Dexcom and App report different numbers

[altintx]

Right now, I've got double arrows up. Plugged in phone and it reports 130(up)(up) but Dexcom reports 116(up)(up). I don't doubt the 130 is right. What I wonder is, officially, Dex checks once ever 5 minutes. Can that frequency be upped, or otherwise controlled?

[scottleibrand]

Did you just calibrate? I've only ever seen this mismatch in the first 15m after a calibration. After that, it seems that the Dexcom only displays and reports every 5m, and always displays and reports the same data.

Scott

On Mar 28, 2014, at 8:04 AM, altintx [email protected] wrote:

Right now, I've got double arrows up. Plugged in phone and it reports 130(up)(up) but Dexcom reports 116(up)(up). I don't doubt the 130 is right. What I wonder is, officially, Dex checks once ever 5 minutes. Can that frequency be upped, or otherwise controlled?

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[altintx]

I'd calibrated perhaps 30 minutes before. Within a few minutes of posting this, Dexcom showed the same 130, and app moved onto 138 or something. Might not be that they're actually different numbers, but instead that they each update on a different 5 minute interval?

[scottleibrand]

In my experience, the Dexcom updates the screen immediately, and then the
app picks up that new data point the next time it refreshes (every minute
or so). Pay close attention to the timestamp in the app: that's the time
the Dexcom put on the data point, not the time it read it.

On Fri, Mar 28, 2014 at 9:05 AM, altintx [email protected] wrote:

I'd calibrated perhaps 30 minutes before. Within a few minutes of posting
this, Dexcom showed the same 130, and app moved onto 138 or something.
Might not be that they're actually different numbers, but instead that they
each update on a different 5 minute interval?

Reply to this email directly or view it on GitHubhttps://github.com//issues/3#issuecomment-38936145
.

[hackingtype1]

The dexcom has a few other flags that the code doesn't currently grab - something along the lines of "is special" and "display only"
I can parse those from the pages, but I will make those changes on the cgm-data-liberator repo - I will not be making future changes to this repo, but I will merge pulls, etc if folks want to keep working on this code.

My reason for not handling these is simply because I evaluate each bg value on the backend: if it's special (<39) or I get different values at an identical time stamp - or a value that is offset from the normal 5 minute interval, all of that business logic is after upload, and didn't require their specific flags (to identify).

On Mar 28, 2014, at 12:33 PM, Scott Leibrand [email protected] wrote:

In my experience, the Dexcom updates the screen immediately, and then the
app picks up that new data point the next time it refreshes (every minute
or so). Pay close attention to the timestamp in the app: that's the time
the Dexcom put on the data point, not the time it read it.

On Fri, Mar 28, 2014 at 9:05 AM, altintx [email protected] wrote:

I'd calibrated perhaps 30 minutes before. Within a few minutes of posting
this, Dexcom showed the same 130, and app moved onto 138 or something.
Might not be that they're actually different numbers, but instead that they
each update on a different 5 minute interval?

Reply to this email directly or view it on GitHubhttps://github.com//issues/3#issuecomment-38936145
.

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[altintx]

If app is getting it first, seems like that'd be a flag not that you're ignoring but that Dexcom proper is filtering out, right?

[scottleibrand]

When a new number comes in, you have to wait for the Dexcom screen to time out, or hit the center button twice to refresh the graph. Is that what's going on?

I'm struggling to understand the behavior you're describing, because I've never seen it before.

On Mar 28, 2014, at 10:10 AM, altintx [email protected] wrote:

If app is getting it first, seems like that'd be a flag not that you're ignoring but that Dexcom proper is filtering out, right?

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[hackingtype1]

This is all based on my "best guess," and is not based on any info given to me by dexcom, and therefore could be entirely wrong:

Some of the confusion may be from how the Dex system behaves. The G4 Receiver is only a "listener," it only receives. The 5 minute update interval is strictly tied to the transmitter. The transmitter broadcasts approximately every 5 minutes (once removed from it's factory packaging, which appears, i'm only guessing, to use a magnet to engage the transmitter battery). To my knowledge, this interval cannot be change, the clock is internal to the transmitter electronics.

When a calibration value is entered, the receiver logs it to its internal database as an a Meter Record, and a "Display Only" estimated glucose value is sent to the screen. Their calibration algorithm appears to use some flavor of a Kalman filter, and will report adjusted values over the next 10-15 minutes.

For example, the school nurse calibrated at snack time today (grey dot in image below), 10 minutes later, the dexcom logged two unique "Estimate Glucose Value" records - one meant for display, the other is the calculated value. On this image, you can see a yellow and red dot at the same time:

As I mentioned, since they are the display only and calculated value are part of the "estimated glucose value" record set, they both get uploaded - I personally chose to show both on our chart, it helps me identify any unplanned or poorly timed calibrations by caregivers.