Reference Guide

CGM Glossary

A comprehensive reference of continuous glucose monitoring terminology. 102 terms covering sensor technology, accuracy metrics, AID systems, and diabetes management.

510(k) Clearance: FDA regulatory pathway for medical devices that are substantially equivalent to existing approved devices. Most CGM improvements use this pathway.

AID (Automated Insulin Delivery): Systems that automatically adjust insulin pump delivery based on CGM data. Also known as closed-loop or artificial pancreas systems. Examples include Tandem Control-IQ, Medtronic 780G, and Omnipod 5.
AGP (Ambulatory Glucose Profile): A standardized CGM report showing glucose patterns over time. Displays median glucose with percentile bands, helping identify trends and problem areas.
Alarm Fatigue: Desensitization to CGM alerts due to frequent alarms, leading users to ignore or disable warnings. Can be dangerous if genuine alerts are missed.
Acetaminophen Interference: Older CGM sensors gave falsely high readings with acetaminophen (Tylenol). Modern iCGM systems like G6/G7 and Libre 3 have resolved this interference.
Auto Mode: The automated mode in AID systems where the algorithm controls basal insulin delivery based on CGM readings. Users may need to enter Manual Mode for certain situations.
A1C (HbA1c): A blood test measuring average blood glucose over 2-3 months by detecting glycated hemoglobin. Target is typically below 7% for most adults with diabetes.

Biofouling: The accumulation of proteins, cells, and tissue around an implanted sensor that can reduce accuracy over time. A major challenge for long-term CGM sensors and implantables.
Beta Bionics iLet: A unique AID system that only requires the user's weight for setup, automatically adjusting all insulin dosing. FDA approved 2023.
Basal Rate: The continuous background insulin delivery by a pump, expressed in units per hour. AID systems automatically adjust basal based on CGM readings.
Bolus: A dose of insulin given to cover food intake or correct high blood glucose. In hybrid closed-loop systems, users still enter meal boluses manually.
Bluetooth Low Energy (BLE): The wireless protocol used by most CGMs to transmit data to receivers and phones. Enables low-power continuous communication.

CGM (Continuous Glucose Monitor): A device that continuously measures glucose levels in interstitial fluid through a small sensor inserted under the skin, providing real-time readings and trends throughout the day and night.
Closed-Loop System: An automated diabetes management system that combines CGM, insulin pump, and control algorithm to automatically adjust insulin delivery. Minimizes user intervention required.
Control Algorithm: The mathematical software that determines insulin dosing in AID systems. Common approaches include PID (Proportional-Integral-Derivative) and MPC (Model Predictive Control).
CV (Coefficient of Variation): A measure of glucose variability calculated as standard deviation divided by mean glucose. CV below 36% indicates stable glucose control.
CE Mark: European conformity certification indicating a medical device meets EU safety and performance requirements. Required for CGM sale in Europe.
Clarity: Dexcom's cloud-based software platform for viewing and analyzing CGM data. Generates AGP reports and provides trend analysis and pattern detection.
Compression Low: A falsely low CGM reading caused by lying on the sensor, compressing blood flow and glucose transport to the sensor area. Common during sleep.
Clarke Error Grid: A graphical analysis tool for assessing CGM accuracy by plotting CGM vs reference values and categorizing results by clinical significance.
Consensus Error Grid: Updated accuracy assessment tool developed by experts, replacing the Clarke Error Grid. Features five risk zones for evaluating CGM performance.

DIY Loop: Open-source automated insulin delivery systems created by the diabetes community, including Loop, OpenAPS, and AndroidAPS. Not FDA-approved but used by thousands worldwide.
Dexcom: A leading CGM manufacturer based in San Diego. Products include G6, G7, and the integrated Stelo system. Known for accuracy and AID integration.
Dexcom G7: Dexcom's current flagship CGM with 10-day sensor life, 30-minute warmup, all-in-one disposable design, and 8.2% MARD. Approved for ages 2+ and pregnancy.
Dexcom G6: Previous generation Dexcom CGM with 10-day sensor life, separate transmitter, and factory calibration. Still widely used and compatible with most AID systems.
Data Sharing: CGM features allowing real-time glucose data to be viewed by caregivers, family members, or healthcare providers remotely via apps like Dexcom Follow or LibreLinkUp.
Dual-Hormone AID: Advanced closed-loop systems that deliver both insulin and glucagon to prevent hypoglycemia more effectively. In clinical trials but not yet commercially available.

Electrochemical Sensing: The method used by most CGMs to measure glucose. An enzyme reaction with glucose produces electrons that create a measurable electrical current proportional to glucose concentration.
Eversense: Senseonics' implantable CGM system with a sensor inserted under the skin for up to 6 months. Uses fluorescence technology and requires on-body transmitter.
Early Termination: When a CGM sensor stops working before its intended wear period ends. May be caused by sensor failure, skin reaction, or physical damage.

Factory Calibration: A calibration method where CGM sensors are calibrated during manufacturing, eliminating the need for users to perform finger stick calibrations. Used in Dexcom G6/G7 and FreeStyle Libre systems.
Full Closed-Loop: An advanced AID system that automates both basal and meal insulin delivery without requiring meal announcements. Still in development and clinical trials for most use cases.
Fluorescence Sensing: An optical glucose sensing method used in Eversense implantable CGM. Uses fluorescent molecules that change light emission based on glucose concentration in interstitial fluid.
Foreign Body Response (FBR): The body's natural immune and wound healing response to an implanted sensor. Can create a fibrous capsule that reduces glucose diffusion to the sensor, affecting accuracy.
FreeStyle Libre: Abbott's flash glucose monitoring system that requires scanning to see readings. The Libre 3 provides real-time continuous monitoring. Known for 14-day wear and low cost.
FreeStyle Libre 3: Abbott's smallest CGM sensor with real-time glucose monitoring, optional alerts, and 14-day wear. Features 7.9% MARD and no scanning required.
Flash Glucose Monitoring: A type of CGM that stores continuous glucose data but requires users to scan the sensor with a reader or phone to see readings, rather than continuously transmitting.

GMI (Glucose Management Indicator): An estimate of A1C derived from CGM data, based on the relationship between average glucose and A1C. Formerly called eA1C (estimated A1C).
Glycemic Variability: The degree of glucose fluctuation over time, measured by metrics like CV (coefficient of variation) and standard deviation. CGM reveals variability that finger sticks cannot capture.
Glucose Oxidase (GOx): The enzyme most commonly used in CGM sensors to catalyze the glucose reaction that produces a measurable electrical signal. Highly specific for glucose but sensitive to oxygen levels.
Glucose Dehydrogenase (GDH): An alternative enzyme used in some glucose sensors. Not affected by oxygen levels but may react with other sugars like maltose depending on the cofactor used.
Guardian 4: Medtronic's latest CGM sensor designed for use with the 780G insulin pump system. Features 7-day wear and integration with Medtronic's SmartGuard technology.
Glucose Target: The blood glucose level that AID systems aim to achieve, typically 100-120 mg/dL. Some systems allow user-adjustable targets for activities or sleep.
Gestational Diabetes: Diabetes diagnosed during pregnancy. CGM provides detailed glucose data to help maintain tight control for maternal and fetal health.

Hybrid Closed-Loop: A type of AID system that automates basal insulin delivery but still requires user input for meal boluses. Current commercial systems like Control-IQ, 780G, and Omnipod 5 are hybrid closed-loop.
Hydroxyurea Interference: A medication used in sickle cell disease that can cause falsely elevated CGM readings. One of few remaining interferences in modern CGM systems.
Hypoglycemia: Low blood glucose, typically defined as below 70 mg/dL, or severe below 54 mg/dL. CGM predictive alerts help prevent hypoglycemic episodes.
Hyperglycemia: High blood glucose, typically defined as above 180 mg/dL. CGM helps identify hyperglycemia patterns and triggers for treatment adjustments.

Interstitial Fluid (ISF): The fluid between cells in body tissue where CGM sensors measure glucose. ISF glucose lags behind blood glucose by approximately 5-15 minutes due to the time required for glucose to diffuse from capillaries.
iCGM: Integrated Continuous Glucose Monitor - an FDA classification for CGM systems that meet specific accuracy and reliability standards allowing integration with other devices like insulin pumps.
Interoperability: The ability of CGM systems to communicate with and be used by other medical devices, particularly insulin pumps and AID systems from different manufacturers.
Implantable CGM: CGM sensors that are surgically placed under the skin for extended wear periods (3-6 months). Senseonics Eversense is currently the only available implantable CGM.
IBOA (Isobornyl Acrylate): A chemical component in some CGM adhesives that can cause allergic contact dermatitis. Abbott and Dexcom have modified formulations to address this issue.
Insulin Pump: A small device that delivers rapid-acting insulin continuously through tubing or a patch, controlled by CGM data in AID systems.
In Silico: Computer simulation studies used to test CGM algorithms and AID systems before human trials. Regulatory agencies accept in silico data for some submissions.

Lag Time: The delay between blood glucose changes and corresponding CGM readings, typically 5-15 minutes. Caused by glucose diffusion from blood to interstitial fluid and sensor processing time.
Lingo: Abbott's over-the-counter biosensor system for metabolic health insights, based on Libre technology. Marketed for weight management and energy optimization.
LibreView: Abbott's cloud-based platform for viewing and sharing FreeStyle Libre CGM data with healthcare providers. Generates standardized reports.

MARD (Mean Absolute Relative Difference): The standard metric for evaluating CGM accuracy, calculated as the average percentage difference between CGM readings and reference blood glucose values. Lower MARD indicates higher accuracy. Current CGMs achieve MARD of 8-12%.
MPC (Model Predictive Control): An advanced control algorithm that predicts future glucose levels and optimizes insulin delivery accordingly. Used in systems like Control-IQ and 780G.
Membrane: The outer coating of a CGM sensor that controls glucose flux and protects the sensor chemistry. Also serves to improve biocompatibility and reduce interference.
MRI Safety: CGM sensors must be removed before MRI scans due to metal components and potential for burns or sensor damage. Some newer sensors have improved MRI labeling.
Microneedle CGM: Emerging CGM technology using tiny needles that penetrate only the outermost skin layer, potentially offering minimally-invasive monitoring with less foreign body response.
Multi-Analyte Sensing: Future CGM technology that measures multiple biomarkers (lactate, ketones, alcohol) in addition to glucose, providing more comprehensive metabolic insights.
Medtronic 780G: Medtronic's advanced hybrid closed-loop system with 5-minute auto-corrections and SmartGuard technology. Uses Guardian 4 sensors.

Non-Adjunctive: FDA designation allowing CGM readings to be used for diabetes treatment decisions without confirmatory finger stick tests. Indicates high accuracy and reliability.
Non-Invasive Glucose Monitoring: Experimental technologies attempting to measure glucose without breaking the skin. Approaches include optical, RF, and bioimpedance methods. None are FDA-cleared for medical use.
Near-Infrared Spectroscopy (NIR): An optical technique that shines near-infrared light through tissue to measure glucose based on light absorption patterns. Challenged by water and tissue interference.
NFC (Near Field Communication): Wireless technology used for CGM scanning and activation. FreeStyle Libre uses NFC for scan-to-read and sensor activation.

Optical Glucose Sensing: Non-invasive glucose measurement using light, including near-infrared spectroscopy, Raman spectroscopy, and photoacoustic methods. Major technical challenges remain unsolved.
Omnipod 5: Insulet's tubeless, waterproof patch pump with integrated Dexcom G6 AID capability. The first tubeless AID system approved in the US.

PID Control: Proportional-Integral-Derivative control algorithm used in some AID systems. Reacts to current glucose levels, recent history, and rate of change to adjust insulin delivery.
Physiologic Lag: The inherent delay between blood glucose and interstitial fluid glucose, typically 5-10 minutes. A fundamental limitation of CGM that cannot be eliminated.
Predictive Alerts: CGM alarms that warn users before glucose reaches high or low thresholds by predicting future glucose based on current trends. Helps prevent hypo/hyperglycemia.
PMA (Premarket Approval): FDA's most stringent regulatory pathway for high-risk medical devices. AID systems typically require PMA approval.
PISA (Pressure-Induced Sensor Attenuation): False low glucose readings caused by pressure on the CGM sensor, often during sleep. Results in inaccurate low alarms and can disrupt sleep quality.

Receiver: A dedicated device that displays CGM glucose readings and trends. Many users now use smartphone apps instead of dedicated receivers.
Rate of Change: How quickly glucose is rising or falling, typically measured in mg/dL per minute. Rates above 2 mg/dL/min are considered rapid changes requiring attention.
Real-time CGM (rtCGM): CGM systems that automatically transmit glucose readings to a display device at regular intervals (typically every 1-5 minutes) without requiring user scanning.
Remote Monitoring: The ability for caregivers or healthcare providers to view CGM data in real-time from a distance. Particularly valuable for parents of children with diabetes.
Reference Measurement: Laboratory-quality blood glucose measurement used to assess CGM accuracy. Yellow Springs Instrument (YSI) analyzers are the gold standard.

Sensor: The component of a CGM system that is inserted under the skin to measure glucose levels. Contains electrochemical or optical elements that detect glucose in interstitial fluid.
Sensor Life: The approved duration a CGM sensor can be worn before replacement. Varies from 7 days (Guardian) to 14-15 days (FreeStyle Libre/Dexcom G7) to 6 months (Eversense implantable).
Sensor Drift: Gradual change in CGM accuracy over the sensor's wear period, typically showing less accurate readings as the sensor ages due to biofouling and enzyme degradation.
Stelo: Dexcom's over-the-counter CGM designed for people without diabetes. Provides glucose insights without prescription, targeting the wellness market.
Signal Loss: Temporary interruption of CGM data transmission, often due to sensor-transmitter-receiver distance, Bluetooth interference, or sensor issues.
Sensor Error: A CGM message indicating the sensor cannot provide accurate readings, often requiring waiting or replacement. May be caused by biofouling, bleeding, or sensor malfunction.
Skin Reaction: Allergic or irritant response to CGM sensor adhesive or components. Symptoms include redness, itching, rash, or blistering. May require barrier products or sensor changes.
Smart Contact Lens: Experimental technology to measure glucose in tear fluid using a sensor embedded in a contact lens. Google/Novartis project was discontinued; research continues elsewhere.
Smartwatch CGM: Proposed technology to measure glucose from a watch-like device without any implanted sensor. Multiple companies claim development but none have achieved clinical accuracy.
Suspend Before Low: An AID feature that automatically stops insulin delivery when the CGM predicts glucose will drop below a set threshold, helping to prevent hypoglycemia.

Transmitter: The electronic component that wirelessly sends glucose data from the sensor to a receiver, smartphone, or insulin pump. May be reusable or integrated into disposable sensors.
Time in Range (TIR): The percentage of time glucose levels stay within a target range, typically 70-180 mg/dL. A TIR of 70% or higher is a common goal. CGM enables accurate TIR measurement.
Time Below Range (TBR): The percentage of time glucose is below the target range (typically below 70 mg/dL). Goal is less than 4% TBR, with less than 1% below 54 mg/dL.
Time Above Range (TAR): The percentage of time glucose is above the target range (typically above 180 mg/dL). Goal is less than 25% TAR, with less than 5% above 250 mg/dL.
Trend Arrow: Visual indicator showing the direction and rate of glucose change. Arrows indicate if glucose is stable, slowly rising/falling, or rapidly changing. Critical for dosing decisions.
Tandem Control-IQ: Tandem's hybrid closed-loop system using Dexcom G6/G7 CGM. Automatically adjusts basal and can deliver automatic correction boluses. FDA approved 2020.
Type 1 Diabetes: An autoimmune condition where the pancreas produces little or no insulin. People with Type 1 require insulin therapy and benefit significantly from CGM and AID.
Type 2 Diabetes: A metabolic condition characterized by insulin resistance. CGM is increasingly used in Type 2 for medication optimization and behavior change.

User Calibration: Traditional calibration method requiring users to enter finger stick blood glucose readings into the CGM system to maintain accuracy. Typically required 2-4 times per day with older systems.
Urgent Low Soon Alert: A predictive alarm that warns when glucose is predicted to drop below 55 mg/dL within 20 minutes, allowing time to treat and prevent severe hypoglycemia.

Warm-up Period: The time after sensor insertion before the CGM provides readings, allowing the sensor to stabilize. Ranges from 30 minutes (Dexcom G7) to 2 hours (Guardian Connect).

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