Dexcom Calibration

Does anyone know how important it is to wait 12 hours between the first calibrations of a new sensor? My first 12 hours always read way too low, so much so it’s basically a wasted day (or night) of false alarms.

I tend to think that less in more with dexcom calibrations, but if it’s way off, I will do it more often. The only time I’ve found that to be problematic is if it’s done at a time of rapid change.


I will calibrate my Dex CGM anytime it is off by more than 20% but only when it presents a level arrow. I usually calibrate more on the first day using that 20% rule but the sensors seem to learn quickly. I like to insert a new sensor in the morning so that it has a chance to be reasonably accurate for the first night’s sleep. This is what the Dex customer service has advised me and it seems to work fairly well.


I had been starting them in the evening but started this one in the morning. For some reason I thought it was important the first day to wait the full 12 hours for the second calibration on a new sensor, don’t know how I got that idea because normally I will recalibrate if it’s off by more than 20%. You’d think after 8 months I’d have this down, not! Thanks @Terry4 and @Lorraine


I am using a Vibe. At the end of every bolus for which a meter reading has been entered , the pumps asks whether to use the meter reading to calibrate the CGM. I usually do so. I typically start (or re-start) my sensor in the morning, so in addition to the two start-up calibration readings, it will typically get another two over the next few hours. It doesn’t seem to cause a problem. I estimate that around 95% of my sensor readings are within 10% of a fingerstick BG, so accuracy doesn’t seem to be a problem.

I agree. @Lorraine is correct when she says that it can be inaccurate during rapid changes. I will try to calibrate first thing in the morning–when everything is pretty stable.

I follow this, Directly from Dexcom :

You should calibrate when the system requires it or if the CGM reading is inaccurate

Inaccuracy is defined as when the difference between your sensor glucose reading and blood glucose value is greater than 20% of the blood glucose reading and blood glucose value is greater than 20% of the blood glucose value for sensor readings > 80mg/dL or greater than 20 points for sensor readings < 80 mg/dL.

Also, as others have stated, I never calibrate after a meal, exercise etc. I always calibrate during a stable period.

Agree with these comments. I was told recently by Dex representative to start a new sensor calibration (assuming stable bg levels) with about 3 measurements 15-20 minutes apart. This has worked well for me.

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That seems not unreasonable. It strikes me a tad too rushed since what you want is to allow the underlying data from the sensor to vary a bit as a reflection of mild variation in your BG between the calibrations. But at least it allows for some time between the calibrations for that to happen.

The most unreasonable approach to calibration I have seen was a YouTube demo vid where the person said that Dexcom says to do two calibrations after you insert. So that’s exactly what she did. She tested with her meter & entered the result as the first calibration. Then without missing a beat she immediately tested again and promptly entered that one as well. There was certainly less than a minute between the two.

How someone could think that two back to back BG values which obviously would differ only by the inherent random error of the BG meter & strips would help to calibrate a CGM is something I doubt I will ever get my mind around. :open_mouth:

Stephen Ponder devotes an entire chapter to CGM calibration in his book Sugar Surfing. It’s definitely worth the read!


I’m going to try that when I start my next sensor @truenorth I’m not sure why it reads low the first day but from now on I’ll coax it more and see if I can get more accuracy during those first 24 hours, thx!

I always assumed a new sensor start asked for two readings for precisely that reason: to average two back-to-back readings, thus reducing the “random error” of the strips. Even with very clean fingers, I find back-to-back, strip-to-strip differences are commonly significant – 10% or more (using OneTouch).

Two back-to-back fingerstick readings is exactly what the Dexcom User Guide instructs (page 71, Dexcom G4 User Guide) when starting a new sensor session.

On the first day of your sensor session, you must enter 2 blood glucose values into your receiver.

I recently had a Dexcom rep suggest that I separate my two start-up readings by 5 to 10 minutes. He said it gives the Dex a sense of whether you are stable or rising or falling. Definitely different advice than manual.

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That’s interesting, @Laddie. Do you do that? Does it seem to help?

I can’t help but think that we’ll look back on this time in a few years and marvel at the tech-gains we’ve made!

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interesting. I always do the back to back sticks at startup but I also don’t enter them when my BS could be rising or falling fast.

I like to think it helps, but frankly I have no idea.

Another thing this same rep told me is that I don’t have to calibrate when a sensor is tracking well. I think that we have all experienced putting in a calibration when the sensor is close to our meter value and then have it skew off to be more inaccurate. I definitely delay calibrations when the sensor is going well, but after a while I get annoyed with the blood drop and finally calibrate it. At least with Dexcom you continue to get values when you don’t calibrate. My memory from Medtronic SofSensors is that they would stop giving you numbers if you didn’t calibrate.


I do the same thing often. I’ve gone for 5 days without calibrating.

how do you go for 5 days w/out calibrating? what do you do when the receiver asks for a blood reading twice a day? how do you bypass this process? and why? are you just trying to save $$ on test strips? don’t the regular calibrations keep your cgm accurate? isn’t that the point?

I would disagree. There is nothing in that sentence which implies that the 2 blood glucose values should be back to back. Actually, more the other direction since it says the two calibrations must be done sometime during the first day of the sensor session implying … at least to me … that some time should elapse between the calibrations.

If you pause to reflect, doing the tests one after the other will only get you readings which differ only because of the meter’s intrinsic error. So what would be the point?

The intent of calibration is to come up with some sort of a “smoothed” or averaged correlation between the sensors data and what your meter reports your BG to be. Back to back meter readings won’t help to accomplish that. It’s better to wait a while and allow both the sensor data and/or your BG level to change and then calibrate.

Which I believe supports what I am saying. Though, I still think that is not leaving enough time between the calibrations. I would think waiting at least a 1/2 hour but an hour would be better.