To calibrate, or not!

I’m on Day 3 of my G6 sensor. It seemed to settle down by day 2, with 1 point diff from finger stick. Today was a different story.

After biking for 3 hours Dexcom showed BG in the 70s. Did finger stick when I returned home. I was 38, so i calibrated. Sensor seems to be close to the finger stick since.

Do others find big diffs with finger sticks after exercise despite accurate sensor performance other times?


I’ve found calibrating when rising to a high or dropping to a low is counter productive. It seems to significantly increase the difference between the G6 and meter, I think due to the lag time of the G6’s responsiveness opposed to the meters being much more close to current. Calibrating when running relatively stable provides much better “fixes” to a sensor, even day 1 when it’s most likely to be wonky (technical term for not quite up to par!).

Of course different sensors with different people = different results!


Yes, I’ve noticed that, too. I think I probably over reacted when my BG was so low and Dexcom so off. Probably Dexcom would have caught up with the finger stick. Typically Dexcom trends lower than my finger stick not higher though.


I always calibrate mine. I do wait until I am in my preferred ranges to do so. I also make sure I have been at a steady level for at least 15 minutes before I do. But I don’t remember having had any issues with exercise and off readings. I stay really well hydrated though and I feel it makes a difference? Could you have gotten a little dehydrated biking a long time and that helped make it so off? I also really like restarts, for me they stay much more accurate after the initial calibration corrections…


That’s a good point! I did restrict my water intake to just a few sips, but i had 10 oz of grapefruit juice, taking small sips for the duration. After the ride i drank 24 oz of water and about 30 minutes later i did the finger stick that was so off. I’ll have to pay more attention to fluids on my next long ride!


I don’t know if I’ve ever recorded a 38 - that must have been scary.

I find that usually my body is ahead of the dexcom in letting me know when I’m going low. I’ll feel it. That said, sometimes I don’t notice as much when I’m exercising - and there are times when I go low and don’t get any warnings. Then I have to collapse someplace and recover.

I also find that I can control somewhat going low by the pace of my exercising. I will go lower if I’m really pushing hard, and if I go more easily my levels can re-stabilize. So I do some of that too.

I have a lot fewer lows with the Control-IQ than I did before starting that. It works pretty well - assuming of course that your dexcom is working right.

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That is a very helpful observation. I think I noticed that, too. I’m not an experienced biker, so I need to be more aware of how my body reacts to a long ride!

A hard run (and I mean legitimately hard, like 5k race pace) will make your BG spike.

For a bike ride, you have to go much faster for an equivalent spike.

In general, for cycling you will see drops more often than you will for running. In cycling it takes a ridiculous fast effort to see any spike.

In running, you can see a spike with almost any hard effort. (Hard meaning fast, not long. Long runs rarely spike you.)


Is the spike caused by the increase in beats per minute (BPM)?


Thanks, Eric! I’m a slow biker and use a tiny fold up bike, not a road bike! My HR doesn’t go up much even on long ascents. Avg HR over 3+ hours was 83 and ascent was about 1100 feet, so I guess this would be considered an easy ride (tough for me though!). Hence, the eventual BG low.


The whole relationship between heart rate (HR) and adrenaline is kind of tricky.

In the simplest terms, you would see a person’s HR go up with more intense exercise, and you would also see an increase in adrenaline when they get to a certain level.

But on the other hand, adrenaline also increases HR. And it changes blood pressure too.

So that’s where it can be a little bit tricky because there is some cause and effect there between the two things. Both exercise and adrenaline will cause an increase in HR.

Adrenaline can also come from just mental stress.

So with exercise, the feeling of exertion will cause a release of adrenaline, which will increase HR. And the physical stress will do the same thing. But the HR is also increasing from the exercise too. So the two things are kind of at the same time.

And to further confuse things, the rate at which adrenaline is cleared from the blood is actually reduced during exercise.

So the simplest thing to say - but not completely full of all the details - is that an increase in HR during exercise causes an increase in the release of adrenaline.

But more accurately, we would say that the perceived psychological stress of higher intensity exercise as well as the increased physical stress causes adrenaline to be released, which in turn causes a higher HR (along with the exercise increasing HR).

Sorry @ClaudnDaye, it’s just never a simple yes or no with me. :smiley:


This is also interesting. I have noticed this in my own body. When I am mentally stressed, I notice that my HR can sometimes go up, even though I’m just sitting at the computer!! Of course, my BG often goes up from just sitting long hours at the computer! :frowning:


With intense exercise I always fingerstick. Most often they are close but sometimes waay off. I try to only calibrate first thing in the am when I wake up or right before bed (if I have not eaten for at least a few hours).


My experience with the Dexcom 6 is that if you are high or low, the Dexcom won’t agree as well with the meter as in the middle ranges. And it’s usually counterproductive to calibrate in those high or low ranges. (Although I am guilty of “rage calibrating.” :joy_cat:)


Me, too!!

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We also need to remember that the G6 measures interstitial fluid and your calibration measures blood. I have gotten some very wacky readings of late - G6 tells me I’m at 40, finger says 120…
This is an interesting experiment: Take both measurements and then eat something that you know will up your sugar a bit. Wait 15 mins, take both measurementa again. Then do it again at 30 mins.
Often the G6 is just catching up (or down) to your actual blood measurement.