High and low alert thresholds

After 11 days I am starting to develop my method. Today I will write about how I use the high and low alerts. I imagine all this is very similar to the 3 day system.

HIGH THRESHOLD

The default setting for the high threshold is 200 mg/dL. I want to avoid reaching 200, not treat myself once I'm there, so I immediately lowered it to 160. You can only work in 20 mg/dL increments for the high threshold. I've had some accuracy issues with high blood sugars in certain situations, and I quickly realized that even 160 was too high. I'll write a separate post about accuracy, but what is working best for me is to set the high threshold as low as it will go (140 mg/dL), and hope the receiver will alert me before my actual blood sugar rises over 200. When the alarm goes off, I decide whether I need a finger stick to confirm the reading and/or calibrate the meter, and whether I need a correction bolus. Then I reset the high threshold to 160 mg/dL and repeat the process until I see the curve start to trend back down. One really great thing about CGMS is you can see when the rising blood sugar starts to plateau, and reverse back down. This really helps avoid stacking correction boluses.

This works fairly well. The accuracy problems I'm experiencing, though, lead to frequent instances in which the Dexcom shows me stable at a reasonable blood sugar level, but a finger stick shows we well above my target range (for instance, yesterday the receiver said I was stable at about 115, but the finger stick put me at 173). I call these "Type 2 errors" (that's statistics Type 2, not diabetes Type 2), i.e. times when the Dexcom fails to alert me to a bad blood sugar. The other type of error, "Type 1", or times when Comrade Dex warns me that I'm out of range when the finger stick shows me at a decent level, hasn't happened in the past 10 days that I can remember. When I can get back on my computer I'll check my records and I'll keep count of Type 1 and Type 2 errors over time. Maybe I'll correlate them with how many days I've been using the sensor. Oh, the games I get to play.

LOW THRESHOLD

I use the same technique with the low threshold, but with better success because so far the Dexcom Seven is much better at catching lows than highs. In the past 10 days, I haven't had a single serious low blood sugar. I think I dipped into the 40's a couple times, but that was as much due to me experimenting with the system as anything else.

The default low threshold is 80 mg/dL. On tight control, I drop below 80 regularly and it's no big deal, so I usually keep it set at 70 mg/dL. You can work in 10 mg/dL increments with the low threshold. When I get the alarm at 70, I might or might not do a confirmation finger stick, calibrate the meter, and eat or drink something. Then I reset the treshold to 60 and repeat the process. There is also an alert at 55 mg/dL that is hardwired, and the user cannot change it.

THRESHOLD SUMMARY

The low threshold is working great for me. It has definitely prevented serious hypoglycemia episodes. I give it an A.

The high threshold works pretty well when my sugar is rising quickly, but works very poorly when my sugar is on a slow sustained rise. If this problem is widespread and not just a fluke of my body chemistry, my suggestion to Dexcom is to allow even lower high thresholds. I would really like to set it at 120 mg/dL. I think at that level I could catch most of the highs I want to prevent. For now, I give the high threshold/alert system a C, or even a C-.