Accuracy of the Dexcom

How much is normal for the Dexcom to be off from the BG meter? I just started my very first sensor yesterday so I am still in the first 24hours, but it seems like the sensor readings are 20 points or more off of my meter results. Does it help to continue entering meter readings to the dexcom to help it be more accurate? Or should I just do the 2 per day and hope the accuracy improves? Any help is greatly appreciated.

Laramie, WY

Hi Renea,
I’m a pretty new Dexcom user too, just a few months now, so others might have better info for you. So far, I’ve noticed that several of my sensors have been off in the first 24 hours, but after that things settle in and they are pretty accurate. But, I’ve also had several sensors also be totally accurate from the start, too. So, I think it just depends. But I think general consensus is to expect a bit of inaccuracy in the first 24 hours. Some people say that if you start up a sensor when your blood sugars are relatively stable, then you get better accuracy. Also, my dexcom reading seems to be about 10 minutes behind a finger stick reading. Anyway, I’m sure more experienced Dexcom users will pipe in with helpful info for you soon. I love my dexcom and have found it so helpful, I hope you do too!

I sometimes see poor agreement until the third day. I think the site chosen on my body may have something to do with that. If there is any scar tissue at all hidden beneath the skin’s surface, it can interfere. There are other things that can interfere as well.

I agree with the others. In the first couple of hours we pretty much ignore it completely, and after that, it still sometimes takes a day or two to be more accurate. But there are the sensors that are almost dead on from the beginning.

I find that they usually do have to ‘marinate’ for a few hours up to a day. You will hear arguments for and against calibrating more than twice a day.

Remember not to base any dosing decisions on the dex reading - always do a fingerstick first. The technology is good but not great. I tend to think of mine more as an indicator or warning than an actual reading. That being said… I LOVE my dex :wink:

I’ve used Dexcom for 4 years. The accuracy of the sensors vary hugely, though as everyone says, they get better after a day or two.

Here what I’ve learned. From Dexcom, only recalibrate if he reading is off by more than 20%, otherwise don’t do another recalibration. Really try not to calibrate when your Blood Sugar is going up or down. I’ve found it better to wait an hour or two when blood sugars are stable. If the sensor is off by 100% up or 50% down (particularly more than once), call Dexcom and get a (free) replacement. They’ll ask for a download of your data, so do that before you call.

The next generation of Dexcom is said to have MUCH better accuracy, but the idiots at Obama’s FDA are stopping most medical devices from being improved since it is politically safer for them to withhold technology from the market. Europe is getting new medical devices 2 to 3 years before the US. I’m not sure what the downside is to having a more accurate Dexcom in the US market, but you’d have to ask the FDA morons that question.

Ok, here is a dumb question, but at the moment I just can’t wrap my head around it. How do you calculate what percent the dexcom is off?

Re: recalibration guidelines. A Dexcom support rep told me that the 20% guideline applies to readings of 80 mg/dl or higher. For readings lower than 80 mg/dl, she said to recalibrate if the reading is +/- 20 units (which is more than 20%).

How close are your initial two startup BG values? That alone was an eye opener to me: the meter itself is not as perfect as you might think. Even so, you really should be thinking in terms of percentages and not values. 20 points is not a big deal when it’s 240 versus 260 but 60 versus 80 might be. To me even 60 versus 80 isn’t a big deal. It just means that I’m on the low side of normal and might want to eat something before long. The trend is far more important to me. Always think to yourself : Does this value make sense? If the Dexcom is clearly showing a drop then it makes sense that the meter might show a lower value than what the Dexcom currently shows because of the slight delay between blood glucose and interstitial fluid glucose. Likewise, if you’re moving up then the meter might likely show a higher value.

I think I’m more fortunate than most here. The only time I see accuracy problems is at the very end of a sensor’s life. By comparing calibration values to what the Dexcom reads at calibration time I find that on average the Dexcom is within 8% of the meter. Keep in mind that both the Dexcom and the meter have a margin of error so there’s no such thing as perfect accuracy.

By the way, I’m now on day 28 of my current sensor and will restart it again soon. This is the longest I’ve ever gone.

Thanks for the info. The first 2 calibration readings were about 4 points off. It seems to be doing ok now, withing a few points of my meter. Guess it takes a day or so to level out.

WOW! 28 days. Thant is amazing. I plan on restarting my sensor once the 7 days are up, but how do you tell when it has come to the end of it’s life?

Usually if I get an Error #1 or a ??? after 7 days it’s time to change.

For me the curve becomes somewhat erratic towards the end of the sensor life. By that I mean that there are outlying values. I just took a picture of mine - once again this on day 28 - and I’m just now starting to see what will likely get worse in the next day or two. I don’t know how well this will show up in the picture but I’ve added some arrows showing a couple of the values that are not consistent with where they should be.
4229-Dexcom.png (595 KB)

How to tell when end of sensor life -

when there are 6 hours left, you will get an alert saying 6 hrs. Again at 2 hrs and 30 min. At 0, it will alert End of sensor. Then you just need to enter Start new sensor.

You can either wait for that, or restart at your own convenience. Before it expires, just select Stop Sensor. Then immediately select Start new sensor. It will repeat the 2 hours ‘blank screen’, and then ask for 2 BGs again.

If you can’t remember when you started it, you can download to DM3 and check.

Wow Mike, 28 days is fantastic! Mine just failed last night at 10 days. Where do you have it?

I just stick with the recommended abdomen and alternate between sides. Based on recommendations on this site I also add a small strip of Flexifix to the top edge once it starts peeling off which is usually around the two-week mark.

may I ask if you sleep on your sides and if you get false lows at night?

Does where your insulin pump is affect the accuracy of the dexcom?

I do sleep on my sides but I’ve not noticed false lows. Is the assumption here that sleeping on the sensor causes it to read lower than it should? What is your experience?

Mike, I think you meant that you put the sensor on your abdomen, on either side of the belly button? That’s what I do. Not on your sides? I never sleep on my stomach. I also use the Flexifix when it starts to come loose.

Closer to the side than the belly button but not completely on the side.