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Is Manual Calibration Necessary for Accurate Readings?

You might have heard people with type 2 diabetes who wear a continuous glucose monitor (CGM) talk about the idea of manual calibration. Manual calibration is a practice that some people use when they believe their CGM readings are not aligned with their blood glucose readings done by using fingersticks.
This method might have been recommended to them as a safeguard for use for a CGM, but with a Dexcom G6, CGM manual calibration is not necessary. This is because the Dexcom G6 self-calibrates in an internal process during the first 24 hours of wear.
Although CGM readings of the interstitial fluid might not match the readings of the blood glucose meter during the first 24 hours of wear, they should be accurate after that time because the Dexcom G6 will have self-calibrated.
(If you have questions about the differences between a CGM and blood glucose readings, you can learn more from this article.)
How Do You Know if the Dexcom G6 CGM is Accurate?
The Dexcom G6 CGM System is the first real-time, integrated CGM that does not require fingersticks or manual calibrations to give accurate readings. (That’s pretty awesome!) So how do you make sure the G6 gets working and calibrating correctly?
First, you must enter the unique Dexcom G6 sensor code that is printed on each sensor’s adhesive label during the startup period for the CGM. As long as you have entered the code, no manual calibrations will be required.
In the first 24-hour time period when the newly-inserted Dexcom G6 is self-calibrating, the glucose readings sometimes are less consistent with those of a blood glucose meter (the fingerstick method). However, the readings become more aligned with each other as the sensor becomes more accustomed to your body. If your readings in those first 24 hours of wear are not matching how you feel, you can always double-check using fingersticks. If you are making medication or dosing decisions, always double check with a fingerstick.
Although your CGM is never intended to perfectly match a blood glucose meter (because they measure different things), they are intended to be close. After the 24-hour first-wear period, you can check the CGM’s accuracy by using the “20 rule.” Your Dexcom G6 device should be within:

  • 20% of the meter value (blood glucose meter) when the meter value is 80 mg/dL or higher
  • 20 mg/dL of the meter value (blood glucose meter) when the meter value is under 80 mg/dL

According to Dexcom specifications, the G6 sensor should display a value that is +/- 20% of a blood glucose meter value. If blood glucose is not changing (rising or falling) quickly, and the trend arrow is horizontal then the sensor glucose is expected to be within the 20% range of tolerance.
If your sensor readings are in fact not lining up after the 24 hour-time period, contact Level2 Support at 1-844-302-2821, TTY 711, and they will help troubleshoot or order a replacement device if necessary.