The accuracy of CGMs may vary under some conditions and for unclear reasons. Earlier CGM devices were less accurate than current devices. All commercial CGMs available today must be extensively tested for accuracy before being commercially available. In the U.S., all FDA-approved CGMs (as well as blood glucose monitors (BGMs)) must be within 15% of the actual laboratory blood glucose value at least 95% of the time. In addition, values must be at least 20% of the actual value at least 99% of the time. Accuracy studies of most CGM units typically have shown that glucose readings are within about 10% of the actual blood glucose value.
As previously indicated, fingerstick testing using a BGM or glucometer is considered more accurate for blood glucose levels than CGMs. This is the reason CGMs are calibrated and results verified using fingerstick readings.
Several factors and conditions can affect the accuracy of CGMs and may include one or more of the following:
- CGMs measure interstitial glucose: Physiological differences between the interstitial fluid and capillary blood may result in differences in glucose readings between the CGM and results from a fingerstick test using a blood glucose meter. Differences in glucose readings between interstitial fluid and capillary blood may be observed during times of rapid change in blood glucose, such as after eating, dosing insulin, or exercising.
- Interfering substances: Certain substances can interfere with the sensor and create measurement errors. Interfering substances may include acetaminophen, ascorbic acid, hydroxyurea, mannitol, and tetracycline.
- Compression: If an individual lies on top of their sensor overnight, the CGM may display a lower overnight reading.
- Other Factors: Extreme temperatures and very humid conditions can interfere with the CGM sensor filament. Sweating could possibly loosen the sensor, interfering with accuracy.
