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A Continuous Glucose Monitoring System (CGMS) is a device that records blood sugar levels throughout the day and night.

Abbott Freestyle Navigator Abbott FreeStyle Navigator®
Paradigm Continuous Glucose Meter MiniMed Paradigm® REAL-Time System
Dexcom Continuous Glucose Monitor Dexcom SEVEN Plus
Medtronic Guardian RT Guardian REAL-Time
Continuous Glucose Monitoring System

Diabetes CGMS issues

There is currently no CGMS on the market that can measure the absolute level of sugar in your blood, they can only measure change relative to a known amount. That is why they constantly need recalibration to the results of your meter. If you don't recalibrate them (by entering a real finger prick result every 3 hours or so), then the CGMS result will drift from the real figure. They work best when the rate of change is small e.g. when you are not eating (ie asleep!).

Also, CGMSs do not measure the sugar in capillary blood (the blood from your fingertips), they measure the sugar in interstitial fluid (between the cells), typically in fat or muscle where the probe is inserted. The sugar in these areas can lag up to 30 minutes behind what your finger pricks will tell you e.g. your finger prick might show 4.0 (USA 74), but the CGMS might show 6.3 (USA 113).

CGMS ARE useful for recording overnight/sleeping blood sugars, especially in young children.

They are not a replacement for fingerstick testing.

The results also require interpretation in conjunction with real readings, as the following excerpt shows:

An example ... our endo is FABULOUS and has always been supportive of the technology. W. has been on the CGM since spring 07 when the MM transmitters became small enough for a little guy so this is not new to us or to the doctor.

But at our last appointment we were looking at the print-outs the doctor had generated from the pump download and he pointed to a page that showed a clear pattern of highs throughout the night. Now I KNEW that wasn't the whole story (nighttime numbers had been quite good the past couple of weeks) so I took out the print out I had brought in. It was one using the CGM data, showing the sensor readings overnight for the past two weeks. You could see that while there were high blips that came right down, overall things were steady.

So why the discrepancy? His print out was of just BG tests ... which only showed BG tests we did overnight when the CGM was telling us there was a high coming. So the report was skewed to show almost all high readings. Not the whole story at all. But had I not brought in my own printouts from Carelink and explained why the BG report was incomplete, we could have been given a mess of basal adjustments to correct a pattern of highs. Eek.

As I said, he has been really supportive of the CGM use but as I think about it, how I use the data and reports has been from self-training, a little from MM manuals, and ideas from other CWD people.

I wonder if MM and the other companies are spending time teaching endos and CDEs the full capacity of the data reporting tools.

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