A continuous glucose monitoring (CGM) system is a medical or wellness device used to continuously track interstitial glucose levels in real time, replacing the need for periodic finger-stick blood draws. Tracking interstitial glucose is a valid way to monitor blood glucose due to their high correlation, despite a lag time in glucose levels between the interstitial space and the bloodstream.
While historically reserved for clinical diabetes management, such as tracking by individuals with type 1, type 2 or gestational diabetes, CGM systems are widely utilized by non-diabetic populations. These include individuals with metabolic conditions like prediabetes or polycystic ovary syndrome (PCOS), elite athletes optimizing endurance fueling, and health-conscious consumers tracking personalized dietary responses, weight management, and everyday metabolic fitness.
The early generations of these systems typically consisted of a sensor patch (worn on the back of the upper arm or the side of the torso) paired with a separate, portable handheld reader to display results. Modern sensors use Bluetooth to transmit data directly to a smartphone app, allowing users to view trends, receive high or low alerts, and share their metabolic data with healthcare providers or coaches in real time.
Most CGMs are electrochemical systems that utilize an enzymatic reaction with glucose molecules in the body's interstitial fluid. This reaction generates electrical current on the nanoampere (nA) scale that is directly proportional to the localized glucose concentration. The resulting data is then wirelessly transmitted from the patch to a paired device, where the real-time readings are displayed to the user. The FreeStyle Libre 3 by Abbott, AccuCheck Smart Guide by Roche, Dexcom G7 by Dexcom, Guardian 4 by Medtronic, SIBIO by Sibionics are examples of electrochemical CGM systems. These systems are known as minimally invasive, since sensor application requires a physical insertion of the sensing filament in the subcutaneous skin tissue. Operational time of electrochemical CGMs spans 14-16 days.
Some CGMs utilize an optical detection (fluorescence) of glucose to provide a real time reading, e.g., the Eversense 365 by Senseonics. However, the installation of such systems requires a minor surgery, so the device is installed by professionals in the clinic and not by the user. Operational time of optical CGM systems is much higher and can last up to one year.
Some CGM systems must be calibrated once at the activation, or calibrated periodically with traditional blood glucose measurements to establish the baseline for the measurement and compensate for the drift, whereas others do not require calibration by the user (factory calibration).
Benefits Continuous glucose monitoring is gaining in popularity for a variety of reasons. Traditional fingerstick testing measures blood glucose levels at only a single point in time. CGM enables users to see blood glucose levels continually, as well as trends in blood glucose levels over time. This results in a higher patient's lifespan by reducing devastating chronic complications and timely treatment of hyper or hypoglycemia events. CGM is more convenient and less painful than traditional fingerstick testing: a single insertion every two weeks versus multiple finger pricks a day. Possibility of insulin pump integration for automated diabetes treatment (not in all CGM models). Advanced CGMs are able to communicate with insulin pumps in real time for precise and automated diabetes management. Integration of predictive algorithms and immediate safety alarms for better glucose control.
Data analysis A Nature paper by Lutsker and colleagues showed that the patterns measured by CGM can be used to predict health risks, including diabetes and heart conditions, years in advance . This was achieved by training a Transformer architecture on 10M CGM samples.
Limitations Physiological Fluid Lag: Because CGMs sample interstitial fluid (the fluid between fat cells) rather than direct blood, there is a minor 5-to-10-minute physiological delay before changes in blood sugar reflect on the screen. This lag is generally clinically significant during rapid spikes or crashes (e.g., during intense exercise or immediately after a fast-acting carbohydrate meal).
Compression lows or pressure-induced sensitivity attenuations (PISA) are false hypoglycemic readings resulting from pressure applied at the CGM site. These can be caused by the user sleeping or sitting on the sensor, and may result in false hypoglycemia alarm.
Chemical Interferences: Certain substances, such as ascorbic acid, gentisic acid, acetaminophen, hydroxyurea, methyldopa can cross the protective sensor membrane and cause electrochemical interference. Electrochemical oxidation of these compounds adds a false-positive readings to the real glucose levels, and the contribution of false signal is highly depends on the real glucose levels - at low glucose the contribution of interferences is much higher, compared to high glucose. In optical CGMs interferences are different - structurally similar to glucose molecule (e.g., mannitol or sorbitol) Initial "Warm-Up": Upon inserting a new patch, the body's natural localized immune response requires a brief stabilization window. While early models required hours of blindness, modern systems still require a 30-to-60-minute automated warm-up period where users must temporarily rely on traditional finger pricks if a reading is needed. During this period, the system is reaching the steady state. Atypical Baseline Mismatch (Sensor Drift): Glucose readings are highly dependent on the immune body response, which may cause a signal drift over time, e.g., due to the encapsulation of sensor filament by body proteins, which leads to the baseline drift and false low readings. Such events can be detected by the sensor itself, and ask for an additional calibration from the user, using a blood glucose test strips.
Flash glucose monitoring Flash glucose monitoring is a historical subset of continuous glucose monitoring technology where data is transferred on-demand via Near-Field Communication (NFC) rather than continuously streamed wirelessly.
Mechanism: First introduced commercially in 2015, early FGM systems utilized a factory-calibrated subcutaneous sensor that recorded glucose levels in the interstitial fluid continuously. However, rather than utilizing a continuous Bluetooth link, the sensor stored up to eight hours of data internally. To view readings and log data, users had to physically scan or "flash" an external reader or smartphone within a few centimeters of the patch. Historical Impact: The introduction of FGM was a significant milestone in diabetes care because it pioneered mass-market factory calibration, completely eliminating the traditional requirement for daily finger-stick blood tests. Regulatory and Insurance Distinctions: In the late 2010s, the operational differences between FGM and traditional CGM created distinct insurance and reimbursement pathways. Because FGM systems lacked continuous, unprompted alarms and were less expensive to manufacture, healthcare networks like the UK's National Health Service (NHS) and US insurance providers initially granted wider coverage to FGM devices over standard CGMs. Convergence with CGM: By the early 2020s, the technological distinction between flash monitoring and continuous monitoring largely dissolved. Subsequent generations of these devices integrated automated Bluetooth transmitters to stream real-time data and predictive alarms every minute, effectively transitioning the product category entirely into mainstream continuous glucose monitoring.
History
United States The first CGM system was approved by the FDA in 1999. Continued development has extended the length of time sensors can be worn, options for receiving and reading data, and settings for alerting users to high and low glucose levels.
The first iteration of the Medtronic MiniMed took glucose readings every ten seconds with average readings reported every five minutes. Sensors could be worn for up to 72 hours.
A second system, developed by Dexcom, was approved in 2006. The sensor was approved for use for up to 72 hours, and the receiver needed to be within five feet for transmission of data.
In 2008, the third model was approved, Abbott Laboratories' Freestyle Navigator. Sensors could be worn for up to five days.
In 2012, Dexcom released a new device that allowed for the sensor to be worn for seven days and had a transmission distance of 20 feet. Dexcom later introduced an app allowing data from the sensor to be transmitted to an iPhone. This system was approved for pediatric use in 2015.
In September 2017, the FDA approved the first CGM that does not require calibration with fingerstick measurement, the FreeStyle Libre. The Libre is considered a "flash monitoring" system (FGM), and thus not a true ("real-time") CGM system. This device could be worn for up to ten days, but required 12 hours to start readings. It was followed by an updated device, the FreeStyle Libre 2, that could be worn for up to 14 days, and needed only one hour to start a new sensor.FreeStyle Libre 14-day Flash Glucose Monitoring system The FreeStyle Libre 2 was approved in Europe in October 2018, and enabled configuration of alerts when glucose is out of range.
In June 2018, the FDA approved the Eversense CGM system (manufactured by Senseonics Inc) for use in people 18 years of age and older with diabetes. This is the first FDA-approved CGM to include a fully implantable sensor to detect glucose, which can be worn for up to 90 days. The Eversense XL, a 180-day version of the system, was approved in Europe in October 2017.
In February 2025, the FDA alerted patients of a safety concern continuous glucose monitors (CGMs) that rely on a smartphone to deliver critical safety alerts, because reports showed that people were missing alerts due to technology configuration issues.
China China develops and produces CGM systems. The first CGM system to be approved for the European Union is manufactured by Medtrum Technologies. The sensor's intended use is up to 14 days and measures glucose levels every 2 minutes via a smartphone application. Medtrum was founded in 2008 and is based in Shanghai, China.
At the end of 2017, Medtrum introduced the TouchCare A6 CGM (later A7 or Slim in some countries) which measures glucose levels in the interstitial fluid up to 14 days. The TouchCare system comes with mobile applications, including a remote view application. The TouchCare system has glucose alerts and requires calibration every 24 hours.
At the end of 2021 the Medtrum Nano was announced, a very slim device not requiring calibration, approved for up to 14 days use, with customizable glucose alerts.
Medtrum makes both CGM and insulin pumps, both controlled by a single smartphone application which enables the user to monitor glucose levels and trigger insulin delivery in a closed-loop system.
United Kingdom
UK NICE guidelines introduced for the NHS in March 2022 in England and Wales advise that all Type 1 diabetic patients should be offered either flash glucose monitoring or CGM. People with Type 2 diabetes should be offered flash glucose monitoring or CGM if they use insulin twice daily or more, are otherwise advised to finger-prick eight times a day, have recurrent or severe hypoglycemia, have impaired hypoglycemia awareness, or cannot monitor their own blood sugar levels but they or a caretaker could use a scanning device. Details differ in Scotland and Northern Ireland.
Device characteristics