Dexcom accuracy while exercising (60 minute runs)

I run at sunrise every other day, and check BG with my meter just before I start stretching. I follow stretching with a 1/2 mile warmup walk followed by my run. After the run I walk ~ 3/4 mile ± depending on how hot it is (Florida) to cool down, do a few minutes of stretches, then check BG with my meter… Time is 1 1/2 to 2 hours between BG checks.

The meter BG is the morning calibration for Dexcom. Dexcom and meter values are usually very close, especially after the recent Dexcom software update. But, they are almost always very different at the second check. While the Dexcom reading barely changes at all during the runs the Meter BG is almost always ~ 50 mg/dl higher higher and only rarely lower.

Once I recalibrate the Dexcom to the new value it is accurate when checked against the meter during the day and very close again at the evening calibration. This happens in any weather, 45F being the coolest and 83F (95%+ Humidity…it is Florida!) being the warmest. I drink a zero carb sports drink, a few sips every 2 miles or so.

Anyone else see this; have any ideas?

The short answer is yes, I have seen this or something similar, along with the standard Dex caveat of it’s the trending that’s more important than the number you’re seeing.

The longer answer is I find my Dex is frequently lower than my finger-stick meter, but most of the time it is within 20%. This morning was pretty classic for me, I woke to a Dex reading of 64, but my finger-stick reading was 85. After calibrating, the Dex decided it was going to read 69 … sigh … Last night, another good example, pre-dinner the Dex was 75, meter said 89 (just inside of 20%), after dinner, the Dex was reading 77, with the meter @ 90 (again just under 20%).

When I ride my recumbent trike, I definitely pay less attention to the numbers and way more to the breadcrumb trail on the Dex. If I am seeing the trail trending down, I will pop a few glucose tablets, then a few more if the trend doesn’t flatten or start trending up. If I get into the below 55 range and the Dex starts alarming like crazy (this happens a little too frequently, btw), I will completely ignore the Dex for the next 30+ minutes, but only AFTER I have confirmed with the meter AND I have taken appropriate action to raise my BG with glucose and a protein bar.

Sorry, I guess that was a long-winded way of saying the interstitial fluid is just going to lag behind my actual BG, especially when low. I use my experience to make the best adjustments possible.

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Time of day makes a huge difference for me with this. I bike commute, about 1/2 hr each way, and I try to keep a good strenuous pace going. In the a.m. the Dexcom will show my BG dropping fairly steadily, from say 130 down to 90 or so, but when I get to my office and fingerstick I’m usually right about where I started, sometimes even higher. Dunno if this is just something to do with dawn phenom taking a while to wear off or what. I have a near-zero carb breakfast, coffee w/light cream, but I find I have to bolus for it anyway (the mystery of the coffee bolus, much discussed around here) when I’m not exercising, and even when I am exercising I still do, just not usually as much. Just a lot of upward pressure on my BG in the a.m., and the Dex behaves as if it’s not there. Afternoons: the opposite. Even if I start out with 140, I frequently end up in the 60s with double down arrows. Those numbers do seem pretty reliable, especially the down-trend: when it starts, it’s usually quite sudden. Usually have some carbs at lunch & some fat, but I’ve been trying to also do a temp-bolus, backing it down to 5% for an hour at about 3:30 or 4pm in anticipation of the ride and that seems to help quite a bit with the lows. Probably I’m producing a bit of endogenous insulin at that time–that’s been my long-time assumption since well before I started bike commuting. The CGM seems to be more accurately reflecting what’s going on then as well.

I have a similar response. The Dexcom frequently shows a much lower bg than my meter, even more than 20% different. I also run daily for 1.5 hours. I used to bring my Dexcom with on runs but it would frequently show ??? so now I leave it behind and wait for it to reconnect when I get back home. Once it connects it is usually pretty close to the meter bg. However, after my shower for some reason it shows ??? again or it will be higher than my meter bg. I was told by the trip that this is to be expected as the hot water effects the cgm sensor response.