A continuous glucose monitor (CGM) is a wearable medical device that tracks glucose concentrations throughout the day and night. Unlike traditional fingerstick devices that provide a single snapshot, the CGM offers a continuous stream of data, allowing users to visualize trends and patterns. This technology provides users and healthcare providers with real-time information on how diet, exercise, and medication affect glucose levels, supporting informed management decisions.
Understanding the Sensor’s Physical Structure and Location
The sensor is the part of the continuous glucose monitoring system that interacts with the body. It is a small, disposable unit attached to the skin by an adhesive patch. The sensor contains a fine, flexible filament, or electrode, inserted just beneath the skin’s surface using an applicator tool. This electrode resides in the interstitial fluid, the thin layer of fluid surrounding the body’s cells and tissues.
This placement is intentional because glucose diffuses from the bloodstream into the interstitial fluid, allowing the sensor to measure its concentration. The sensor’s housing also contains electronic components to process the initial signal, often including a silver/silver chloride (Ag/AgCl) counter electrode to complete the electrochemical circuit. Since the glucose concentration in the interstitial fluid naturally lags behind the blood concentration, the system must account for this time difference.
The Chemical Reaction: Converting Glucose to a Signal
The sensor’s function relies on a biochemical process involving a working electrode. This electrode is coated with an immobilized layer of the enzyme glucose oxidase (GOx), which acts as a catalyst. When glucose from the interstitial fluid reaches the sensor tip, it interacts with the GOx enzyme.
The enzyme catalyzes the oxidation of glucose, converting it into gluconolactone and simultaneously producing hydrogen peroxide (\(H_2O_2\)) as a measurable byproduct. The amount of hydrogen peroxide generated is directly proportional to the glucose concentration in the interstitial fluid. The sensor’s working electrode is designed to detect this byproduct.
The hydrogen peroxide is then electrochemically oxidized at the electrode surface, causing an electron transfer. This transfer generates a small, measurable electrical current. The magnitude of this electrical current is directly proportional to the initial glucose concentration, converting the chemical concentration into a quantifiable electrical signal for the glucose reading.
Transferring Data: From Sensor to Receiver
Once the electrical current is generated, it is relayed to the user via a small transmitter. This transmitter snaps onto or is integrated into the adhesive patch. Its role is to amplify the electrical signal and convert the analog current into a digital value.
The digital glucose reading is then transmitted wirelessly, often using Bluetooth Low Energy, to an external device. This receiving device can be a dedicated handheld receiver, a compatible smartphone application, or an integrated insulin pump. Transmission occurs automatically at regular intervals, often every one to five minutes, providing a stream of data.
Sophisticated algorithms within the system refine the raw data. These algorithms smooth the data points to filter out electronic noise and fluctuations that could cause erroneous readings. They also apply a calibration factor to translate the interstitial fluid measurement into a value that approximates the plasma glucose equivalent, accounting for the physiological time lag.
Factors Affecting Sensor Accuracy and Lifespan
CGM sensors are designed to function accurately for a limited duration, typically lasting between seven and 15 days. Performance is impacted by the initial warm-up period after insertion, which allows the sensor to stabilize and the glucose oxidase to become fully functional. This period can range from 30 minutes to two hours, during which the sensor does not provide reliable readings.
External pressures, such as lying directly on the sensor (a compression artifact), can temporarily restrict blood flow and alter the interstitial fluid, leading to falsely low readings. Accuracy can also be compromised by certain medications. For instance, high doses of acetaminophen can interfere with the electrochemical reaction, creating an artificially high current and an inaccurate glucose reading.
The physical integrity of the sensor affects its lifespan. Repeated micromovements or poor adhesion can cause inflammation at the insertion site, which affects the sensor’s ability to interact correctly with the interstitial fluid. To maintain accuracy, users must adhere to proper application techniques and avoid areas prone to excessive pressure or movement.