Continuous Glucose Monitors: Definition and Brief History

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The page below is a sample from the LabCE course Continuous Glucose Monitors: Control and Monitoring of Diabetes Mellitus. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

Learn more about Continuous Glucose Monitors: Control and Monitoring of Diabetes Mellitus (online CE course)
Continuous Glucose Monitors: Definition and Brief History

A continuous glucose monitor (CGM) is a device that automatically estimates an individual’s blood glucose level throughout the day and night (continuous). The device determines the glucose level every few minutes and tracks and records the results over time. A CGM typically consists of three components: the sensor (a small wire inserted under the skin), a transmitter attached to the sensor, and a handheld receiver and/or smartphone that displays the glucose reading in real time. The minute disposable sensor is inserted under an individual’s skin and held with a patch. The sensor estimates the glucose level in the interstitial fluid between the cells. The disposable sensor is typically replaced at specific times.
Continuous glucose monitoring was introduced in the late 1990s and early 2000s as an alternative to traditional glucose self-monitoring methods requiring frequent finger punctures. The early CGM devices were rather large, cumbersome to wear, and challenging to set up and interpret. In addition, the first CGMs had limited clinical utility since they experienced significant drift in sensitivity. Moreover, the early devices could only record a patient’s glucose values over three days and required a sensor calibration with a finger-stick glucometer every 6 to 12 hours.
In the late 2000s, Dexcom introduced its CGM model, allowing patients to monitor their glucose levels continuously for seven days. In 2014, Abbott released the FreeStyle Libre CGM, which had several innovative improvements, including eliminating the need for initial and subsequent finger-stick calibrations. In 2016, Senseonics introduced the Eversense CGM, which increased the sensor's lifespan to 90 days. One year later, Senseonics launched the Eversense XL device, which increased the sensor lifespan to 180 days.
In 2018, the FDA approved the first CGM, the Eversense CGM system for use in people 18 years of age and older with diabetes. The system was the first approved CGM with a fully implantable glucose sensor and compatible with a mobile app for adults with diabetes, which can be worn for up to 90 days. Over the past two decades, CGMs have significantly improved and are now widely accepted for managing diabetes. Today, CGM systems are considerably smaller and are worn comfortably for extended periods. In addition, they offer seamless data streaming, improvements in real-time insights, more accuracy, less frequent calibration, and the ability to integrate with apps to provide valuable information for the person with diabetes.
The image on this page shows a CGM with a transmitter attached to the patient’s upper arm above the skin. The sensor part of the transmitter is inserted under the skin. A handheld device (cell phone) is a receiver that receives and displays the patient’s glucose reading.

Figure 1. Continuous glucose monitor