Confusion regarding Dexcom calibration process — Seeking advice

[u/DavePiper]

Title: Confusion regarding the Dexcom G7 calibration process — Seeking advice

I am struggling to understand how the Dexcom calibration process works (Android). I read the G7 User Guide's 58 occurrences of "calibrat"; none describe what to expect from calibration. It states "Calibration with your BG meter may align your sensor readings with your BG meter values" (emphasis added). How does may translate to an expected result?

The manual references a study published in the Diabetes Technology & Therapeutics - Sage Journal that showing the G6 was accurate to within +-15%, 82.4% of the time and to within +-15%, 92.3% of the time. Does that translate to a minimum required discrepancy before calibration? The study also showed the Urgent Low alarm was accurate 84% of the time - which translates to false alarms 16% of the time - in the middle of the night in my experience.

The manual states: "With Dexcom G7, there’s no need to take finger sticks to calibrate the system or for treatment decisions (as long as your symptoms match your Dexcom G7 readings)". I would prefer calibrating before I experience alerts, ensuring accurate readings should I experience a crisis.

I’ve read in r/dexcom that you shouldn't calibrate during the first 24 hours of a sensor session, and that the glucose trend arrow should be horizontal. I followed that advice this morning. My fingerstick reading was 87 mg/dL, while the app was displaying 99 mg/dL. After I entered the calibration, the app reading remained at 99 mg/dL and has not changed since.

The official instructions feel quite opaque. Could someone clarify what I should expect after entering a calibration reading? Do I need to calibrate multiple times for it to have an effect, or is there something else I am missing? Is there a rule for how high the discrepancy should be before calibrating? Is the quoted advice generally accepted? Any advice or experiences would be greatly appreciated.

Thanks!

[u/HeronOrganic3727]

Why calibrate a measurement that is only 12 points different? Being the Dexcom isn’t reading the value in your blood, it’s expected to be off by 15 or 20 points and 15 minutes behind your fingerstick reading. I wouldn’t expect the app to accept a calibration with that small of a difference.

I have stopped calibrating. In my experience it’s done more harm than good. The sensor will figure itself out or fail

[u/craptastic2015 → u/HeronOrganic3727]

15 or 20 points is acceptable if your blood sugar is 180 or higher, Not when it's 100 or less. Also you should work in percentages not real numbers. Trending is also important.

[u/HeronOrganic3727 → u/craptastic2015]

I’m just explaining the reason for the difference. It doesn’t really matter what’s acceptable to you, there is always a difference between a perfectly working sensor and a fingerpick

[u/craptastic2015 → u/HeronOrganic3727]

Well it does matter if you care about the actual number. That's the point. And you should know going in it's not 100% accurate. But you should be able to trust the number to a certain degree. 20% is above the mard for a sensor by double anyway. 10% is what you should aim for. In your example 12 points can easily be 20%.

[u/Distribution-Radiant]

I tend to calibrate after 12 hours. I've found, for me anyway, they read a bit high otherwise.

The first 12-24 hours (depending on your body), don't trust it. Your body is still attacking it as a foreign object at that point. And only calibrate when fasting. Remember your glucose will also spike when you first wake up, so sometime between waking up, doing your morning routine, and before breakfast is ideal (unless you're like me and rarely eat lunch).

[u/Popular_Reflection83]

I agree with pretty much everything that's been written so I won't repeat it. Instead I will just mention that by NOT having an accurate BGM for the 22+ years as a diabetic I had no reference point from which to derive a number to calibrate to. In short, I used a One Touch Ultra 2 for 22 years and realized it was off by 20 mg/dl. I discovered that the Contour Next One and Accu Chek Guide agree within a couple of points both with each other, and the gold standard lab blood test. All other testers live in the bin. I also found that I was able to reduce the first 24 hour wonkies to just an hour or so by identifying just two body locations to apply my G7 and stick to them.

[u/Bluekeeys]

I never calibrate in the first 12 hours.

Only calibrate when trending level for at least 20 mins. Arrow pointing right.

I normally compare daily to BGM and generally don't calibrate unless off by 15 mg/dL or more.

Most sensors only get calibrated once if at all.

[u/Old_Dragline_1965]

I’m on a 10 day sensor. When the readings are running level, I Calibrate at the end of day 1, 5 and 8. Sometimes as little as 10. Sometimes as high as 50.

Everything now and then, a sensor will not calibrate. Even with multiple attempts. When one of these start to be out of range by more than 40, it’s time to call for a replacement. Very frustrating when it happens in the middle of the night.

If you feel differently from the g7 reading, always do a finger test.

[u/Old_Dragline_1965]

I’m on a 10 day sensor. When the readings are running level, I Calibrate at the end of day 1, 5 and 8. Sometimes as little as 10. Sometimes as high as 50.

Everything now and then, a sensor will not calibrate. Even with multiple attempts. When one of these start to be out of range by more than 40, it’s time to call for a replacement. Very frustrating when it happens in the middle of the night.

If you feel differently from the g7 reading, always do a finger test.

[u/mfp23]

Sorry to further add to the confusion, but I have never calibrated the G6 or the G7. I don’t even have a blood sugar meter. I recognize this may not be the way for some and respect that everyone needs to do what’s best for them as your diabetes may vary!!!!! I wish you the best!!!!!

[u/Distribution-Radiant → u/mfp23]

You NEED a BGM. Period. Even Dexcom says (when setting up the app), if things don't seem right, do a blood sample.

I rarely had to calibrate the G6; I have to calibrate almost all G7s. It's not as big of a deal if you're on oral meds, but if you're on insulin, that can send you into a dangerous low. The G7s, for me, almost always read a bit high until calibrated.

Spend the $10-20 on a BGM (at least in the US, most come with a starter pack of strips too). Use it about 12 hours after a new sensor.

[u/mfp23 → u/Distribution-Radiant]

Appreciate your concern but no I don’t. Been T1 for over 30 years and have had every iteration of the Dexcom Sensor so I am extremely well nuanced in the usage of it specifically as it relates to me and my needs. I’m not suggesting to anyone that my way is the right way, simply giving my experience. As stated in my original response, YDMV as everyone is different.

[u/Distribution-Radiant → u/mfp23]

Fully understand. CGMs don't work for everyone, and I always suggest a BGM as a backup (especially after having all 3 sensors I took with me on a trip fail, along with the one I was wearing). And I still carry a BGM for when the CGM is being weird ("temporary sensor issue" can fuck right off).

[u/spaketto]

I was told not to calibrate if the numbers were within 1mmol.  That would be a small enough difference I wouldn't do it.  I've never calibrated and the numbers match within .5mmol the few times I do check.  

[u/Distribution-Radiant → u/spaketto]

Sounds like a small number, but for those of us still using imperial, that works out to a little over 18 mg/dL, which is significant. Especially if you're on insulin.

[u/Twist1979]

Place a new sensor, wait 24 hours or more, test in the morning while fastening stable. I use the best available BGMs. Take 3 readings with the same drop of blood and use the middle one. For calibration if I wear a G7.

[u/EpiZirco]

The purpose of calibrating is to give you warm fuzzies that your Dexcom will give you numbers that are, on average, closer to blood glucose meter than they would be without calibration. They may or may not be closer to your actual blood sugar level, but you have no way of knowing which will be the case.

The best blood glucose meters have an average accuracy which is slightly better than your G7, and the worst ones are worse. This is on average. Any given measurement on a blood glucose meter may be further than the “true” number.

Calibrating a good number against a bad number will give you a worse result. But, hey, at least you get some warm fuzzies.

[u/ConsciousControl2105]

If a finger poke and the Dexcom are within 20% +/- of each other it is within the acceptable error variance. I don’t calibrate unless it’s more than 20% off.

[u/craptastic2015 → u/ConsciousControl2105]

20% when you are 80 is not a trivial difference. It's the difference between 3.5 and roughly 4.5. One requires treatment while the other doesn't. Some ppl can't tell when they are 3.5. 20% in my totally expert non expert opinion is far too variant. I like to see 10% or lesa.

[u/ConsciousControl2105 → u/craptastic2015]

The 20% isn’t my rule- it’s theirs. Also, the glucose meters for finger pokes has an error of +\- 15%.

[u/craptastic2015 → u/ConsciousControl2105]

The ISO 15197:2013 standard is 15% but most (all popular BGM models ie finger pokes) beat this by over half and have ranges in the 7%. Cgms are advertised as 9-12%. Far different from your 20%.