Accuracy and How to Manually Calibrate your Dexcom G7

Overview: Here’s how to test the accuracy of your Dexcom G7 CGM with a fingerstick meter reading and manually calibrate the CGM if desired. 

⚠️ Please note: The information listed here is applicable to the Dexcom G7 CGM users within the US only.

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 G7 CGM; a 20% difference can be expected between CGMs and fingerstick tests. You can compare readings from a fingerstick meter and your Dexcom G7 to see how closely the numbers match. If your G7 reading and meter value is within “The 20 Rule” (also known as the %20/20 rule), they match closely. Be sure you are comparing numbers when glucose is in a stable state, It's expected that glucose levels will not match fingerstick readings after a meal or intense exercise as glucose is fluctuating rapidly. 

📺 Click this link,  Dexcom G7 Accuracy Video, to understand CGM accuracy.

To Manually Calibrate the Dexcom G7 sensor: 

  1. Open the Dexcom G7 app
  2. Tap + on the top right corner 
  3. Under Add Event, Select Blood Glucose
  4. Under Entry Type select, Use as Calibration 
  5. Enter the Blood glucose meter value (be sure to enter this value within 5 minutes of sticking your finger).

Calibration best practices

⚠️ Calibrating your Dexcom G7 with a fingerstick meter is never required. Calibration can help make the Dexcom G7 more accurate or less accurate compared to the lab result, and it should bring the G7 readings closer to the meter values.

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

FAQ: Is it normal for my CGM data and fingerstick data to be different? 

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 G7 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 G7 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 

    G7readings. The match should get closer after the pressure is relieved.

To determine what to do, watch your G7 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 G7 and meter numbers don’t match and your expectations or symptoms fit the meter value, not the G7 reading. 

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