Dexcom reading delay

Please help me understand this. So I am of the understanding that the dexcom reading is about 15 to 20 min behind( let’s say 15 mins for this example). Now the novorapid takes 20 mins before it takes effect. So in theory if I take novorapid at 1pm, my insulin takes effect at 1:20pm and given my dexcom is 15 min delayed, i won’t notice a drop in my BG until 1:35pm. Right?

But the thing is, I notice a drop in my BG on the dexcom exactly at 1:20pm. Consistently 20 mins after injecting insulin all the time. So how come there is no dexcom delay?:sweat_smile:

Wrong - There is no Dexcom delay. The delays are the time it takes for insulin to kick in. Same delay as you have with a finger stick. It is not a BG meter delay, it is the insulin. Dexcom displays bg value with dots every 5 minutes so there can be a minor delay if you are between 5 minute readings but they are minimal and not worth considering.


While this is not a new article (2009, I think) here is a technical article about delays in IF glucose vs capillary BG.

It indicates that it is NOT just a single delay. While it can often be 8-10 minutes, if BG is falling, it indicates that IF readings can sometimes PRECEDE the fall seen in BG … at least the way I read it as a non-Medical person:

Maybe someone has a newer, better reference as this DOES seem to be an old paper that has likely been re-examined more recently.

A bit more digging turned up a more recent article on the topic of CGM sensor delay from 2015:

Stay safe!


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There actually is a delay. But as someone pointed out it’s not what you described.

The real delay is the time it takes glucose to move from your blood to your interstitial fluid.
That is generally 5 min.

Dexcom readings adjust for that.

However the time between bolus and drop in sugar is the time it takes insulin to travel from your subcutaneous fat into your blood stream.

That has nothing to do with your dexcom.

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i dont know what I was thinking then. I always got the impression that there is a 15-20min lag between the sensor reading and the meter BG reading… and that was also the main reason behind calibrate only when the BG is stable. Or so I thought…

I think the tandem support also told me at one stage there is a delay.

I tried to read the articles posted above and I must honestly say it went over my head :smiley:, didn’t understand it at all :sweat_smile:

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hmmmm. @tedos, I very much appreciate this thread, question. I had exactly the same impression that there was a 15min ‘delay’ between the measure that Dexcom provided (@ 1:30pm) & what BG meter would provide ( @ 1:30pm).
I’m sure I’d had it explained that if the number was trending down, then you probably already had entered hypo land, because of the delay.
A false understanding.


I thought the same too @tedos, from conversations/posts on this forum and others.

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:sweat_smile: I know right?!? :woman_shrugging:t2:. There’s definitely been communication out there is a lag, it looks like it’s not put in the right context. I’m still not quite sure what the correct context is to be honest.:sweat_smile:

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I think to simplify it think of it this way.

The cgm has a 5 min delay because of the time it takes sugar to move in and out of your tissue where the sensor is. Finger sticks test your blood directly. So that is a real delay.

There is a 15-20 min delay after you inject insulin before it makes it to your blood. That’s not a delay in cgm. Your cgm is still only 5 min off even while you wait for your insulin to take effect.

But also know that the cgms build that delay into the system so the most current reading the second it comes up is actually 5 min old. That’s why you should be stable when calibrating. However since it calculates with that delay in mind it’s not that noticeable.

So just know that those are two totally separate things.

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Thanks for explaining that. For me, it’s clear that the insulin delay is a real thing. Its not really a delay, it just takes 15-20 mins to kick in. That in my mind is accounted for.

The dexcom lag was what was throwing me off as I was always of the understanding there is a lag here - more than just 5 minute - more like 15-20mins. But I get what you are saying. Thanks.

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