Guardian 3 sensor low SG problem


(Kathy) #1

I had a frustrating session this morning with my sensor. I’m wondering if anyone has run into this and has any idea what causes it and how to resolve it.

My pump started alarming about approaching low and low SG at about 2 this morning. Got up and checked my BG a few times over the next hour. BG was fine - in the 115 - 125 range. SG was insisting on 60 - 75. Did not want to try to calibrate it out as auto was going to be a major headache because of the difference in numbers. Took the pump out of auto so I would get some insulin and finally ended up turning off the sensor and going back to sleep. When I woke and reconnected the sensor the SG as in the 40s, BG in the mid-90s. Tried a calibration, had a rejection, did a second one, calibration was accepted (after an “updating sensor”). The sensor finally “sorted” itself out about an hour after getting up. It made for a really long night.

Any idea what causes this sort of sensor behavior. The site area I’m using has provided good sensor results in the past and I do rotate in this area.


(joe) #2

@kkavolinas hi Kathy, The big causes of sensor error include drift, calibration error, and delay of the interstitial sensor value behind the blood value (lag).

you are using a finger stick glucose meter to calibrate your cgm sensor. if your last calibration was with a BS of 150 mg/dl the basis of error for your blood glucose meter is +/- 30 mg/dl. this error can be much worse if you calibrated while your blood sugar was moving, especially moving downward.

the CGM SG is based on interstitial fluid which can lag by 30 minutes.or more. so if you calibrated while your bs was dropping… then at later times, perhaps much later, your cgm SG can be significantly lower than your BG.

the observation that your CGM sorted itself out really points to (in my opinion) a calibration error that occurred much earlier than this episode.


(Kathy) #3

HI @joe My BG was stable when I did the prior calibration, right before bed. No arrows, no active insulin, dinner was 4 hours prior, hands were clean and dry, and given that I have challenges with the sensors in the past, my “pre-bedtime” BG (about 30 minutes earlier) was similar to the bedtime (and calibration (BG)). The sensor had been tracking well prior and appeared to be tracking well after the bedtime BG. I know I’m being defensive but I really don’t know that I messed up.


(joe) #4

@kkavolinas

your bg meter has an actual accuracy of +/- 20% if you are above 75 mg/dl. so the error you input to your cgm can be as follows
BS-------actual blood glucose
80-------64-96
100------80-120
150------120-180

so if you were completely stable for over 30 minutes (covers interstitial lag) and your bs meter said 100, your actual blood sugar (between 80 and 120) and your SG could have been “off” by 20 mg/dl. The SG accuracy is 10% behind the BG meter, so that’s another 10 mg/dl at the worst case off by ~30 mg/dl.

in the AM you tested 95 mg/dl, again actual BS could be 76-111 mg/dl, and your SG calibration “error”, if the sensor didn’t also drift, could actually read in the 40’s.

you didn’t do anything wrong, the errors are inherent to the devices. you are calibrating a interstitial device with errors, with a blood glucose device with errors, and if the errors add… you will get readings significantly different than reality. this by the way, is my reason for not caring about the dex, the libre, or the guardian, for another couple “generations” or at least until we give up on interstitial fluid and start measuring actual blood sugar.

when I am wearing my cgm I typically don’t care all that much what it thinks my blood sugar is, I care more about if it says I am rising or dropping.

sorry if you thought “calibration error” meant you… certainly not intended


(Kathy) #5

@joe Thanks, that helps. I understand what you are saying and agree.