I often look at my son’s CGM wondering what is really happening with his BG, and how it correlates to his CGM. Yesterday morning (Saturday), around 3:00am, I had a chance to catch a microspike that I was able to measure on his regular glucose meter, in parallel with his CGM:
I measured 4 BGs by hand over approximately 25 minutes, from 2:58am to 3:23am (the blue crosses on the trace).I guesstimated a few points that I missed at the beginning of the spike, to show what I think really happened. These are the points that I found interesting:
As expected, you can see a delay of 7-17 minutes (sometimes more), variable, between the real data and the CGM
the CGM calibration is about 12 points low +/-
The true BG varies very fast. It must have gone up and down like a flash: look at how fast it is dropping as I have just caught it in the process of moving down ahead of the CGM.
The CGM never sees the full high. This reflects my experience with most peaks.
the CGM somewhat amplifies the low (not a low here, just a low point). This reflects my experience as well. In fact, it only catches up with the final higher number more than 25 minutes afterwards.
the CGM trace is fairly close to the real trace (although a little behind), but does not quite map it perfectly. This also reflects my experience.
Finally - what a weird spike, even though it is tiny and contained. What I find most interesting is not the rise, but the fall - what caused this spike to come down? HIs last Novolog injection was at 10:53pm the night before, and his DIA is about 5 hours, so he has precious little IOB left beyond his basal.
By the way, his final drop, past 5:00am, is due to a correction. This brings another interesting point: his most active insulin phase normally starts about 40 minutes after injection these days - but at night it comes quite a bit slower. He injected a correction at 3:28am, on that little plateau, but you don’t see him really turn the corner until about 5:20.
I love data I spend some time every day looking at his trace and wondering what we could do better tomorrow…
I agree with these two points completely and this is part of the reason so many people are surprised that their actual A1C is not as low as the Dexcom A1C prediction! The Dexcom doesn’t catch the full extent of highs, and over-amplifies the lows.
As for why this happened, I’d be willing to bet that he had a hard practice earlier that day, and his body was pulling the blood glucose to replace his carbohydrate stores. Yes on practice?
Not this time Another sign that this is not the case, btw, is that you don’t see a slow but progressive drop for multiple hours prior. I really don’t see a good reason why this microspike resorbed itself on its own, one of the daily mysteries of diabetes.
I think, however, that it may be related to something that I have argued over with our endo. It appears to us that not all BG peaks are created equal, and that some types of peaks require much less insulin than others. Our endo totally disagrees.
For us, big night peaks are typically heavy users of insulin, but some day peaks (NOT related to food) seem to require much less. Night correction factors are typically 1:38 right now for us, as well as food peaks resulting from carb miscount, but many day peaks (stress related typically) require a 1:70 or a 1:80 correction for us. This may be related to something I read in Steve Ponder’s Sugar Surfing, where he mentions that some of his stress peaks only need 1/2 as much insulin as “regular” peaks.
I see frequent little ups-and-downs on my CGM. Part of that could be anomalies in the readings, and the other part I do think for me is just related to normal biology, the body pulling sugar out of the blood and storing it.
Just a brief reply to say this is a tremendous catch…and so reflective of my wave pattern too. The CGM often cuts the peaks off for me. The peaks and the valleys, actually, and presents much flatter than I usually am. Thank you so much for this detailed post!!
I spend some time every day looking at his trace and wondering what we could do better tomorrow…
Another sign that this is not the case, btw, is that you don’t see a slow but progressive drop for multiple hours prior. I really don’t see a good reason why this microspike resorbed itself on its own, one of the daily mysteries of diabetes.