Accuracy Metrics (MARD)
Mean Absolute Relative Difference (MARD) is the standard metric for CGM accuracy. A lower number indicates the sensor reading is closer to a simultaneous lab blood draw (YSI).
| Sensor | Adult MARD | Pediatric MARD | Warmup Time |
|---|---|---|---|
| Dexcom G7 | 8.2% | 8.1% | 30 minutes |
| FreeStyle Libre 3 | 7.9% | — | 60 minutes |
| Medtronic Guardian 4 | 8.7% | — | 120 minutes |
Sensor Lag and Interstitial Fluid
CGMs do not measure blood glucose; they measure interstitial fluid glucose. There is a physiological lag of 5-15 minutes between blood and interstitial fluid. Algorithms in the closed-loop systems attempt to predict and offset this lag, but during rapid changes (like during exercise or fast-acting carb absorption), the sensor will always trail a fingerstick.
Common Interfering Substances
- Dexcom G6/G7: Hydroxyurea (cancer medication) causes significant false highs. Acetaminophen (Paracetamol) interference is blocked up to 1000mg every 6 hours.
- FreeStyle Libre 3: High doses of Vitamin C (>500mg/day) can falsely elevate readings.
- Medtronic: Acetaminophen causes false highs.
Integration and Connectivity
The utility of a CGM is bound by its connectivity. Dexcom uses a standard Bluetooth Low Energy (BLE) protocol that allows concurrent connections to a smartphone and an insulin pump. The Libre 3 streams real-time data but has historically restricted third-party integrations, pushing users toward DIY workaround systems for data freedom, though official integrations are expanding.
FAQ
- Why does my sensor fail on day 9?
- The enzyme coating on the sensor wire degrades over time in the subcutaneous tissue. Immune response (biofouling) also encapsulates the wire. See troubleshooting for mitigation strategies.
- Can I calibrate a factory-calibrated sensor?
- Dexcom G7 allows optional calibration. Libre 3 does not accept manual calibrations. Only calibrate when the arrow is flat, fasting, and the difference is >20%.