Continuous Glucose Monitoring (CGM) systems track glucose levels throughout the day and night. These devices provide continuous readings. This technology helps users understand how various factors influence their glucose levels. CGM is designed to offer a more comprehensive picture compared to traditional single-point measurements.
System Components
A typical continuous glucose monitoring system includes three main physical components. A small, disposable sensor is inserted just beneath the skin. The sensor measures real-time glucose levels in the interstitial fluid, the fluid surrounding the body’s cells.
Attached to the sensor is a small, reusable device known as a transmitter. The transmitter’s role is to collect data from the sensor and wirelessly send it to another device. This data is then displayed on a compatible device, which can be a dedicated receiver or a smartphone or tablet application.
The Sensor’s Core Mechanism
A CGM sensor detects glucose in the interstitial fluid, not directly in the blood. This fluid, found in the spaces around cells, contains glucose that has diffused from the bloodstream. The sensor usually incorporates an enzymatic technology, often involving glucose oxidase, which is immobilized on the sensor’s tip.
When glucose molecules from the interstitial fluid encounter the enzyme on the sensor, a biochemical reaction occurs. Glucose oxidase reacts with glucose and available oxygen to produce gluconic acid and hydrogen peroxide. The amount of hydrogen peroxide generated is directly proportional to the concentration of glucose present.
Following the enzymatic reaction, the hydrogen peroxide is then detected through an electrochemical process. This detection generates a tiny electrical current. The strength of this electrical current is directly proportional to the amount of hydrogen peroxide, and thus to the glucose concentration in the interstitial fluid. This electrical signal is then converted into a digital format.
Signal Processing and Data Delivery
After the sensor generates an electrical signal, the transmitter, physically connected to the sensor, processes this raw electrical signal. This processing includes digitizing the signal and performing initial calculations before transmitting the data.
The processed data is then wirelessly sent from the transmitter to a display device. This wireless communication often occurs using technologies like Bluetooth Low Energy. The display device, whether a dedicated receiver or a smartphone application, receives this continuous stream of glucose readings.
The display device interprets the data and presents it to the user. This typically includes real-time glucose numbers, trend arrows indicating the direction and speed of glucose changes, and historical data graphs. While many systems are factory calibrated, some may allow or require user calibration with a traditional blood glucose meter to ensure accuracy.
Sensor Placement and Operational Aspects
Users typically apply the sensor to areas of the body with adequate subcutaneous fat, such as the back of the upper arm or the abdomen, using an applicator. After insertion, CGM sensors usually require a “warm-up” period before they begin providing glucose readings. This warm-up can range from 30 minutes to up to two hours, allowing the sensor to acclimatize to the body’s tissues and for the chemical reactions to stabilize.
The typical wear time for a CGM sensor varies by brand and model, commonly lasting between 7 and 14 days. Some newer sensors are designed for 10 or 15 days of continuous use, and certain implantable models can last for months or even a year.
It is important to note that CGM sensors measure glucose in the interstitial fluid, which has a slight physiological lag compared to blood glucose levels. This time lag, typically ranging from 5 to 25 minutes, occurs because glucose must first move from the bloodstream into the interstitial fluid. The lag can be more noticeable when glucose levels are changing rapidly, such as after a meal or during exercise.