Accuracy Expectations on the First Day of a Sensor

I’m reading about sensor technology this morning, and I wanted to ask you all a question about your own first day accuracy for comparison purposes.

From my understanding, there is an FBR (foreign body response) effect that can cause some rocky performance in the first hours following sensor initiation. It is also my understanding that the warm up period is really a period designated to allow for the worst of it. I can’t remember what Dexcom’s warm up is, but Medtronic has a 40 min-2 hr window, and the Libre has a guaranteed 12 hours on the dot. I also understand that the first 24 hours, with any given CGM, are potentially affected by this response. However, in my own experience, I rarely see any kind of difference on my first day compared to any of the other days UP UNTIL day 5 or 6. At this point, I believe what I see is a sign of sensor degradation and not FBR. The study I found this morning, if only I could access the entire thing and not just the abstract, seems to confirm this.

So many people in my 670G group talk about these crazy first day accuracy issues, and I just don’t experience them. It leaves me wondering… Could it be we really do get different mileage first day? Could the drastic decrease in accuracy be something more specific to Medtronic Guardian technology? And here is the conspiracy theorist in me, are 670G wearers learning to sweep all accuracy issues on the first day under this explanation without considering any others…just as we tend to sweep all other accuracy issues under hydration, sensor placement, and whether or not we have well-behaving interstitial fluid? You couldn’t see me, but I was wincing when I added that because I understand all of these are true factors, but I wonder if we’re not overapplying many of them as a way to justify poor accuracy performance on the Guardian. I feel I understand my sensor very, very well, and I, also, possibly coincidentally, just don’t see a lot of these things. It really could just be a stroke of good luck, but I’d love to hear from other CGM wearers on their own experiences.

I’ve had a lot of coffee and realized the question may have not been clear (or maybe is not even in there)… How is your accuracy on your first day? :smiley: :coffee:

I think you are just lucky.

The only CGM problem I’ve ever had has been a consistent “bounciness” of the trend line for the first few hours. I go through about a vial of test strips to wrench it into accuracy the first day.

But after the first few hours I find the Dexcom to be accurate enough to calculate doses from for the next ten days.

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This is a bad weekend for me to try to weigh in on this, but normally, I find the first 12 hours or so to be a little wacky, but not usually off by too much. More than anything else, in the first 12-24 hours it will be generally accurate but will invent random little lows that aren’t actually happening and then bounce back up. At least with these I know it’s just my sensor being nuts and I can usually ignore it. I don’t usually find the readings to be completely unpredictable or all over the place though, if that makes sense.

Any weirdness doesn’t seem to follow any pattern based on location, hydration, etc. either.

My last couple sensors have had a real rough breaking in period, which is unusual for me. Thankfully I’m about 24 hours in to my current one and it seems to be leveling out.

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We have good Dexcom cgm numbers right out of the gate. Although, the first day the graph is a little “shaky” as compared to on Day #2 and going forward. So we can tell the difference between day #1 and day #2. But not enough of a difference to in any way impact our use of the data. Even a little bit.

I am under no illusions that our control is so good that a minor variation on the cgm data will cause us an impact.

I believe the individual response as to how accurate a cgm is upon initial insertion varies quite widely.

When a “warm-up” period is mentioned, is that the technically enforced period of no readings or are you referring to how long it might take each individual before they feel comfortable with the numbers?

The technically enforced period before readings are available (assuming using FDA approved apps - lol):

Abbott Libre has a recent label change (last month I think?) to reduce this from 2 hours down to the now FDA approved Libre warm-up period of 1 hour.

Dexcom continues to have a 2 hour warm-up period which is for the current G6 and was/is the same for the G5 and G4.

Senseonics Eversense requires a 24-hour warm-up.

EDIT: Dexcom considers first day accuracy to be a big-enough deal that this is one of the major advertising points of the G6 cgm system. Improved first day accuracy as compared to the G5/G4.

Yes. Absolutely. No doubt. 100%.


The individual difference in how long a sensor last for any given individual would appear to be significantly greater then as compared to the “out of the gate” period.

This is a HUGE difference (in sensor longevity) from individual to individual.

That would be okay. I’m not a particularly “lucky” person. I’d take it in this department. :grin:

But either way, you’re still talking about the first few hours… not the first 24 hours, right? Dexcom has a 2 hour warm up, too?

That’s really interesting. I’ll have to keep an eye out for something like that. Lately I’ve had some sensors let me calibrate right out of warm up but then take a nosedive down to a 40 and remain there for hours. Most of them end up recovering and being halfway decent, but I’ve never seen this before. I was thinking of it as a sign of a faulty sensor, but maybe that’s not entirely right.

Having worked with pacemaker leads placed in people’s hearts. The difference between people is truly astounding, i.e. one person would have a little injury current for a few minutes, and others have an body response that lasts weeks. There really is a huge difference in foreign body reactions between individuals.


This is me.

I can experience some big swings on day 2 but always thought of them more as a result of not having access to CGM during the warm up. I’m trying to get out this sentence but am falling ssleep.

I do believe this. I know it. It’s actually true in every last aspect of diabetes, and I remind myself of it every single day. I’ve been hypersensitive to Medtronic’s response to people’s sensor issues, and I was just curious whether or not our “first day inaccuracy” was proportionate to that of the general CGM using population. I have a hard time believing I’m just this lucky (luck is not my thing), but, as I said up there, I’ll take it. :smiley:

That can happen depending on how much tissue trauma was done at insertion. The first stages of local healing around the sensor suck up glucose to power the repairs. As you observed, this false low generally recovers all by itself as the first stages of emergency healing pass.

As far as the “first day inaccuracy” of a sensor, with me it really varies. (I’m using Dex, not Medtronic.) Sometimes the sensor seems to work pretty well right from the start. Other times I get that big drop for several hours after insertion. Some sensors have significant jitter when new, and it can continue up to 3 days before the sensor settles in and reads steady and true. In all these cases, after the sensor finally settles, it runs well until it starts to reach what I treat as its end of useful life, at which point it is developing jitter in the CGM graph, plus increased delay in detecting trend changes.


This is just interesting as I’ve never seen this until the last couple of months. My first day has always been smooth and accurate (as accurate as one can hope anyway). :grin: I have a few old logs available to look through. I think I’ll go see what ACTUaLLY has been happening first day. :thinking:


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@Nickyghaleb It takes about 5-6 hours before I have confidenice in it. But within that 5-6 hour period I probably calibrate about 20 times.

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@docslotnick, and you’re Dexcom??

I’ve never won a thing in my life, but it looks like I’m a big winner in first day accuracy. I’ll stop complaining about anything.

For now.

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