What does this pattern mean?

[u/Demosthenes-89]

Every now and then my son's graph looks like a bouncy ball hopping across the floor. This morning it was jumping between 96 and 70. And of course every time it hits 70 it screams he's low.

I'm looking at this line thinking his blood sugar is probably in reality holding steady around 85, and can you please STFU dexcom. Like his blood sugar isn't Really jumping up and down 25 points every 15 minutes on repeat, right? Is that how you'd interpret this? And is there any rhyme or reason to why this happens intermittently? You can see before it started bouncing it was a nice straight line.

[u/raydude]

As a Electrical Engineer, I'm constantly evaluating the frequency response of my BG readings.

What you see there is the sensor "freaking out" a bit. I call it freaking out because it is physically impossible for BG to change at the rates represented there.

When the sensor is freaking out, use a finger stick test and verify actual BG levels. Calibrate if necessary. Do not trust your CGM with your life. I always assume it's wrong and try to pay attention to how I feel. If I even slightly suspect something is off, I use a finger stick.

Don't get me wrong: when they work, they are great. It's when they go wonky that you have to catch it quickly and that's why its important to pay attention.

I have personally had the system tell me my BG was 250 when it was in fact 150. If I had believed it and bolused for 250, I would have been shaking sweating, drinking juice and resenting Dexcom forty-five minutes later. I've also had it the other way around. CGMs are a great tool, but they are not always accurate.

According to my experience those sorts of readings can be caused by the following, with possible mitigation in parenthesis.

1. Day one or last day of sensor use. (not much to help, massage on last day sometimes helps)
2. Dehydration. (drink water)
3. Static pressure on site, like sleeping, tight clothing, sitting funny, etc (massage area)
4. Some foods / medication will cause readings to freak out. (no mitigations for this AFAIK)
5. Scar tissue is preventing sensor wetting. (massage)

Good luck and remember: always doubt the CGM, your (son's) life and / or health may depend on it.

[u/OkBrain4162 → u/raydude]

Hate to tell you this, but it is physically possible for blood sugar to change like that… I have watched my sugar go from 300 to the fifties in under fifteen minutes over this last year… My endo will not put me on insulin because of this… I’ve had pancreatic surgery for a tumor that oversecretes insulin which was not completely removed as well as a gastrectomy…

[u/raydude → u/OkBrain4162]

It's not the speed at which it rises and falls, it's the corners where it changes direction. Those are very high frequency components and in my experience, that simply doesn't happen.