Hi - I’ve been using the G5 for a month or so - I apologize if this has already been addressed, but I couldn’t find it in the search. I’m curious to know why there is sometimes a large discrepancy between the G5 and the reading I get on my meter. Tonight for example - about an hour after dinner the G5 required a calibration - the G5 said 91 and steady, the glucose meter said 168. The sensor and transmitter are both new this week. Is there a lag time after eating where the G5 doesn’t pick up the rise in glucose right away? Do you still do a finger stick test 2 hours after eating or rely on the meter? Thanks in advance.
There can be discrepancies, yes. There should, normally, be no more than 20% difference between your glucose meter and your CGM when readings are > 80mg/dL or more than 20 points for sensor readings < 80 mg/dL.
There is always a “lag” between glucose meter readings and CGM readings, not just after eating. This is because our glucose meters read blood while our CGMs read interstitial fluid, which takes longer to register changes in blood sugar. The lag time isn’t as noticeable when we haven’t done anything that would change our glucose much, but in times when our BG is in flux, the discrepancies become more noticeable. One hour post-meal would be a time when you are in one of these states of rapid flux, and so any discrepancy between your Dexcom and glucose meter are greatly magnified.
I’ve found the “steady” arrow to be somewhat of a liar, personally. It just means you aren’t rising super-fast. Even a -> means you could rise 15 points in 15 minutes. If, after you happened to glance at the ->, it switched to a diagonal arrow, that indicates you could be rising as fast as 30 points in 15 minutes.
Did you happen to go back and look at the trend line later to see if it took a sudden, sharp upwards turn?
An hour after dinner your blood sugar could be changing fast. That is not the time to calibrate. Calibrate when your blood sugar is flat, the flatter the better. As @BeastOfGevaudan said your dexcom will lag your meter by 15 minutes which means that when you have a double up arrow you meter will likely read “at least” 45 mg/dl higher than the dexcom simply from the lag. This is why you calibrate only when flat or these time lag differences will actually corrupt the calibration and your dexcom and meter differences may be much worse than the +/-20%.
When blood sugar is changing the dexcom can lag your meter and they may diverge, but that is transient. So be patient. Also, remember not to calibrate when you have up or down errors can since this will introduce errors into the calibration and the readings for the dexcom.
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Thank you @Brian_BSC and @BeastOfGevaudan. I didn’t know about the lag or not calibrating when there is a large discrepancy. My bg did seem to go up on the G5 eventually but I tweeked it with a little insulin. By bed time the numbers were within 1mg of each other. A couple more questions, if I may - when the G5 says to calibrate and its not the right time, like last night, it’s OK to wait? Does the G5 continue to work? What would cause discrepancies >20% in fasting # s ? Thank you.
I actually think the exact time is not particularly important. It is far better to put off calibration for a couple hours than to calibrate with up or down arrows. You are also supposed to calibrate when you find your Dexcom and meter differ by more than 20% (when you have a flat arrow again). I actually use the flexibility in timing to make my calibrations at the most convenient times for me with a flat arrow, generally 5am when I awake and 5pm right before dinner. I’ll even calibrate an hour early if I want to shift the schedule.
And there may be a variety of reasons for an error of more than 20%, but generally people find their Dexcoms when calibrated are actually pretty accurate.
I regularly put off calibrating by an hour or two until I’m on a straight arrow left. I watch the number for a couple of readings and try to pick a time when I see it hasn’t jumped more than 3 points or so between the last two readings. This is probably a little easier for me as I use an additional android phone app called xDrip, which puts my CGM readings on my Android phone (it only works with Android) and I’m able to click on the individual dots and actually see what past readings were rather than guestimating from the graph.
Generally I try to calibrate before a meal.
Discrepancies in fasting #s could possibly be caused by:
- Your sensor is slightly off (Not unheard of in the first 24 hours of a new sensor)
- Bad finger stick - Remember, even glucose monitors have a +/-20% accuracy 95% of the time, and are allowed to be wildly inaccurate 5% of the time! If my Dex and meter are wildly different, I’ve learned to wash my hands and do a second finger stick.
- Do you deal with Dawn Phenomenon? If so, there could be a lag issue. Before I switched to Tresiba, my DP rise was swift and ruthless - I had one day where I suddenly jumped 30 points within 3 readings. Suffice to say, I do any morning calibration before my feet hit the bedroom floor!
Thanks again. Lots to learn!