CGMS Percent Off Table

[Funnygrl]

I made a table of my readings for meter vs. cgms on what I consider a somewhat decent day for CGMS use. This is all the readings for a 24 hour time period.

Am I expecting too much?

I think it's weird how far off it is sometimes, and how dead on it is others.

Am I calibrating when it's two far off maybe? I don't think I'm over calibrating.

I couldn't figure out how to get it to calculate percent off, so I did it by hand, and I'm not sure I did it correctly.

And no, before anyone asks, I don't have too much time on my hands.

[Coppernob]

Hi there Funnygirl - I have been on the sensor for just about 6 weeks now. I think that when your blood glucose is on the move even without trending arrows, then you will notice the bigger differences. There is a lag, I believe, between the BG measured in the blood becoming the BG measured by the sensor. When my BGs are flatlining (?!) as I call it, then I am usually almost exactly on target. I try only to calibrate when I have a nice flattish line. That has been my experience so it may be different for you. Hope this helps even a little.

[morrisma]

I wonder if the meter results would be more accurate if you took a sample from an area adjacent to the sensor. Or if that is not practical, use a forearm test site so you get a bit more lag like Anne thinks the sensor is seeing. Interesting info.
Mike

[Funnygrl]

I really don't think it's lag. I feel like the sensor never moves unless I calibrate it.

[Coppernob]

This is what I was told when I started using the sensor - the sensor measures the glucose in the fluid between the cells and it takes a while for the same glucose reading to appear there as in the blood and that they only really match if the BGs are even. I understood that if your BG is trending up then a fingerstick will be higher and if trending down it will be lower. I could have completely misunderstood of course ... it has happened before! :)

[jeggeman31]

I calibrate 3 times a day. I do it when I wake up at 445am. I eat lunch at around 1030am, so I calibrate as soon as I get home around 245pm. Then I also calibrate when I go to bed around 10pm. Most of the time I eat dinner around 6pm and maybe a small snack at times around 730p or 8p. I have never had the problems you have had, and I find it to be very accurate. In fact I will use the CGMS reading about 90% of the time, reducing my finger sticks.

[Alice]

I don't understand the reason for the calibration unless it is some manner to "boost" the numbers between two totally different sources. (Of course, I understand that you must calibrate...but what correction is the calibration doing to the meter?) Is it attempting to speak like a finger tip sample? Or, is it bringing the finger tip sample back to the tissue number?

I don't get it...but it's interesting you have tracked this so well.

[solox316]

You percents are very intriguing... I would like to try something similiar, seeing yours... I was going to say that they seem to be more accurate when you are in better control, but your numbers look great, so that is out the window...

To determine if it is lag, one would have to test like every 30 minutes, who wants to do that, or waste the strips....

Hmm...

[jen_slc]

Remember also that meters are only 20% accurate. Taking that into consideration reduces the sensor's "percent off." So the question is, who's right, the meter or the sensor? Or who's more accurate? What is the accuracy of the sensor, by the way?

[Funnygrl]

Remember also that meters are only 20% accurate. Taking that into consideration reduces the sensor's "percent off." So the question is, who's right, the meter or the sensor? Or who's more accurate? What is the accuracy of the sensor, by the way?
I know in theory meters are only required to be within 20%. However, in my experience they're way better than that. The meter readings are via Freestyle Lite. I compared that to the lab a few weeks ago and it read exactly what the lab read.

[someone]

Hi there Funnygirl - I have been on the sensor for just about 6 weeks now. I think that when your blood glucose is on the move even without trending arrows, then you will notice the bigger differences. There is a lag, I believe, between the BG measured in the blood becoming the BG measured by the sensor. When my BGs are flatlining (?!) as I call it, then I am usually almost exactly on target. I try only to calibrate when I have a nice flattish line. That has been my experience so it may be different for you. Hope this helps even a little.

The sensor definately has lag sometimes, but there are a number of variables that can contribute to this lag. One being scar tissue.. from what I've heard, the longer you wear the sensor, the less scare tissue you have therefore making it more accurate and less laggy. Another thing that seems to affect the lag for me is activity level. When I am active, there usually isn't much of a lag.

However, in this case, it doesn't sound like lag is the problem. You have to look at the sensor isig to see if it is changing. The isig is basically the raw data from the sensor, not converted to a standard BG value. If the isig isn't moving at all, then calibrating isn't going to solve anything.

[dionh]

Two readings on my freestyle today...21.3 then 12.4 20 seconds later. Both with sufficient blood... is it a science? should be but appears to be more of an art. Did you take multiple tests with your freestyle then average them to give a more accurate reading (not to be done continuously but to help give you an accurate comparison)

[NoelD]

I calibrate 3 times a day. I do it when I wake up at 445am. I eat lunch at around 1030am, so I calibrate as soon as I get home around 245pm. Then I also calibrate when I go to bed around 10pm. Most of the time I eat dinner around 6pm and maybe a small snack at times around 730p or 8p. I have never had the problems you have had, and I find it to be very accurate. In fact I will use the CGMS reading about 90% of the time, reducing my finger sticks.

That sounds a lot like my experiences too, and it's been about a year now that I've been using it.

[Tony]

Hey funny I have a question for you. When are you calibrating? How long after your last bolus?

I ask because you could be calibrating when your bg is on the move from your last bolus. Like Anne said your bg needs to be steady other wise it's not going to be accurate.