Variations in BG readings in Dexcom, Omnipod PDM and OneTouch ultra

Hi All,
I've been using OmniPod for two months and have been a Dexcom G4 user for about year and a half. Prior to getting onto OmniPod I used to calibrate my Dexcom from fingerstick readings taken from OneTouch glucometer. But now that I have the PDM with me all the time, I use its built-in glucometer for fingerstick readings.

What I've been noticing is that my Dexcom readings drift a lot more from the fingerstick readings when compared to the Onetouch glucometer. Onetouch and Dexcom readings were almost always very close to each other (+/- 10 points), but that's not the case with PDM & Dexom readings.

I test BG on Omnipod PDM 3-4 times a day and the reading is usually 20-25 points higher than what my Dexcom shows. I re-calibrate the Dexcom every time but it drifts +/- 20 points in next 3-4 hours.

I'm not sure what's the reason for the difference in these values? The possibilities are
1) My Dexcom is misbehaving and not able to track the BG closely even after multiple calibrations
2) OmniPod PDM is showing incorrect BG values
3) The error ratio of Dexcom and OneTouch is similar

Anybody experiencing similar issue? I know that conventional wisdom is to always trust your fingerstick value, but do you trust one glucometer over the other?

I have to be able to trust a device to keep my sanity :).


I'm on Dex and Omnipod and I find the Dex can drift a lot and be horribly off one day and the next day be right on. I also find that the longer the sensor is in, the better the readings. I can do a bg check on my pdm, find my Dex is perfect, enter the new number into my dexcom and then my Dex jumps up or down so now it's not correct!? Is it showing that it's dropping/rising? Who knows. Can't see how "what" you use to re-calibrate would affect the drifting you're getting. My reading with the dexcom are always so erratic that I don't rely on them at all. I know some people are always right on but I don't find that to be.

If you are using the glucometer built-in with the OmniPod, I would suggest you only use the OmniPod to calibrate your Dexcom. If you need to use your other glucometer, then just know that it will be slightly different.

I use the built-in glucometer with the OmniPod too and find it to be very accurate with my Dexcom. I do see a variance when I test on a different glucometer. I only use the value from a finger stick/glucometer (OmniPod) to treat any high or low or to eat for that matter. I use the Dexcom to show me trends and to warn me of an oncoming low or high.

when I have the omnipod PDM calibrated to 17 I find the dexcom is more closely accurate

There are discussions here on the Omnipod forums that suggest the PDM meter is somewhat high in it's readings. It seems to me this variance might push a wider envelope for the G4 to work in. Just an opinion...

I don't use a cgm wish I did , if I had the variances that you do I would check the accuracy the next time I had lab work done , as soon as you get out of the chair see what your cgm reads and do a finger test make a note of the results and compare to the lab work , trust what ever device is closer

Hi Whoami:

This is not uncommon. Rest assured, it's not the Dexcom, it's the PDM meter. Firstly, make sure you're using the regular Freestyle strips (not Freestyle lite). Secondly, the PDM is calibrated high. If you switch the code to 17 (from 16), you will find you will get almost the exact same readings on the PDM as on the OneTouch from the same drop of blood. You can also validate this by comparing the PDM on 17 to the reading you get on a FreeStyle Lite meter using FreeStyle lite strips too.

Once you do this you'll find much better consistency in readings between the PDM and your Dexcom.


Were you notified by Abbott of the FreeStyle test strip recall? If you've only been using the PDM for a couple of months you may not have been. If you haven't done so already check *every* box of FreeStyle test strips to ensure that the use-by date on them is 2015/08 (August *NEXT* year) or later.

Make absolutely sure the date is right - immediately after the recall (about 3 months ago) - Walmart supplied me with test strips dated 2014/08 and I made the mistake of thinking they were OK. Walmart did this even though Abbott were supposed to have informed them. IMO it's still necessary to check every new box.

The bad test strips could consistently read higher, at least with some PDMs.

In addition with any meter it's necessary to use test strips from the same manufacturing lot. Changing to a new lot will shift the readings and the shift can easily be 10%. When this was tested (between 2010 and 2011) the figures for FreeStyle Lite (with the Abbott meter) and OneTouch Verio Pro using four test strip lots showed FreeStyle reading on average 12.8% *lower* than OneTouch. However the variation between lots of FreeStyle was 7% while that for OneTouch was only 1%.

Those are different systems from the Omnipod and other OneTouch meters of course, even though the manufacturer is the same. What underlies them is good inter-lot consistency in OneTouch combined with a bad system error - OneTouch consistently read about 9% high. On the other hand FreeStyle was more accurate, reading 4% low, but showed a much higher inter-lot variation, from about 1% low to about 8% low.

Both systems showed even bigger variations between individual readings - both would read 20% wrong about every test in 40.

One thing CGM users should probably do is double check *every* reading that is entered into the CGM. A large error 2% of the time can be handled without this if it only causes a correction bolus - the bolus will be too high or low, but then next reading will correct this. A large error entered into the CGM will either cause the CGM to abort or will consistently result in high or low values until the next test is entered. It's the consistently incorrect numbers that cause the problem.

Not *higher*, *lower* (if I remember correctly.)

Thanks everyone for your replies! Once again I'm pleasantly surprised by the amount of insight one can gain on this forum from others' experiences.

@Tapestry: I don't calibrate my Dex with both PDM and OneTouch, I used to take BG readings from OneTouch and now only from the PDM.

@Jbowler:Interesting fact about the variations between the lots. I'm using the FreeStyle (not Lite) with the use-by date of 08/2015. I'm a new FreeStyle user so didn't have any old batches to worry about. I enter ALL the finger stick readings into the Dex, but the next reading is always so far out. For example I entered my PDM reading last(150) night into Dex. The overnight trend in Dex was near flat and it showed the morning BG as 156, where as the PDM BG showed 190. So I entered 190 into PDM, gave myself a small bolus. Three hours later the Dex graph trended slowly to 155(as expected due to the bolus), but PDM reported 203. This never used to happen with OneTouch.

@Christopher, @Dishers: Interesting bit about using code 17 on PDM. I suppose that only works with "Freedom Lite" strips, which don't have a code printed on them. The Freedom (non-Lite) strips have a code 16 and thus I assume that the PDM should use that code too. Right?

I am in the process of investigating a cgm system. I have an animas pump and had beenwaiting for their combined system, but they have no firm release date. The only two transmitter system I would consider is the omnipod with Dextronic system. I don't want to have to carry two things in my pocket all the the time and I figure with this system I could park the omnipod system in an alternate spot. I was wondering how the rest of you have found this two transmitter system especially men and older men who who if like me tend to be a little forgetful.

No. Let me clarify. I’m talking about using regular freestyle strips in the pdm meter and changing the code to 17. At the suggested 16 code they run high.

Ah, got it. I'll definitely give that a try. Thanks!

Where did you find out about changing the code to 16? It seems to me that this is a manufacturer issue which would affect all users of the Freestyle system.

when the new PDM’s came out, when on 16 it seemed very high to me, someone mentioned to try 17 or 18 see if it made a difference, when on 17 for me I find that it is correct. others it is better on 16 or 18, I am just glad we can change it to be able to have more accurate reaadings. If Manufacturer’s had to make it more accurate than a 15 - 20% accuacy margin then maybe we would not have to change things. so I would assume yes to it is a manufacturer issue

I also changed mind to 17 and found that it worked a lot better.