I use a medtronic guardian 3 sensor. I use the back of my arm for it. My accuracy and ISIG goes way down on about the 2rd or 3rd day of wearing it. Does anyone else have this problem? I wonder if I were to put it in a different area would make any difference. What’s your experience?
It should go down. That’s why you have to calibrate it every12 hours
It seems that when I calibrate and the isig is low it has trouble being close to the bloodsugar I calibrated it with. I know the interstitial fluid is different from the blood but it should be close. I get alot of false lows when the isig is low.
When you calibrate, it should result in an adjustment to the isig factor, to align the current ratio (isig) used to calculate BG from the sensor value.
But if sensor value is not accurate (or recently inserted), it will often lead to sensor errors.
In general, calibrate when bg is level for best calcs of isig factor.
My understanding is that what Medtronic refers to as “ISIG” is an acronym for “I SIGnal” or “current signal”. (“I” is one of the standard abbreviations used for electrical current). The electrical current coming from the sensor is assumed to be directly proportional to your interstitial glucose level. An increasing ISIG implies increasing glucose. Decreasing ISIG implies decreasing glucose.
If your ISIG decreases while your glucose remains the same or increases, it is an indication that your sensor probe is failing for some reason. Reasons can vary, but one possible reason could be if the sensor probe is disturbed because of a poor insertion site or because it was not taped down properly. Or the sensor could be failing for some other reason.
Calibration can help to offset this to a certain extent. But the weaker the current signal from the sensor becomes the less calibration will be able to compensate. The lower the ISIG value is when you calibrate, the more erratic the resulting sensor glucose values are likely to be.
Have you spoken to the Medtronic cust reps about this? I’m not sure how much help they might be, but maybe they might have a suggestion to help. 
I am paying more attention to the times that I calibrate. No insulin on board, no recent exercise or food affecting my bloodsugar to see if it makes a difference. I have trouble going the full 7 days with these sensors. I usually get at least 5 days. The last 2 days it has been inaccurate. The numbers I was getting weren’t even close to what my actual bs was. I haven’t called Medtronic about it. I just wish these sensors would work better and not be so fussy. Maybe the Zeus sensor will alleviate alot of the problems. We’ll see.
D’Etta Meloche