Continuous Glucose Monitors (CGMs) are wearable devices that provide automated, real-time tracking of glucose levels throughout the day and night. These systems offer a dynamic view of how diet, activity, and medication affect the body’s glucose response, unlike the single-moment snapshot provided by traditional fingerstick meters. A CGM system consists of a tiny sensor inserted under the skin, a transmitter, and a receiver (often a smartphone application or a dedicated handheld device). The device functions as a biosensor, continuously sampling body chemistry and translating that information into a numerical reading and a trend arrow.
Interstitial Fluid: The Measurement Site
The continuous glucose monitor sensor measures glucose in the interstitial fluid (ISF), not directly in the blood. ISF is the fluid surrounding the body’s cells in the subcutaneous tissue. The sensor’s filament is inserted into this fatty layer beneath the skin, where it is bathed in the ISF. Glucose must first diffuse from the bloodstream into this fluid before the sensor can detect it. This necessary diffusion step creates a slight delay, known as lag time, between the blood glucose concentration and the ISF measurement. Lag time typically ranges from five to 20 minutes when glucose levels are stable. When blood glucose changes rapidly, this lag can become more pronounced, meaning the sensor provides a picture of the immediate past.
The Enzymatic Reaction That Detects Glucose
The CGM sensor relies on a precise electrochemical reaction initiated by an enzyme. The sensor filament is coated with glucose oxidase, which acts as the biological recognition element. When glucose from the interstitial fluid enters the sensor, it reacts with this enzyme and oxygen. This reaction transforms the glucose and oxygen, resulting in the production of hydrogen peroxide and gluconic acid. The sensor is a miniature electrode that measures the generated hydrogen peroxide. This hydrogen peroxide is oxidized at the platinum electrode, causing a transfer of electrons. This electron transfer produces a measurable electrical current, a process known as amperometric detection. The strength of this electrical signal is directly proportional to the amount of glucose present in the interstitial fluid. The sensor measures the electrical byproduct of the enzyme’s reaction, not the glucose molecule itself.
Data Transmission and Interpretation
Once the electrical current is generated, it is captured by the transmitter. The transmitter converts the analog electrical signal into a digital data packet. This digital signal is then wirelessly transmitted, typically using low-energy Bluetooth technology, to the user’s display device. The sensor takes readings frequently, often every one to five minutes, generating hundreds of data points daily. Algorithms within the transmitter or receiver software process the raw electrical current data, accounting for factors like temperature and lag time. The result is a calibrated, real-time glucose value, displayed to the user in a standardized unit like mg/dL or mmol/L. Clinicians and users interpret this data stream using reports like the Ambulatory Glucose Profile (AGP), which summarizes glucose trends and metrics such as Time in Range (TIR).
Sensor Insertion and Practical Usage
The application of a continuous glucose monitor is simplified using a single-use, spring-loaded applicator. This device houses the sensor filament and an insertion needle, which quickly pierces the skin and retracts, leaving only the flexible filament embedded in the subcutaneous tissue. Common insertion sites include the back of the upper arm or the abdomen. The sensor is held in place by an adhesive patch and is designed to be worn continuously, typically between 7 and 14 days before replacement. Proper site preparation, including cleaning the skin with an alcohol wipe, helps maximize the sensor’s lifespan and accuracy. Users should rotate the insertion site with each new sensor to prevent skin irritation or the formation of scar tissue, which can interfere with accurate readings.