Discrepency between numbers

Started using a dexcom CGM about a month ago.  So far have been having really good results with it matching my glucometer, even on the first day after starting.  Not so with this last sensor.  Changed my sensor on Wednesday.  It has been really off for the last 4 days.  Saying I'm low (39) and I'm 80.  Saying I'm under 80 and I'm 130.  Saying I was 350 and I was 200.  Since I was so far off from what it was saying, I entered these blood glucose numbers to try to help it calibrate.  Didn't seem to make any difference.  Finally decided to stop sensor and restart it.  Still not very accurate.  I'm fed up with the alarms that aren't true.  Last night I ate something even though I was 120 something because it thought I was under 55 and would not stop alarming!  I tried calling support but it's outside of regular office hours-do I bother the on call tech support people?  Is there something else I could try?  Sensor and transmitter are still attached correctly from what I can see.  Thanks for any ideas :)


I always go with what the sensor says.  Glucometer readings are usually not as accurate because the interstitial (sp?) fluid under the skin is what the sensor reads, therefore giving the CGM the more accurate number since it reads the fluid every five minutes.  I had hypoglycemia unawareness without even knowing it until I got the sensor.

that's exactly opposite of what the dexom rep and my diabetes educator and endocrinologist said.  They all said to make any dosing based off the glucometer, that the CGM is NOT to be used for dosing information.

Hasn't really been an issue as they have been pretty close to the same for the last couple of weeks, it's just this sensor that seems off...

Well sometimes the sensor can be off, if you leave it in for more than a week etc. but I hadn't heard anything about the glucometers having a plus or minus 20 mg/dL discrepancy until the dexcom rep told me. I think since the site is closer to the fluid, it also delivers a more accurate reading.  My A1c went from a 9.3 to a 7.4 so I definitely think the sensor helps!!! I hope you like it too!

I have the "off" reads when my blood sugars spike or drop at a rapid rate. It takes forever to for the sensor get back to normal.  I aim for gently rolling numbers not just because it's better for me but because Dexy (my Dexcom) gets really confused at one straight arrow up or down. And double arrows...super confused! Eventually it fixes itself, but sometimes it's 24 hours before I get a good read. Try calling Dexcom to see what they say -- you could get a replacement sensor if this one's bad.

I was told to always correct based on the glucometer and never based on what Dexy says. The readings from Dexcom tend to be 5 to 20 minutes behind your actual blood sugar readings. This lag can be crucial if you correct based on the CGM  -- if you are dropping rapidly and Dexcom gives you a 150 reading when your are actually 80, taking insulin for Dexcom's 20 minute behind reading can drop you to a horrible low.

Good luck!

I have the same problems.  I just started using the Minimed Guardian (6/30/11) and it has been a complete pain in the A$$.  My One touch results are usually more than 50 off +/_ and the beeping is driving me nuts!!  But... my blood sugars have always jumped around.  and can drop like a brick or climb for unknown reasons.  

But on a positive note... I can see the results anytime and not have to prick my fingers so much.  and it seems to keep me on a more even keel ( maybe because I am more psychologically aware of what I'm doing/eating) My BS is hanging around the 150 - 200 mark most of the day!!  

I was told the opposite as well.  I was told the CGM is up to 30 minutes behind what your blood sugar really is.  Joslin has an afterhours hot line you can call if you need further validation or information. let me know.  I am having the same issues as you with the difference in numbers.  It is driving me mad.  but it is at least within the ball park when the numbers are above 100.  The only problem is when the CGm says Im 97 and Im really 42.. that is a problem!!!

Also... just found out that the Minimed Guardian has a predictor rate alarm and a rapid rate change alarm... it will tell you if your BS is dropping fast ( you can set it for 1-5 mg per minute.  Mine is set at 4 units per minute so I know if it is dropping that fast that I am going to run into a problem.    ~~GREAT~~ More beeps, but it is better than getting the EMTs called on you while you're in the middle of getting dressed, pass out  and are naked!!! LOL... trust me!!

There is a big difference between the Dexcom and Minimed's sensors so they are like comparing apples and oranges. However, the physiology initially with insertion is the same. You insert the sensor and your body initially has a mild inflammatory reaction. This skews the numbers for the first 12-24 hours of use somewhat and increases the error rate. This mild reaction goes away and the sensor then has access to the actual interstitial fluid. I find that the first 12-24 hours, the readings on the Dexcom are lower and the correlation between spikes and crashes is blunted. After my body adapts, it picks up. In fact, I think the sensor is MORE accurate the 2nd week of use!

The Dexcom also has rise and fall rates, which work the same as the Minimed sensor. However, Dexcom's typical lag time between blood glucose and sensor glucose is 5 minutes whereas Minimed's is approximately 20. Though the studies demonstrate equivalent accuracy, this is accounting for the lag time. As an example, say your blood sugar drops to 50. 5 minutes later the Dexcom will alert you - "hey, your blood glucose is low!" where as it can take 20 minutes for the Minimed to catch up (and hopefully you aren't comatose on the floor).

FINALLY, no CGMS system is approved for replacement of the glucometer. Insulin dosing must be based on the glucometer readings and when the two are discordant, trust the glucometer (unless you tested your blood sugar with syrup on your finger. . . then wash your finger and test again!).