Why isn't this working better?

When I was last on a CGM in 2007-08, I was pregnant, so Dexcom kept telling me it was less accurate b/c of being pregnant. So, I went back on it about 3 weeks ago. I'm using Dexcom's new system which definitely has some nice updates (i.e., a warning if your BG is dropping very quickly) and fewer errors. But, I;m still not getting very accurate readings. (e.g., reading 54 when I was 102, reading 248 when I was 178). It seems to go crazy saying I'm very low or high even if I'm not.

So, my question is, are there ways I should be calibrating it better? I saw someone with a medtronic CGM on another site saying she only calibrates when it's close to her meter reading and only when her BG's have been steady for a bit. My trainer just suggested I calibrate before breakfast, before lunch, and before bed. But, also I've been calibrating when the CGM is really off from my meter to try to "fix" it.

Any advice on how you calibrate? I'm testing almost as much on my meter with the CGM which is fine. But, I'm getting frustrated with worrying I'm high or low all the time when I'm not -- I'm having a hard time seeing these 250 and 300 numbers even if I know they're aren't true.

Thanks all!!

I just started on the CGM about a little over a month ago, and I'm on the Medtronic one, so I'm not sure how helpful I'll be - but here goes: 

 - I definitely try to only calibrate when my blood sugar has been stable for the last half hour or so.  And almost never within 2- 2 1/2 hours of giving myself a bolus.

- After I enter a calibration value, I don't eat anything or give myself any insulin for 15 minutes (that's how long it take the medtronic system to readjust itself based on your calibration)

- The medtronic system uses the last 4 calibration values for the algorithm to determine your sensor glucose.  So when I start a new sensor, it takes until I've enter 4 readings to get really good accurate sensor glucose values.  My CDE said that some people chose to test and calibrate every 15 minutes for the 1st hour after starting a new sensor - or anytime you see your sensor and meter values are way off of each other (and you know that blood glucose is stable). I don't do this all the time, but I have tried it a few times and it seems to work.  Not sure if the Dexcom is similar or not.  I've heard that it actually tracks much closer and more accurately than the medtronic.

I know that even with the best calibration, value are going to be "off" when you're at the high/low extremes or when blood sugars are rapidly changing (like after eating a meal) but I know what you mean, it's WAY irritating to see numbers that you know or wrong.  It's like, what's the point of the technology then?  We WANT it for the highs and lows!  Hopefully, someone with more experience will have much better insight than I do...

I wrote to my Dexcom trainer with this problem and thought I'd post her reply for anyone else on the dexcom:

Hi Sarah,

There are no tricks for calibrating. The calibration tips for best results are on page 26 on the quick start guide. If you notice a discrepancy, you can add a calibration and it should help bring a wide spread in numbers closer to each other. You should not have to do that all the time but it may happen on occasion. Calibrating a bunch does not make it anymore accurate though so most people just do 2-4/day on average. The first day is always the most varied with the sensor so you can expect more discrepancies then on any CGM. It takes time for it to learn the trends and adjust to the surrounding tissue.

Of course, make sure you are not taking anything with acetaminophen because calibration will not help correct the wide spread that comes from that. Another time you will see a wider spread will be after a low glucose occurs. It will take longer for glucose to get back to the sensor area after a low. This is not the sensor not working, it is the body not filtering the glucose back to the skin area because it needs it in the blood for the brain, heart, lungs, etc. The meter will read higher 10-20 minutes after treatment and the sensor will take another 15 minutes or so to come back up as the glucose slowly filters back to the sensor area.

I hope that makes sense.

Thanks for posting this, Sarah!  I had never heard that part before about it being off right after a low - but that does make sense.