How accurate are CGMs?

We currently partner with Dexcom and use the Dexcom G6 CGM. Once calibrated (after the first 24-48 hours of use), we have found them to be highly accurate. From third-party studies, the Dexcom G6 CGM utilizes an automated sensor applicator that provides accurate glucose readings in adults, children, and adolescents throughout the 10-day sensor life.

The Dexcom G6 has a MARD (error rate) of 9% and the data follows the %20/20 Rule. For example, if your glucose meter reading is 100 mg/dL, your G6 reading is a close match if it’s between 80 and 120 mg/dL. Dexcom calls it the 20 Rule.​ As seen in that article, the Dexcom G6 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 Dexcom G6 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.