Dexcom Calibration: How Much Is Too Much?

I will admit that I never got fully trained on how to use the Dexcom. I sort of just jumped in (thanks to all the info I learned in the DOC and on TuDiabetes). Thanks to y’all I have learned that you shouldn’t calibrate when the arrows are anything but steady.

And I recently read someone on a Facebook group give advice about calibrating “too often.” This advice was a first for me. I sort of just assumed that calibrating would help with accuracy…so I have been punching in my BGs every time I did a finger stick.

The Dexcom website says that you should calibrate when the red blood drop shows up and also:

whenever the sensor 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.

But there’s no real info on any harm that would be done by calibrating more often. What do you guys think??

I love how the quote runs contrary to the advice of MM tech support! I would prefer that Enlite calibration followed the same “rules” as what you quoted, as I get really aggravated with MM for constantly telling me not to calibrate when the discrepancy is great than as follows: (their advice is needed due to how MM CGMS’s processes the calibration data, so I UNDERSTAND why I have to follow those rules, but it makes it hard to deal with difficult sensors. :slight_smile: )

Ok to calibrate if
discrepancy is LESS than the following:

20 for under 100 (all of these are BLOOD sugar numbers)

30 from 100 to 150

40 from 151 to 200

50 from 201 to 300

60 over 300

I’m sure the dexcom users here will answer your question. I just wanted to share how things are supposed to work on my side of the CGMS field. sigh.

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I calibrate regardless of the arrow state - I am pretty sure that is a minimed thingy. Granted, if you have double arrows in either direction, your meter value could be suspect as well. I frequently will do two fingersticks (one on each hand) if the Dex has double arrows, and I am calibrating.

Somewhat counterintuitively, the Dex is less accurate if you calibrate more frequently. Without getting into all the technical stuff, it essentially learns as it goes along. So if your readings are within the “20/20” rule, don’t calibrate unless prompted to do so by the red blood drop. (of course, the “red” is sooo confusing to those of us who are blue-bloods :smile:).

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Like you, I just jumped into Dexcom use. I tend to do what you do – pretty much punch in my BG anytime I do a finger stick. I also test when the blood drop(s) appear (I refer to that as its Seymour mode – that’s the theater geek in me coming out). I don’t think there is any harm in doing it more often. I have found that the longer I wear a sensor the more it seems to get to know me – accuracy improves.

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I was told by a Dexcom support person not to calibrate if the Arrows are straight up or down, 45 angle is OK. Also the manual says not to calibrate if ??? is showing. I’m not sure what the advice is if the arrows are absent, probably don’t calibrate.

The support person also said 3 or 4 times a day was OK, but every hour was not good. The algorithm uses not only the last BG entered but a couple of the previous ones also when it recalculates the BG reading.

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I don’t calibrate every time, but more than the required two times. I’ve read in the manual that you shouldn’t calibrate during periods of significant increase or decrease. Although, I have entered a calibration on the 45 arrow, I don’t on the 90 arrow.

It’s fairly accurate for me, before and after the 505 update. I can get weeks out of one sensor; my last sensor was in for 28 days and accurate up to the end.

Where I have problems, if it is a problem, is “every 12 hour calibration” time. If I calibrate at 10:00 AM, I shouldn’t have to calibrate again until 10:00 PM, but that isn’t the case. It seemingly goes by when the sensor session started, regardless of when I last calibrated. If the sensor session started at 4:00 PM, then at 4:00 AM it wants a blood drop, even if I calibrated it before I went to bed at 10:00 PM. According to Dexcom tech support, that isn’t suppose to happen. They had me reset the Dexcom receiver, but that did not help. So, I just deal with it and plan a little better when I start a new sensor session.

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Like you, I usually calibrate whenever I check my blood sugar, especially if there is a discrepancy between my meter and the CGM. If the CGM and my meter are very close, though, then I don’t bother to calibrate until it is time. I haven’t had any problems with doing it this way.

I’ve had my CGM do the calibration timing thing to me as well, but not consistently, only twice. I don’t know what caused it but it wanted a blood drop exactly 12 hours after my last timed calibration, even though I had calibrated already. And there was no problem with the sensor or accuracy at the time, so I have no idea why it didn’t update the calibration time. But it fixed itself, I guess.

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With G4, it’s usually so accurate, I don’t like to mess with it. It will ask for a bg for hours and I may not calibrate. I’ve always known that less is more, so I think I have a predisposition to fewer calibrations. We calibrate if it’s off unless it’s a period of rapid change and dexcom’so number can be justified by a delay. And yes, every so often we acknowledge the blood drop and calibrate.

With seven plus, over calibration miscalibrarion was obvious to me. I could tell that I messed things up by inputting too much. But it seemed to need more calibration as it was less accurate. That could be a vicious cycle at times. It’s just not an issue w G4 cuz that thing it so stinking good.


This seems so confusing to me. The explanation, that is.

If you could describe what aspect confuses, perhaps we could describe more clearly.

So basically, how do you calculate the 20% rule? For example, if my blood sugar on the meter reads 165, and my CGM says 216, would I take the difference (51) and divide that by 165 or 216 to get the percentage?

If your meter read 165 mg/dL, then if your CGM read outside of +/- 20% of 165, then you should calibrate. If CGM was within 132-198 mg/dL, then you would not need to calibrate. If outside that range then do calibrate.

165 - 20% = 132
165 + 20% = 198

Okay. And for the <20 rule, if your BG is 79, it can’t be higher than 99 or lower than 59. Correct?


Okay. Thank you.

I’d encourage you not to over think it… it’s not like 20% is precisely the number where recalibrating becomes necessary. You could just as easily replace that “20%” with the term “significantly off” in your mind and accomplish the same thing… it’s not like you need to actually add it up mathematically to see if it’s truly 20% off and time to calibrate yet or not


I concur @Sam19. I understand why we all have a propensity towards obsessive compulsive behavior. We all want the same thing, control. I personally have gotten better results just calibrating when it asks and every once in a while if it is off in the mid zone. After all, bloodletters (meter) can be off too.


Yeah that makes sense. It’s a good rule of thumb, but I’m not sure how many people do it.

The most important point is not to calibrate when the rate of change is rapid (as indicated by a directional arrow that is NOT horizontal) regardless of the percentage off. Bottom line: Calibrating when your BG is not steady is asking for trouble.