CGM Diaries: Day 1

Following on from yesterday's post, today was CGM pickup day. This involved visiting the hospital to collect the first batch of supplies for the CGM, as well as set it up, and learn how to use it. This is more of a "first impressions" post, as it will take a while to be able to assess its usefulness in managing glucose levels overall.

The smaller box contains the Dexcom transmitter. The larger box contains the sets. 

The smaller box contains the Dexcom transmitter. The larger box contains the sets. 


Surprisingly simple, there are two parts to the Dexcom G5 CGM. There's the transmitter (shown above as the smaller of the two boxes), and the "sets" - the piece holding the cannula that attaches to your skin. The Dexcom iOS app has a straightforward onboarding process which ends with you linking the transmitter device by scanning a barcode on the back of the box.

This is used to fix the set to the body. It looks scarier than it is. 

This is used to fix the set to the body. It looks scarier than it is. 


Sensor insertion

Nearly lifetime of needles, nor over eight years of inserting cannulas for an insulin pump meant I felt calm when seeing the insertion device. It worked fine and fortunately didn't hurt, but that doesn't mean it wasn't intimidating. You can wear the set either in your stomach or on the back of your arm. I choose stomach, as I'm comfortable with putting the sets for my insulin pump there and figured it's a safe option. Once it's in, you clip the transmitter device into the plastic enclosure of the set where it sits for the next week. The transmitter lasts three months, while the sets are swapped every week.

Physical size

It's bulky. See the photos above. It's about twice as thick as the insulin pump sets I'm used to. It's not uncomfortable, but my biggest concern is that it'll be easy to knock out. For reference, sensor and the set combined are about a thick as the width of my index finger.


Getting started

Once the sensor is inserted, it takes two hours to warm up. From there, it asks you to do two manual blood glucose readings, one after the other, to calibrate the sensor. You input these into the Dexcom app, and from there it gives you your first reading. After that, it senses your glucose level every five minutes and sends that reading back to the phone. Interestingly enough, Dexcom makes the only CGM sensor that is accurate enough to trust as a replacement for blood readings. Despite that, two manual blood readings are still required per day to keep the sensor calibrated.


The iOS app

The iOS app is fairly simplistic. It shows you your current reading, along with an arrow indicating the trend of the readings. It's worth pointing out that, for those who care, the iOS app is still running at iPhone 5 resolution - not yet updated for 4.7" and 5.5" displays, let alone the 5.8" screen size of iPhone X.


Notification alerts

The Dexcom app offers real-time notification alerts for glucose levels that are either too high or too low. These somehow bypass silent mode, do not disturb mode, and the system volume level on iOS. I've received a few of these notifications already (don't judge - the CGM should help reduce the number of poor readings I have) and it's safe to say they're extremely annoying. My phone has stayed in silent mode from the second I got a smartwatch four years ago, and I don't like the fact that notifications are making noise again. More granular control over this would be appreciated. It may be useful overnight to alert me to hypo (low) glucose levels, but I wish there was a way to silence them. A tap on the wrist from the Apple Watch is all I need to see an alert and action it.


Apple Watch complications

For a variety of reasons that I won't go into here, I've used the "Simple" Apple Watch face (shown in the first image above) almost every day for two years. I am also of the opinion that having an Apple Watch and a Dexcom CGM means you are obliged to use the complication to get your BGL at a glance. Unfortunately, the Dexcom complication doesn't support the Simple watch face. The closest alternative is "Utility" - shown in the second image above. It means I lose having my step count on the watch face, as there's space for one less complication, but I think that's a worthwhile tradeoff for now. The only watch face that allows me to have the Dexcom complication as well the other complications I'm used to is "Modular" (third image above) but I'm not a huge fan of that one. I might use it as my overnight sleeping watch face but will try to avoid it during the day. It's too early to tell for sure, but the complication seems to update every 10 minutes, which lines up with every second reading recorded by the Dexcom sensor, so it's frequent enough. Occasionally I have noticed that it fails to update, and instead shows three dashes "---" (shown in the fourth screenshot above).


HealthKit sync

Another reason for choosing the Dexcom CGM, other than it being the most accurate, was that it's the only one to sync to a phone. If you have an iPhone, not only does it sync to the Dexcom app on your phone, but also optionally can write the data to HealthKit. This is perhaps the most strange thing I've discovered about the Dexcom so far. The Health screen reads, "Dexcom G5 Mobile information posts to Health with a three hour delay." My first thought upon reading this was that it only writes to HealthKit every three hours - i.e. eight times per day, maybe in an attempt to save battery?


As far as I've been able to discover, the Dexcom app writes to HealthKit every five minutes but writes the reading from three hours ago. I'm not sure why this is, and I'll continue to try and find out why. It feels arbitrary. As can be seen in the above screenshots, a reading recorded at 2:29 pm was written to Health at 5:29 pm, and a reading from 2:34 pm was written to health at 5:34 pm.

Wrapping up

I've still got a lot to learn about the Dexcom CGM. The best way to use it, how to interpret results and trends, and how it fits into my life all remains to be seen. Some sections may have sounded a bit like a complaint, but I assure you it's just an expression of my initial impressions. I am in no way complaining about the bulkiness of it, the missing "Simple" complication, nor HealthKit sync. I'm in a very fortunate position to have a CGM, and these are simply the things that have stuck out to me that I wanted to share after six hours of use.