How to test the accuracy of your Dexcom G6 CGM with fingersticks


Here’s how to test the accuracy of your Dexcom G6 CGM with a fingerstick meter reading.

Please note: The information listed here is applicable to Dexcom CGM users within the US only.

In this article:

The fingerstick meter test

It’s important to understand that blood glucose meter readings through a fingerstick will never exactly match the CGM reading. The 9% error margin for the Dexcom G6 CGM is close to the most accurate meters available today at 10%; a 20% difference can be expected between CGMs and fingerstick tests.

You can compare readings from a fingerstick meter and your Dexcom G6 to see how closely the numbers match. If your G6 reading and meter value is within “The 20 Rule” (also known as the %20/20 rule), they match closely.

To use The 20 Rule, follow these steps and use the table below:
  1. Find your fingerstick meter value in the green columns.
  2. The number to the left of your meter value (G6 –) shows the low range for a G6 reading that’s a close match.
  3. The number to the right of your meter value (G6 +) shows the high range for a G6 reading that’s a close match.

For example, the orange highlighted row below shows that if your meter value is 100 mg/dL, your G6 reading is a close match if it’s between 80 and 120 mg/dL.

If you are within 20% points of your fingerstick meter reading, there is no need to calibrate.

Best practices for fingersticks

When using your fingerstick meter, make sure:
  • Your test strips are stored as directed and are not expired.
  • Your hands are clean. Wash your hands thoroughly with soap and water and dry them, then do the fingerstick test.
  • You don’t test during or directly after a meal since readings can be delayed during glucose rises or dips
There are times when the numbers won’t match but are likely to become closer over time. For example:
  • The first day for your CGM. With newly inserted CGMs, the differences between your meter value and the G6 reading may be greater. Generally, the match gets closer over the first 24 hours.
  • When glucose changes quickly. When your glucose is changing quickly, it’s more difficult to compare numbers. The meter and Dexcom G6 measure glucose from different sources (blood and interstitial fluid), and blood glucose changes faster than interstitial fluid glucose. The match gets closer when your glucose stabilizes.
  • When there’s pressure on the CGM. When something is pressing on your sensor it can affect your G6 readings. The match should get closer after the pressure is relieved.

To determine what to do, watch your G6 CGM readings over several hours. If the readings are always higher (or always lower) than your meter values and always outside the 20 rule, consider calibrating. Also consider calibrating if your Dexcom G6 and meter numbers don’t match and your expectations or symptoms fit the meter value, not the G6 reading.

Calibration best practices

Calibrating your Dexcom G6 with a fingerstick meter is never required. If the Dexcom app asks for calibration, ensure you  entered your CGM code from Step 2.

Calibration can help make the Dexcom G6 more accurate or less accurate compared to the lab result, and it should bring the G6 readings closer to the meter values.

When calibrating:
  • Enter the calibration within 5 minutes of taking a fingerstick.
  • Don’t calibrate during a Sensor Error.
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