How CGM technology works
Many CGMs use electrochemical sensing of interstitial glucose. Eversense uses optical fluorescence with an external transmitter. The sensing mechanism alone does not establish clinical accuracy.
Reading, transmission and display intervals differ: original Libre 3 reports every minute; G7 every five minutes; Stelo transmits every five minutes while its app display updates every fifteen minutes.
Layer 1 — Biochemistry
Electrochemical CGMs detect a glucose-related signal in interstitial fluid. Exact sensing chemistry and labeling differ by model; Eversense instead uses optical fluorescence.
Layer 2 — Electrochemistry
The sensor signal is processed to estimate interstitial glucose. An explanation of a generic electrode reaction is not a specification for every CGM.
Layer 3 — Signal processing
Processing and calibration requirements differ by model. Use the exact device’s instructions for calibration and confirmatory blood glucose testing.
Layer 4 — Wireless transmission
Connectivity differs by sensor, transmitter, app and display. Use current model-specific compatibility and connection instructions rather than a universal Bluetooth distance or reporting interval.
The electrochemistry of glucose sensing
Most current CGM sensors use an amperometric enzyme electrode — an electrochemical technology that has been refined over decades from laboratory instruments into something small enough for model-specific wear under the skin. Not every system works this way: the implantable Eversense sensor is fluorescence-based, with an external transmitter that calculates the glucose value (FDA K241335).
The three components of a CGM system
Compare models rather than assumed generations
CGM designs differ in sensing methods, wear time and connectivity. Compare the specifications and intended use of the exact model; design changes alone do not establish improved clinical accuracy.
Factory calibration removes routine calibration for some models, but does not eliminate all need for blood glucose confirmation. Eversense models retain specific calibration schedules.
Understanding CGM accuracy and MARD
MARD is the mean absolute relative difference between sensor readings and reference measurements in a particular study. It is an average across paired readings, not an error bound for each reading or a direct clinical ranking across separate studies.
MARD is an average of absolute relative differences across many paired readings in a validation study — it is not a confidence interval or a guaranteed error margin for any single reading. An 8% MARD means that, on average across the study, CGM readings differed from reference blood glucose by about 8%; any individual reading could be closer to or further from the true value than that average. This is why finger-stick verification remains important for critical treatment decisions, especially when symptoms don't match the CGM reading.
Factors that improve accuracy
Use the approved insertion site and follow the exact device instructions. Performance varies by device, glucose range and conditions; no universal list guarantees accuracy.
Factors that reduce accuracy
Check model-specific interference warnings and instructions for symptoms or readings that do not agree. A low may be real or an artifact, so do not assume compression.
The interstitial lag — why CGM lags blood glucose
CGM sensors measure glucose in interstitial fluid — the fluid that surrounds cells in subcutaneous tissue — not glucose in blood directly. Glucose moves from blood capillaries into interstitial fluid by diffusion, a process that takes time.
This physiological delay, together with the sensor's and algorithm's own processing, means that during periods of rapidly changing blood glucose a CGM reading can lag behind blood glucose. The size of the lag depends on the device, its algorithm and the situation. In direct microdialysis studies under overnight fasting conditions, the blood-to-tissue delay was about 5.3–6.2 minutes in eight healthy people (Basu et al., 2013) and a median of 6.8 minutes (range 4.8–9.8) in six people with type 1 diabetes (Basu et al., 2015); these conditions are not the same as rapid changes after a meal. During fast changes a reading may therefore differ from a blood glucose value; follow your device's instructions if a reading does not match how you feel.
Trend arrows do not remove lag
Trend arrows summarize direction and rate using device-specific definitions. They do not correct the displayed glucose to a simultaneous blood value. Follow your individualized care plan and device instructions rather than a generic arrow-based treatment rule.
How to respond to trend arrows — particularly for insulin dosing decisions — should be determined in consultation with your healthcare provider. Many providers use published trend arrow adjustment guidelines, but these must be individualized. Never adjust insulin doses based on CGM trend arrows without guidance from your diabetes care team.
The future of CGM technology
CGM development includes changes in wear time, sensing methods and connectivity. New features do not by themselves establish greater clinical accuracy.
Frequently asked questions
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