Delay in sensor readings

Those of you on the CGMS, how long do you find the delay between the reading you get when you take your blood sugar and when it shows up on your pump as the same or close to it? I hope you understand what I’m getting at. I may not have worded that well. I am finding a disparity of about an hour between my actual blood sugar reading and the reading I am getting on my pump from the sensor. I find it quite frustrating and wonder if I am alone with this annoyance. I also find the pump reading to never be EXACTLY the same as my blood sugar reading, just sometimes close.

I also experienced a phase of frustration at first, but I have since worked through most of the kinks. On the first day, my MM will usually have a longer lag (like 20 to as much as 30 min) and will rarely go as high or low as my meter says. By the second day, it’s usually down to 10-15 min, though it still may not go quite as high or low as my bg but it has better range. But there are always sensors that just don’t seem to ever come alive no matter how long I wait.

To get to this point, it took a few months of playing with insertion angle and sites. I currently insert at a shallower angle and use my thighs most often and have been playing with manual insertion lately. I also put the sensor in the night before I plan to change it if possible, so that it will be more accurate on start-up. It’s a lot of pain to find what works best for you but I promise that it can be worth it!

Don’t forget that meters aren’t perfect either – they have an error rate of +/- 20%. That means if you meter says 100, it could be anywhere from 80-120 in reality. Typically meters give readings that are reproducible within the same patient, but they are not as accurate as a lab test. So being within 5-10 points or less, where mine is most of the time unless I’m changing rapidly, is pretty good.

Even on sensors that aren’t very responsive, I can usually get trending info. I just adjust the alarm settings so that they’ll be useful – like setting a low alarm at 80 instead of 70.

Studies of the CGM have shown that the delay in glucose in the interstitial fluid and blood glucose is about 17 min. This comes from one if the primary researchers in the CGM study published in 2008 and the soon to start Artificial Pancreas Phase III study coming later this year at Stanford.

Both JohnG and Tom T are right about the other factors that can impact the association between BG and CGM sensor readings. The Revel system is taking the rate of rise or fall into account to give the predictive warning. It is a great indicator, when the system is calibrated accurately, of BG going up or down and out of range the warning will be a good indicator of where your BG may be going.

Even though there is a delay in the sensor readings, I’ve found my Dexcom Seven+ to be very close to my finger stick readings… of course, this all about how well Dexcom has written its algorithms to deal with the lag.

As it is, I find the CGMS as a great way to see my BG trends and make adjustments after about a week of data… unless I see something really bad I’ll take action right away.