtronic’s early Enlite CGM as well as Dexcom G5 and G6 products.

Overall, I’m curious to see if others have experienced the same and why this occurs?

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Andrew_P:

There are lots of things going on that cause this. So to start yes it happens to many of us, you are not alone.

Now the real question is why? first you must understand that the CGM does not measure blood glucose. It instead reads the glucose level Interstitial fluid.

This article has a good explanation of the issue with regard to exercise.

https://journals.sagepub.com/doi/full/10.1177/1932296817699637

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To start, if the CGM and a finger stick were the same, I would WORRY BIG TIME.

First, FS & CGM are only expected to be within 20% of each other according to published standards. 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 100 it is ±20 points (mg/dL) with out the percentage.

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 docs 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 above to take place is usually 15-30 minutes depending on multiple variables including the amount of carbohydrates, proteins, & fats in the consumed food.

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……

Hope this helps. Please share your conclusion. It is how we learn.

How much difference and is the exercise provoking rapid increase/decrease in BG?