Continuous glucose monitor


A continuous glucose monitor is a device for monitoring blood glucose continuously instead of monitoring periodically by drawing a drop of blood from a finger. This is known as continuous glucose monitoring. CGMs are used by people who treat their diabetes with insulin, for example people with type 1 diabetes, type 2 diabetes, or other types of diabetes, such as gestational diabetes.
A continuous glucose monitor has three parts:
  • a small electrode that is placed under the skin
  • a transmitter that sends readings from the electrode to a receiver at regular intervals
  • a separate receiver that shows the glucose level on a display.
Approved CGMs use an enzymatic technology which reacts with glucose molecules in the body's interstitial fluid to generate an electric current that is proportional to glucose concentration. Data about glucose concentration is then relayed from a transmitter attached to the sensor, to a receiver that displays the data to the user.
Some CGM devices must be calibrated periodically with traditional blood glucose measurements, but others do not require calibration by the user.

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.
  • CGM is more convenient and less painful than traditional fingerstick testing.
  • Some studies have demonstrated that CGM users spend less time in hypoglycemia or with lower glycated hemoglobin, both of which are favorable outcomes.
  • Continuous glucose monitors appear to lower hemoglobin A1c levels more than just monitoring with capillary blood testing, particularly when CGM is used by people with poorly controlled diabetes and combined with the use of an integrated insulin pump. However, a Cochrane systematic review found limited and conflicting evidence of the effectiveness of continuous glucose monitoring systems in children, adults, and users with poorly controlled diabetes.
  • A meta-analysis of 25 randomised controlled trials found that CGM-based feedback to support behavior change modestly reduces glycated hemoglobin levels and increases time-in-range in individuals with and without diabetes.

    Limitations

Continuous glucose monitoring has some important limitations:
  • CGM systems are not sufficiently accurate for detecting hypoglycemia, a common side-effect of diabetes treatment. This is especially problematic because some devices have alarm functions to warn users about a hypoglycemic condition, and people might rely on those alarms. However, the Cochrane systematic review showed that the use of continuous glucose monitors did not increase the risk of hypoglycemia or ketoacidosis. Some manufacturers warn users about relying solely on CGM measurements. The National Institute for Health and Care Excellence recommends validating hypoglycaemic values with fingerprick testing.
  • Compression lows or pressure-induced sensitivity attenuations 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 incorrect treatment.
  • Another limitation of CGM is that glucose levels are taken from the body's interstitial fluid rather than from the blood. Because it takes time for glucose to travel from the bloodstream into the interstitial fluid, there is an inherent lag between actual blood glucose level and the level measured by the CGM. This lag time varies by both user and device, but it is usually 5 to 20 minutes.

    Flash glucose monitoring

The original Freestyle Libre monitor introduced by Abbott Diabetes Care in 2015 was described as doing "flash glucose monitoring," with a disposable 14-day sensor probe under the skin, but factory-calibrated without requiring calibration against a fingerstick glucose test. The sensor measures the glucose level of interstitial fluids continuously; up to eight hours of these readings, averaged over each 15-minute period, are stored in the sensor unit, unlike most other CGM systems, which use a wireless link to an external device for each reading. Data stored in the sensor are transmitted on demand to a "reader" held within a centimeter or two of the sensor unit, employing near-field communication technology. As only eight hours worth of data can be stored, downloads must not be spaced more than eight hours apart.
Differences in US insurance coverage favoring "flash glucose monitoring" over "continuous glucose monitoring" were an advantage to early adoption of Abbott's less expensive system. In the UK, flash glucose monitors and sensors are available to many patients without charge on the National Health Service.
The later Freestyle Libre 2 version of Abbott's device uses different, incompatible, sensors. It can be programmed to transmit a low blood sugar or high sugar warning via Bluetooth to a nearby device and, as of 2023, transmits glucose readings via Bluetooth on a 60-second basis effectively making a CGM and not a flash glucose monitor. The following Freestyle Libre 3 is smaller, and transmits its readings via Bluetooth, as other meters do; it is not described as flash 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, and thus not a true CGM system. This device could be worn for up to ten days, but required 12 hours to start readings. and was followed by an updated device that could be worn for up to 14 days, and needed only one hour to start a new sensor. 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 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 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 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

  • Continuous versus flash monitoring: Dexcom, Eversense, and Libre 2 and 3 use continuous monitoring where information on the glucose levels are continuously updated. Continuous monitoring allows to set automatic alarms that are triggered when the glucose level goes out of pre-configured thresholds. In contrast, with flash monitoring such as the Freestyle Libre1, the glucose level is read automatically by the sensor; however, data is only transmitted to the user on user request. The glucose information stored on the sensor contains all the data since the previous read. FreeStyle Libre 2 allows configuration of alarms when glucose reaches a pre-determined level.
  • Implantable sensors: Since the electronics and battery require a relatively large package, most CGM sensors are worn over the skin with the actual sensing probe penetrating the skin. However the Eversense sensor is an actual implant, and receives its power wirelessly from a so-called transmitter worn above the skin. The "transmitter" receives data from the sensor every 5 minutes and forwards that data to a nearby device wirelessly. However unlike the Freestyle Libre, the implanted device is too small to have its own battery and memory, so that no glucose readings are generated during periods in which the transmitter is not being worn. The transmitter must be removed at least once a day for recharging and replacement of the adhesive.