CGM sensor locations

Where we place our CGM sensors is sometimes a fraught issue.

Regulatory “labels”

Dexcom and Medtronic initially registered their CGMs with the regulators (FDA, TGA, etc) saying they should be used on the abdomen. Abbott registered the Libre saying it should be used on the upper arm. Actually the Medtronic Guardian3 sensor lists the abdomen and the back of the upper arm as suitable locations.

As a result the approvals they’ve received say that these are the locations the sensors should be installed. And the companies can’t recommend that we do differently or they risk invalidating the regulatory approval!

But many of us are “rule-breakers”

But many of us don’t follow those instructions. Why would we use the devices “off label” like this?

There can be many reasons:

  • We find the results more accurate when we use different body locations.
  • We find the “labelled” locations more prone to being knocked. Whether it’s from car seatbelts, small children, or a myriad of other causes.
  • We find those locations socially awkward (for example at the beach or in formal dress).
  • We find the sensors stay accurate for longer in other locations. For those of us who have to self-fund CGM, this can make a huge difference to the overall cost.

The CGM suppliers are forced (by the regulators) to say “you should only use them in […] locations”. Unfortunately some people are intimidated by this and hear that as “you must only”.

Is it still accurate in other locations?

Keep in mind that we can evaluate for ourselves how accurate/reliable the CGM is by comparing to fingerprick blood tests, and most CGMs require this to maintain calibration. This does require good calibration technique and a BG meter that’s as accurate as possible, but it’s not hard.

See “CGM accuracy – Calibration is King!” and “Do you trust your meter?

Some people wear their sensors on their legs (both thighs and calves), chest, etc. Personally I usually wear my Dexcom G5 sensors on the back of my upper arms: I find they tuck out of the way neatly, and my body bends a lot less there than on my abdomen which probably helps me get longer sensor life (whenever I’ve tried my abdomen the sensor hasn’t lasted as long).

I have found my own CGM to be acceptably accurate. And I’m dosing my insulin based off the results (via my closed loop system) so I have a big incentive to only use something that’s accurate.

A peer-reviewed clinical trial

An interesting paper was recently published: “Comparison of Continuous Glucose Monitoring Accuracy Between Abdominal and Upper Arm Insertion Sites“. It’s coming out in Diabetes Technology & Therapeutics (DTT).

In this study (which was conducted in Denmark in 2016-2017) they used Dexcom G4 Platinum CGMs (running the “505” software in the receivers, so equivalent to the Dexcom G5 algorithm and in fact using the same physical sensor as the G5) to test the accuracy of the CGM in different locations compared to lab blood analysers (which are the gold standard that home BG meters are compared to).

To quote from the paper:

Conclusion: The accuracy of a Dexcom G4 Platinum CGM sensor placed on the upper arm was not different from the accuracy of the sensor placed on the abdomen in adults with type 1 diabetes.

If anything the numbers they show seem to indicate slightly better accuracy on the upper arm, however I don’t think the difference in this study was statistically significant.

What does this mean?

Although this paper won’t mean that the regulatory approvals for these products will be automatically updated to allow for the use of alternate locations, it might be a step along that path especially if more data appears over time.

It’s good to see peer-reviewed papers that back up the observations we CGM users have made in real-world use of the products!

More flexibility in siting CGM sensors can make a big difference in the uptake and use of CGM. If a future CGM product was to come out with regulatory approval that included a wider range of tested and “labelled” locations, I dare say that could be a competitive advantage for that product.

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