Is my CGM accurate?
How do you know if your CGM is accurate?
Accuracy of your glucose values IS essential. If your CGM tells you that your glucose is 91mg/dL and your finger stick is 144mg/dL, the CGM data is essentially useless. That's a huge range and not helpful. You can imagine what you might think when you wake up in the morning and it's flip-flopped the other way! Your CGM reads 144mg/dL, and your finger stick reads 91mg/dL! If you went by your CGM, then you might think you have a glucose problem and need to see your Doctor. You want an accurate reading.
Your glucose level will be different depending on the location of the measurement.
A CGM measures your glucose in the interstitial space, and that is at the "end of the chain," so to speak, from when you ingest carbs, and the glucose moves throughout your body. Measuring with a glucometer, a finger prick will measure the actual blood glucose, but it's also in an extremity, so it's also delayed. If you are in a hospital, your blood glucose can be measured intravenously, and that's the fastest and "closest" place to measure blood glucose. For example, your intravenous measurement might be 100mg/dL, your finger stick 90mg/dL, and your CGM 80mg/dL.
What should I do to ensure I know my CGM is reading correctly? First off, you need to buy an inexpensive glucometer. You can buy these with the test strips, finger pricker, lancets for under $40 now, and I recommend testing yourself 3 hours after inserting your CGM, then again 24 hours later, and again if you have ANY doubt about the CGM reading. When you do a few tests of your finger stick blood glucose and compare it to your CGM, you will know how much difference there is between them. **IMPORTANT**>> Make sure you take your finger stick reading RIGHT after your CGM reading "updates". Some CGMs will update every 15 minutes, others every 5 minutes, and every 1 minute, so it's super important that you get your blood glucose finger stick right after your update. If your CGM hasn't updated for 14 minutes and it's reading 100mg/dL but you had (2) sports gels 10 minutes ago and your finger stick reads 150mg/dL, then do not confuse this for a bad CGM reading.
What about consistency? "Most" of the time, you won't see much variation throughout the time you have your CGM inserted, and if it's 20mg/dL high or low from your finger stick, then it will likely stay that way for the life of the CGM. The "over the counter" CGMs like Stelo and Lingo are advertised as accurate to +/- 20mg/dL, and I have found that's correct 90% of the time. I recently had a Stelo sensor that was off by 50mg/dL, and it got worse the closer it got to its end of life. I have had other sensors, such as the finger stick and the CGM, for the entire life of the CGM sensor. The prescription sensors, like the Dexcom-G7 and Abbott Labs Freestyle Libre 3, are advertised as +/- 10mg/dL, and I, too, have found this to be correct 90% of the time. For this reason, I predominantly use a Freestyle Libre 3 from Theia. (Our link lets you join our community if you purchase a sensor. Learn more here. )
If your CGM consistently differs from your finger stick, that's fine. Once you know which way it is "off," you'll be able to calculate that in your head to get a clear understanding of your finger stick blood glucose.
My reading was great, and now they are totally off, what happened?
Most of the time, this is because you bumped your CGM, which slightly lifted out of your skin and then settled back into the previous location. In most cases, this messes up your readings, so be careful when entering and exiting through doorways and taking off your shirt or workout jersey. Readings can also become inaccurate if the battery dies or starts dying early. You need to then file a claim with the CGM manufacturer, and they will replace the CGM.
In conclusion, make sure you have a glucometer and test yourself a few times. If you ever "bump" your CGM at a door entrance or when taking off your shirt, and the reading starts to appear "off," then test yourself with your glucometer. If your CGM is off by 20mg/dL and it's consistent, then not to worry; just know that your finger stick values are different.
