What I’ve Learned about my CGM

I’ve been using a Medtronic Minimed continuous glucose monitor (CGM) for a little more than a year and a half now, and here’s what I’ve learned about it.

• Having CGM is the greatest thing. It’s a decision-support tool, a dashboard, and a security blanket. I’ve learned so much about my diabetes from it, and it’s provided the data I needed to make some important changes to my self-management. I wish everyone had it.
• The BG values it reports are delayed by 15 minutes. This is a fact of life. The CGM looks at “interstitial glucose” between cells (ISIG) instead of blood glucose, and there’s a lag time between the two. Don’t expect the BG numbers from your meter and your CGM match unless your having very stable BGs.
• The most important factor in getting believable CGM readings is calibration. Wait until your interstitial glucose (ISIG) is the most stable before calibrating.
• If my BG is on the move but the system says I need to calibrate it, I’ll let it go past the “Meter BG Now” deadline and look at the “Sensor ISIG” values on the CGM information screen. When this value isn’t changing much, that’s when I’ll calibrate.
• Lots of things affect ISIG values and therefore CGM accuracy: the rate of change of BGs, the level of hydration, and the wetness of the sensor (both too wet and not wet enough).
• I find that if I keep my sensor and transmitter covered, I get a longer sensor life and better readings.
• Tegaderm film works the best for me. Unlike IV-3000, it doesn’t come off when I’m swimming or sweating a lot from exercise; it doesn’t leave behind a lot of residue; and it’s stretchy enough to cling nicely and create a good barrier.
• Hydration matters a whole lot to accuracy. If you’re dehydrated, the sensor will read incorrectly, or perhaps not at all.
• Don’t calibrate first thing in the morning. There’s too much going on. Almost everyone is slightly dehydrated when they wake up, and I’ve noticed that the very act of waking up and getting moving causes my CGM sensor to give inaccurate readings for about a half hour.
• Don’t calibrate right before or after exercise. For me, my blood sugar usually is on the move during the first bit of a workout and immediately after it. Plus, the chance of BG swings is greater while exercising, which is bad news for calibration.
• I find that sometimes I can count on my CGM values when I exercise, and other times I can’t. The lag is a little less during exercise, but my hydration is also different so it’s less accurate. Sometimes it catches big BG swings, and sometimes it doesn’t.
• Swimming: I find that I can swim for at least an hour without affecting the sensor or transmitter. But I also find when I swim three or more times in a week, the off-label lifespan of each sensor is noticeably shorter.
• Sensor life: The longest-lasting sensor I’ve used that produced reliable BG estimates is 12 days, which is not too bad for something that’s only supposed to work for three. (Just tell the CGM that you’re starting a “new” sensor when it says “Sensor End,” even though it’s the same one and not new at all.) Because my transmitter battery requires a quick recharge after 6 days, I have to very carefully peel off the Tegaderm I put over the sensor and transmitter, give the transmitter a charge for 15-30 minutes, and then reattach it and apply more Tegaderm.
• Occasionally, the sensor values will just drop out after the three days; they might come back to match my BG, and they might not.
• When the CGM graph gets really noisy, the sensor is dying.
• Sensor freshness matters. A sensor getting close to its expiration date won’t last as long and seems to give less accurate readings.
• Treat the number after finding a “Lost Sensor” with some skepticism. I find I often need to recalibrate 30-60 minutes after reacquiring the sensor.
• To avoid “CAL ERROR” messages, I don’t calibrate if the CGM and BG meter values are very different.
• The Medtronic CGM does some curve fitting, so it often doesn’t “snap” the CGM values to the calibration value when they’re very different. I hate that. Entering another, new BG value usually gets them closer, though.
• Post meal, a period of flat BG values followed by a rise 1.5-2 hours later almost always means that the meal bolus was too small. (There are a lot of other trends you can spot, but this one was the most consistent.)

Do you use CGM? What have you noticed?