I have been using a Medtronic 670G insulin pump together with its Guardian 3 CGM since Feb. 2019. Many diabetics who use such systems have complained on Internet forums about this system and have praised others. When I asked my endocrinologist about the relative merits of the Medtronic CGM and the Dexcom G6 CGM, she offered me a sample Dexcom G6 CGM kit to try out. Later I was sent a second G6 sensor by Dexcom.
Over the past month or so, I have worn a Dexcom and a Guardian 3 CGM. I have about 25 days of tabulated data comparing BG from fingersticks* with readings from a Dexcom G6 and from a Guardian 3. I haven’t yet found a good way to display all the data concisely, so in this report I will write about the main things I saw, and end with a few subjective comments.
I installed the Dexcom G6 app on an iPhone 8 (now running iOS 14.8) to read out the G6 sensors. At 5:15 pm on 8/31/21, I started the transmitter and the first Dexcom G6 sensor with its 4-digit factory code, initiating the 2-hour warm up. At about 7:50 pm the iPhone beeped and its home screen showed a high-glucose alert. The Dexcom app read 325 mg/dl while the Guardian 3 CGM showed 263. I measured my BG by a fingerstick – it was 261.
So the G6 was reading 64 mg/dl higher than the BG measurement. According to the webpage https://www.dexcom.com/faqs/is-my-dexcom-sensor-accurate, a BG reading of 260 could show up on the G6 in the range 208 to 312. (This is +/- 20% – part of their so-called “Rule of 20”.) The G6 reading was 25% higher than the measured BG, while the Guardian G3 was in almost perfect agreement. I asked the pump to give a corrective bolus of insulin and both CGM readings came down over the next hour, but the G6 continued to be about 63 mg/dl above the Guardian 3 reading.
Calibration of the G6
I decided to calibrate the G6 even though Dexcom makes a big thing of saying it’s generally not necessary. (How would I have known whether a calibration was necessary if I hadn’t also been wearing the Guardian 3?)
At first I couldn’t find instructions on how to calibrate the G6, so about 9 pm I called Dexcom’s 24/7 Global Tech Support and waited 15 minutes for an agent to answer. While waiting, I found the instructions, and at 9:13 pm I did another fingerstick. My BG was then 229. The G6 read 293, about 28% higher than the BG, while the Guardian G3 read 221. I applied the BG calibration to both CGMs. Over the next 5 minutes the G6 reading dropped, ending up very close (233) to the BG measurement of 229.
The Dexcom 24/7 agent said I shouldn’t expect perfect agreement with BG and referred me to the “Rule of 20”, but said she would ship me a replacement sensor (perhaps because the G6 had been more than 20% too high).
Tracking the Dexcom G6 against BG
Overall I found that the Dexcom G6 sensors did well except for five things:
(1). As described above, the first G6 read too high by 25 or 28% until I calibrated it.
(2). At 9 pm on 9/02/2, the Guardian 3 read 57 and gave an alarm for low glucose. I was definitely feeling hypoglycemic. My BG measured 69, an uncomfortable level for me. However, the G6 was reading 82 and it gave no alarm of an existing or impending low – not good.
(3). On 9/09/21, 9:52 pm, I re-started the first G6 with its factory code (no BG calibration). Over the next three to seven hours it read too high (by more than 30%), similar to its initial behavior. However, all later readings fell within the “Rule of 20” even though I didn’t re-calibrate. This inconsistency didn’t seem too bothersome as the sensor was past its rated life.
(4). Owing to a delay in delivery of the second G6 sensor, I couldn’t install it until 9/17/21. I didn’t apply any BG calibrations as I wanted to track its performance using only its factory code. Over the first 30 hours the G6 deviated a great deal from measured BGs, reading as much as 40% too high and 19% too low. After this initial period all the G6 readings were within the “Rule of 20” except for the sixth day, when it read 30-34% high during all three of my BG measurements during one 24-hour period. I am not aware of anything I did to make the G6 readings go so high for one day and then return to acceptable values later.
(5). Both of these G6 sensors gave untrustworthy results on the first day and again after the eighth or ninth day. Re-starting the first sensor gave me some readings well beyond its 10-day rated life, but they seemed erratic and were interspersed with hours of no readings at all. I didn’t try re-starting the second sensor.
If the Guardian 3 hadn’t made me realize that something was wrong with certain Dexcom G6 readings, I would have been overtreating highs and missing lows. I find this worrisome. Were I to switch to the G6, I would be sure to do a fingerstick with each new sensor and perhaps again at the middle and near the end of its lifetime.
The sensor wire in the G6 is hair-thin and I was seriously concerned about it breaking off and remaining lodged in my body. Some correspondents at TuDiabetes have spoken of such an occurrence. The Guardian 3 sensors are much more substantial and seem to pose no risk of breakage.
In my data, the Dexcom G6 doesn’t show better accuracy compared with BG than the Guardian 3. Of course, the latter system requires frequent calibrations with measured BG, so the agreement of the Guardian 3 readings with BG may be due to the many fingersticks it requests (although I sometimes cheat and supply a BG guess rather than stick my finger!).
It must be admitted that the Medtronic 670G system has problems and annoyances. It demands frequent BG measurements, thus many fingersticks. It is very fussy and drops out of Auto mode at slight provocation. For example, if the glucose level hasn’t required any basal insulin for 2-1/2 hours straight, it sounds an alert, jumps into Manual mode, and demands a BG measurement to be returned to Auto. Or if the CGM detects a glucose level out of range (below 70 or above 280 mg/dl), it kicks the pump out of Auto mode. This has the result that the pump may not be adjusting the basal insulin just when such action is most needed. These too-strict criteria for staying in Auto mode cannot be modified by the user. I have read that the latest Medtronic pump and CGM system, the 780G, alleviates these problems, but it is not yet approved for sale in the U.S.
For me, the all-in-one nature of Medtronic systems is a real convenience. The Dexcom integration with a smartphone posed some difficulties for me. In my house the iPhone/Dexcom readout often gave a no-data alert when I left it in one room and stayed in another room for more than 20 minutes. When I leave my house I don’t like to carry a smartphone – I use instead a dumb flip phone that fits easily in my pocket; losing or breaking it would not be a major disaster. Dexcom’s G6 receiver appears smaller than a smartphone, so it can probably be kept in a pocket and thus always be within Bluetooth range of the sensor. It might be a good alternative for a person like me averse to carrying a smartphone everywhere.
- My BG meter is a Bayer Contour Next Link, supplied with the Medtronic 670G system. Of 18 meter/test-strip systems studied in 2013-2015, the Contour Next was found to be the most accurate, with 97% of 300 tests coming within 10% of the reference value. See my Sept. 12 note to TuDiabetes, https://forum.tudiabetes.org/t/any-recent-tests-of-bg-meter-accuracy/88266.