What is a CGM glucose sensor?
A glucose sensor - also known as a CGM sensor (Continuous Glucose Monitoring) - is a small, discreet medical device that is applied to the skin and continuously measures glucose levels in interstitial fluid. Unlike traditional glucometers that require finger pricks multiple times a day, a glucose sensor provides real-time data, 24 hours a day, without pain and without additional effort.
As a diabetologist with over 15 years of experience, I can say that the introduction of CGM sensors was one of the most important innovations in the last two decades in diabetes management. My patients who switched to continuous monitoring reported not only better glycemic control, but also a significantly improved quality of life.
How a glucose sensor works - step by step
Understanding how a glucose sensor works is essential to appreciate the advantages it offers. The process involves three main components that work together.
Sensor filament - the "antenna" in the skin
The central part of any glucose sensor is a thin, flexible filament, with a diameter smaller than a hair. This filament is inserted approximately 5-7 mm below the skin surface, reaching the interstitial tissue - the space between cells where glucose-rich fluid is found.
The filament is covered with a special enzyme (glucose oxidase) that reacts chemically with glucose molecules in the interstitial fluid. This reaction generates a tiny electrical signal, proportional to the glucose concentration. The more glucose present, the stronger the electrical signal.
The transmitter - the data interpreter
The electrical signal generated by the filament is picked up by a miniature transmitter located on the sensor surface. It converts the analog signal into digital data and transmits it wirelessly - via Bluetooth or NFC - to a receiver: a mobile phone, a dedicated reader, or even a smartwatch.
Data transmission occurs at regular intervals, usually every 1-5 minutes, which means that in one day you get between 288 and 1440 glucose readings. In comparison, even the most disciplined patient doing traditional testing does a maximum of 8-10 tests per day. The difference is enormous.
The app or reader - data visualization
The data arrives in the mobile app or on the dedicated reader, where it is displayed as a continuous graph. This visualization - called the "glucose profile" - shows not just the current glucose value, but also the trend: whether glucose is rising, falling, or stable. A trend arrow indicates the direction, giving you the ability to act preventively.
What does a glucose sensor actually measure?
It is important to clarify a technical aspect that many patients are not aware of. A glucose sensor does not measure glucose in blood directly, but glucose in interstitial fluid. This closely follows the blood glucose level, but with a physiological delay of 5-10 minutes.
This delay is due to the time it takes for glucose to diffuse from capillaries into the interstitial fluid. In practice, for most patients, this difference is negligible and does not affect therapeutic decisions. Modern calibration algorithms compensate for much of this lag.
However, there are situations where the difference can be greater: during severe hypoglycemia, after carbohydrate-rich meals (when blood glucose rises rapidly), or during intense physical exercise. In these moments, if there is doubt, verification with a traditional glucometer is recommended.
Types of CGM sensors available
There are several types of CGM sensors on the market, classified according to the method of data transmission and the features offered.
Real-time CGM sensors (rtCGM)
These transmit data automatically and continuously, without any intervention from the user. They display the current value, the trend, and can emit alarms when glucose exceeds set limits - both for hyperglycemia and hypoglycemia. They are ideal for people with type 1 diabetes or those with type 2 diabetes who use insulin.
CGM sensors with scanning (isCGM / Flash)
These store data in the sensor and transmit it only when the user scans the sensor with their phone or reader. They do not emit automatic alarms in basic versions, but are usually more affordable. Some recent models have added optional alarm functions.
Implantable CGM sensors
These represent the newest generation - sensors that are implanted subcutaneously and function for up to 1 year (the Eversense 365 model). They are still less widely available, but offer the greatest long-term comfort.
Why is a glucose sensor important for people with diabetes
The benefits of using a glucose sensor far exceed mere convenience. Clinical studies have demonstrated multiple measurable advantages.
HbA1c reduction
Meta-analyses published in specialty journals show that CGM use leads to an average HbA1c reduction of 0.3-0.6%, which translates to a significantly lower risk of long-term complications. For a patient with HbA1c of 8%, a drop to 7.5% can mean a 12-22% reduction in the risk of retinopathy or diabetic nephropathy.
Reduction of hypoglycemia
One of the most valuable benefits is the identification and prevention of hypoglycemic episodes. CGM sensors detect glucose drops before they become dangerous, alerting the patient to take action. Studies report a 50-70% reduction in time spent in hypoglycemia.
Improved quality of life
Last but not least, eliminating repeated finger pricks and the comfort of having information always at hand reduce the stress associated with the disease. Patients report less anxiety, better sleep (thanks to nighttime alarms), and greater confidence in managing their condition.
Type 2 diabetes affects millions of Romanians, and continuous monitoring is becoming an increasingly recommended tool for this patient group as well, not just for those with type 1 diabetes.
Components of a CGM sensor - what is included in the kit
When you purchase a glucose sensor, the package typically contains the following components:
The sensor itself - the small, round or oval disc that attaches to the skin. It contains the sensor filament, electronic transmitter, battery, and medical adhesive.
The applicator - a single-use device with a spring that inserts the filament under the skin quickly and painlessly. The process takes less than a second and most people compare it to a simple touch, not a prick.
Medical adhesive - a hypoallergenic adhesive tape that keeps the sensor fixed to the skin for the entire duration of use (7-15 days, depending on the model). It is resistant to water, sweat, and physical activity.
Where to apply the glucose sensor on the body
Correct sensor placement is important for reading accuracy. The most common application areas are:
Back of the arm - the most popular area, easy to cover with clothing and with good interstitial fluid flow. Most sensors are approved for this area.
Abdomen - the preferred area for some models, especially older ones. Avoid the area 5 cm around the navel and the belt area.
Thigh - a useful alternative for children or for people who do intense sports with their arms. Some newer models are also approved for this area.
It is important to alternate application areas from one sensor to the next to allow the skin to rest and to avoid irritation.
How long does a glucose sensor last?
The lifespan of a glucose sensor varies depending on the manufacturer and model. The most common durations are:
7 days - for some basic models or for sensors used with insulin pumps.
14 days - the most common interval, offered by most manufacturers. This means you will use approximately 2 sensors per month.
15 days - some newer manufacturers have extended the duration by one additional day.
After the period of use expires, the sensor automatically deactivates and must be replaced with a new one. It is not recommended to try to extend the duration of use, as accuracy decreases significantly.
Glucose sensor vs. traditional glucometer
The question I receive most frequently from patients is: "Is it worth switching from a glucometer to a sensor?" The short answer is yes, for most patients. Here is an objective comparison:
A traditional glucometer provides a single value at one point in time - like a photograph. A glucose sensor provides a continuous film of glucose evolution. With a glucometer, you see that at 8 in the morning you have 150 mg/dL, but you don't know what happened overnight. With the sensor, you see exactly whether glucose was stable, whether you had a nocturnal hypoglycemia that you didn't feel, or whether there was a hyperglycemic spike after dinner.
From a comfort perspective, a glucometer requires finger pricks 4-10 times a day, which leads to thickened skin, pain, and discomfort. The sensor is applied once every 14 days and requires no other intervention.
The only situations in which a glucometer remains necessary are: to confirm severe hypoglycemia (when treatment must be administered immediately) and to calibrate some older sensor models that still require this.
Who should use a glucose sensor?
Initially, CGM sensors were recommended exclusively for people with type 1 diabetes. Today, the indications have expanded considerably:
People with type 1 diabetes - primary indication, all international guidelines recommend CGM as a standard of care.
People with type 2 diabetes on insulin - proven benefits in optimizing insulin doses and reducing hypoglycemia.
People with type 2 diabetes without insulin - recent studies demonstrate benefits in changing eating behavior and increasing motivation for a healthy lifestyle.
Gestational diabetes - continuous monitoring provides a clear perspective on the impact of diet and helps protect the health of mother and baby.
Prediabetes - increasingly, endocrinologists recommend temporary CGM use for people with prediabetes as an educational tool.
If you want to explore available options, you can consult our guide on how to apply a CGM sensor to see how simple the process is.
Accuracy of glucose sensors - MARD explained
An important technical indicator you will encounter frequently is MARD (Mean Absolute Relative Difference) - mean absolute relative difference. This expresses, in percentages, how close the sensor values are to the actual blood glucose values.
A MARD of 10% means that, on average, the sensor shows a value 10% different from the actual value. At a glucose level of 100 mg/dL, the sensor might show between 90 and 110 mg/dL. Modern sensors have MARD values of 8-11%, which is considered excellent and precise enough for therapeutic decisions.
Factors that can affect accuracy include: the first 24 hours after application (the "break-in" period), dehydration, pressure exerted on the sensor (for example, if you sleep on the arm where it is applied), extreme temperatures, and certain medications (for example, high-dose vitamin C can influence some models).
