📘 Understanding glucose sensor technology (CGM)

Continuous glucose monitoring (CGM) is a method of real-time glucose level tracking using a small sensor worn under the skin. The system measures glucose concentration every 1-5 minutes and provides you with a continuous graph of evolution, not just point values. You receive information about the direction your glucose is heading, not just where it is right now. This technology has revolutionized diabetes management through access to real-time glucose data.

Unlike classic self-monitoring through finger prick, CGM shows you the hidden trends and patterns of your glucose. You can see what happens at night while you sleep or how the body reacts to different foods. Alarms warn you before glucose becomes dangerous. Current guidelines recommend CGM as standard of care for all people with type 1 diabetes and for many people with type 2 diabetes.

The CGM sensor uses a thin and flexible filament, 4-6 mm long, inserted into the subcutaneous tissue. On the filament surface is the enzyme glucose oxidase, which reacts with glucose molecules from interstitial fluid. The chemical reaction generates a small electrical current, proportional to glucose concentration. A transmitter on the skin transforms this electrical signal into numerical values.

The process is continuous and completely automatic, without your intervention. The electrical signal is processed by an algorithm, which converts it into glucose estimation values expressed in mg/dl or mmol/L. Accuracy depends on enzyme quality, conversion algorithm and local tissue conditions. Technology has evolved significantly in recent years, with increasingly accurate and reliable sensors.

The sensor measures glucose from interstitial fluid (space between cells) because placing a continuous sensor directly in blood vessels would be invasive and risky. Interstitial fluid is accessible through insertion of a small filament under the skin, without affecting vessels or nerves. Glucose from blood continuously passes into interstitial fluid, to feed tissue cells.

Interstitial glucose concentration faithfully follows blood glucose, with a 10-minute delay, necessary for diffusion. During stable periods, values are almost identical. Differences appear when glucose changes rapidly, such as after meals or during physical exercise. This indirect measurement is a compromise between accuracy, safety and comfort.

Real-time CGM automatically transmits values to receiver or phone every 1-5 minutes, without any action from you. You receive alarms when glucose exceeds set thresholds or changes rapidly. Flash glucose monitoring requires voluntary scanning of the sensor with phone or receiver to see values. Without scanning, you have no access to data.

CGM is ideal for people with unaware or frequent hypoglycemia, because the alarm wakes you at night if needed. Flash monitoring is more discreet and can be sufficient for those with stable glucose and preserved warning symptoms. The newest flash systems have added optional alarms for hypoglycemia, approaching CGM functionality. The choice depends on your individual needs and availability in the health system. The future is for real-time systems.

Modern CGM sensors have accuracy comparable to glucometers, with a mean absolute relative difference (MARD) of 9-11% compared to laboratory glucose. In practical terms, a glucose of 150 mg/dl (8.3 mmol/L) can be displayed between 135-165 mg/dl (7.5-9.2 mmol/L). Accuracy is optimal in the 70-180 mg/dl (3.9-10 mmol/L) range and decreases at extremes. Most current sensors meet accuracy criteria for therapeutic decisions (insulin dosing), but not all. Check the product leaflet!

Differences from glucometer don't necessarily mean the sensor is wrong. The glucometer has its own margin of error of ±15%. Different values reflect also the physiological delay of interstitial glucose concentration evolution. For routine decisions, you can trust the sensor. Confirm with glucometer only in critical situations, such as symptomatic hypoglycemia, extreme values or discordance with your clinical state.

The 10-minute delay occurs because glucose must diffuse from blood into interstitial fluid, where it is measured by the sensor. This transit time is a physiological constant, not a technical limitation. When blood glucose rises or falls, the change in interstitial fluid follows with several minutes delay. During stable periods, the difference becomes negligible.

The delay is more important when glucose changes rapidly, such as in the first hour after meal or during physical exercise. The sensor may show 120 mg/dl (6.7 mmol/L) when the glucometer already indicates 150 mg/dl (8.3 mmol/L). Trend arrows compensate for this delay, showing you the direction of evolution. Learn to interpret both the trend and the instantaneous value.

A complete CGM system includes three main components. We have the sensor itself, with the filament that measures glucose concentration, the transmitter, which processes and sends data wirelessly and the receiver, which displays the information. In modern systems, sensor and transmitter are fused into a single disposable unit. The receiver can be a dedicated device, a smartphone, a compatible insulin pump or smartwatch.

Additionally, you need the applicator that inserts the sensor under the skin and the software application for data visualization. For optimal operation, check your phone's compatibility with the manufacturer's application, including operating system version. Consumables include replacement sensors and possibly the transmitter, if it's reusable.

The sensor transmits data wirelessly using Bluetooth Low Energy (BLE) or NFC (Near Field Communication) technology. Bluetooth allows continuous transmission at distances up to six meters. NFC requires bringing the receiver close to the sensor for scanning. During scanning the estimated glucose values, trend and technical information about sensor status are updated.

For Bluetooth, transmission occurs at regular intervals, usually every 1-5 minutes. If the receiver is outside the range, data is stored in sensor memory and synchronized upon reconnection. Energy consumption is optimized for sensor lifespan. Interference is rare, but can occur near powerful medical equipment.

Most CGM sensors use electrochemical technology, based on glucose oxidase enzyme. The enzyme catalyzes glucose oxidation, generating hydrogen peroxide, which produces a measurable electrical current. Current intensity is proportional to glucose concentration. This is the same chemical reaction used by classic glucometers, but adapted for continuous measurement.

An alternative is fluorescence technology, used by implantable systems. A fluorescent polymer responds to glucose presence by modifying fluorescent light intensity emitted. This technology allows sensors with long lifespan. Research continues on non-invasive optical sensors, using infrared spectroscopy and other methods, which have not yet reached the accuracy necessary for clinical use.

For most daily management decisions, the sensor can replace the glucometer, if it has official approval on the leaflet for this. Current guidelines accept insulin dosing based exclusively on CGM values for sensors with validated accuracy (MARD under 10%). This eliminates the need for multiple daily finger pricks and improves quality of life. Closed loop systems function autonomously, without glucometer checks.

However, the glucometer remains necessary in certain special situations, such as hypoglycemia symptoms with normal sensor values, suspected sensor malfunction, first hours after applying a new sensor and whenever sensor values don't correspond with your clinical state. Always keep a functional glucometer and available test strips for occasional checks and emergency situations.