Dexcom G6 and exercise

I have been wearing a G6 since December. Over the last 2 weeks, I have started cycling again (Road bikes get put up during the winter here) and am seeing strange behavior from the CGM. Yesterday, I had breakfast before a 3 hr ride and adjusted my insulin in a fashion that has always worked for me before an intense ride. Early in the ride, ny BG drifted below 70, so I had a high carb snack. It hovered in that range for a while, but continued to drift down. At the end of the ride, my CGM said 46 mg/dL level while my meter said 92 (I definitely felt more like 92). During the winter I tend to exercise 3 times a week running indoors (and some outdoors), but usually only for 30-40 minutes. I have found the CGM to be pretty accurate at those times. A lot of times it shows a small increase in BG during running which is supported by meter readings after. I always wear the CGM on my belly and had it relatively high yesterday. After the ride, I calibrated it yesterday and then a few hours later had to calibrate again because it was reading about 20 units high. I have had good luck with sensors being accurate. Only one has been significantly off since I started wearing them. I’ve sometimes calibrated if they seem more than 5 units off after 12 hours, but sometimes go the full 10 days without a calibration. Any suggestions? Should I not calibrate and wait for the sensor to correct itself after biking?

It did kinda catch up to being accurate. I would still call a reading within 20 points accurate. My guess is that the exercise (the relatively rapid decrease in BG) is hurting its accuracy in the short term. Calibrating during this period might negatively impact its accuracy over the sensor’s life, but since its an important thing to know BG on a 3 hour bike ride, I’m comfortable with you doing it.

Previous sensor models have always shown lag in their accuracy during rapid increase/decrease of BG. Its a thing. I’m sure that is playing some role here.

This might take some experimentation. You might have to do a fingerstick at the half way point, just to identify how the sensor is operating. Let us know what you find.

Remember that that sensor reading is displaying what your BG was 15 minutes ago, not in this exact moment. That gets significant sometimes during rapid increase/decrease and can make accuracy appear much worse than it is.

This shows BGM during the ride from 8:45 to noon. Based on the way I felt, I think BG might have fluctuated between 70 and 100 in that time. I didn’t have my meter with me, so couldn’t check until I was back home.

The increase about 10:30 was after eating a granola bar. I didn’t feel low at the time, but ate in response to the CGM readi g. The jump after noon was from the first calibration.

Well, Yep, its off during that period. Sure is. Its enough that Dex would replace the sensor because when BG < 100, they want it within 20 points of actual value. Keep an eye on it in case it continues to be innacurate.