We currently partner with Dexcom for CGM devices. Once calibrated, we have found them to be highly accurate. From third-party studies, the Dexcom CGMs utilize an automated sensor applicator that provides accurate glucose readings in adults, children, and adolescents throughout the 10-day sensor life.
The Dexcom G6 CGM has a MARD (error rate) of 9% and the Dexcom G7 CGM has a MARD rate of 8.1%. The data follows the 20/20 Rule. For example, if your glucose meter reading is 100 mg/dL, your reading is a close match if it’s between 80 and 120 mg/dL.
The CGM reading must be within: 20% of the meter value when the meter value is 80 mg/dL or higher 20 mg/dL of the meter value when the meter value is under 80 mg/dL As a reminder, the CGM is measuring interstitial fluid, which is the fluid that surrounds the cells of your tissue under your skin, while the glucometer is measuring blood glucose.
It is important to know that the readings can lag behind blood glucose by up to 15 minutes. This is because glucose moves from your blood vessels and capillaries first and then into your interstitial fluid. You can read more about comparing CGM readings with finger sticks here. That being said, if you are comparing the readings between a glucometer and a CGM, please only do so when your glucose levels are stable. This would be either first thing in the morning or 2-3 hours after a meal or exercise.
Other reasons there could be a difference between your BG meter and your CGM are:
- Hand cleanliness: Wash your hands with soap and water (not hand sanitizer) and dry them. Then test. Many inaccurate BG meter values are from hands not being washed thoroughly before testing.
- Sensor’s first day: With newly inserted sensors, the differences between your BG meter and the CGM reading may be greater. Generally, the numbers get closer over the first 24 hours.
- Pressure on the CGM: Sometimes when something is pressing on your sensor, for example, if you’re lying on it, it can affect your CGM readings. Relieve the pressure and the numbers should get closer.
- Glucose changing quickly: When your glucose is rapidly changing, it can be more difficult to compare your meter and CGM because blood glucose changes a little before interstitial fluid glucose. The match should get closer when your glucose stabilizes.
- Test strips: Make sure your test strips are stored as directed and not expired. Also, make sure to use enough blood on the test strip.
For more information about the accuracy of CGMs, check out this video: