CGMs, or continuous glucose monitors, are small wearable devices that track your blood sugar levels around the clock. Unlike traditional fingerstick tests that give you a single snapshot, a CGM takes a new reading every few minutes and shows you how your glucose is trending over time. They’re most commonly used by people with diabetes, though their use has expanded in recent years.
How CGMs Measure Glucose
A CGM doesn’t actually measure glucose in your blood. Instead, it measures glucose in the thin layer of fluid just beneath your skin, called interstitial fluid. As blood circulates through tiny capillaries, some glucose diffuses out of the bloodstream and into this fluid. A small sensor inserted under the skin detects that glucose using an enzyme-based reaction, converting the chemical signal into a number you can read on a screen.
Because the sensor is reading interstitial fluid rather than blood directly, there’s a slight lag. When your blood sugar is rising or falling quickly (after a meal or during exercise, for example), the CGM reading may trail behind what a fingerstick would show by several minutes. During stable periods, the two readings are very close. Modern CGMs like the Dexcom G7 have an overall accuracy margin of about 8 to 9 percent compared to lab-grade blood glucose measurements, which is accurate enough for day-to-day decision-making.
The Three Parts of a CGM System
Every CGM has the same basic components working together:
- Sensor: A tiny filament that sits just under the skin and measures glucose in interstitial fluid 24 hours a day. Sensors are disposable and last a set number of days depending on the model. The Dexcom G7, for instance, lasts 10 days with an extra 12-hour grace period to swap in a new one.
- Transmitter: A small piece that sits on top of the sensor and wirelessly sends glucose data to your display device. On some systems the transmitter is reusable and clips onto each new sensor. On others, like the G7, it’s built into the disposable sensor as a single unit.
- Display device: This is where you actually see your numbers. Most people use a smartphone app, though standalone receivers and insulin pumps can also display the data. The screen shows your current glucose level, a trend arrow indicating whether you’re rising or falling, and a graph of your recent history.
What the Numbers Tell You
The real power of a CGM isn’t any single glucose reading. It’s the pattern. Because the sensor takes readings continuously, it generates a detailed curve of your glucose throughout the day and night. You can see how specific meals, exercise, stress, and sleep affect your levels in ways that occasional fingersticks would never reveal.
One of the most useful metrics a CGM provides is called Time in Range, or TIR. This is the percentage of the day your glucose stays between 70 and 180 mg/dL. For most adults with type 1 or type 2 diabetes, the goal is to spend at least 70 percent of the day in that range, which works out to roughly 17 hours. Other targets include keeping time below 54 mg/dL (a dangerously low level) under 1 percent and time above 250 mg/dL under 5 percent. These targets give you a much fuller picture of glucose control than a single lab test can.
Who Uses CGMs
CGMs were originally developed for people with type 1 diabetes, and current guidelines recommend offering one as early as the time of diagnosis. They’re also widely used by people with type 2 diabetes, particularly those who take insulin or have a history of low blood sugar episodes. Medicare covers CGMs for people who use insulin or have documented problems with hypoglycemia, provided a healthcare provider evaluates you and confirms you’ve been trained on the device.
In recent years, some people without diabetes have started using CGMs out of curiosity about how their bodies respond to food and exercise. This is a growing trend, though the devices are primarily designed and approved for managing diabetes.
Where the Sensor Goes
Most CGM sensors are designed for the back of the upper arm or the abdomen, depending on the brand and model. The Dexcom G7 is approved for both locations. Abbott’s FreeStyle Libre is worn on the back of the upper arm. The Eversense system, which is a longer-lasting implantable sensor, goes exclusively in the upper arm and can stay in place for up to 90 days. Placement matters for accuracy, so each device has specific FDA-cleared locations.
Inserting the sensor is quick and mostly painless. You press an applicator against the skin, click a button, and a spring-loaded needle guides the tiny filament into place before retracting. Most people say they feel a brief pinch and then forget the sensor is there.
Calibration and Fingerstick Requirements
Older CGM models required you to prick your finger several times a day and enter the blood glucose value to keep the sensor calibrated. Current-generation devices have moved past this. Today’s CGMs come factory-calibrated, meaning the sensor is ready to go out of the box with no fingerstick calibration needed. There’s still a brief warmup period after you apply a new sensor (30 minutes for the Dexcom G7) while the device adjusts to your body before it starts displaying readings.
Some people still keep a traditional meter on hand as a backup for moments when a CGM reading seems off or when making a critical treatment decision, but routine fingerstick calibration is no longer part of daily life with newer models.
Integration With Insulin Pumps
For people who use an insulin pump, CGMs can connect directly to the pump to create what’s known as an automated insulin delivery system, sometimes called a closed-loop or “artificial pancreas” system. The CGM continuously feeds glucose data to an algorithm, which then adjusts insulin delivery from the pump every 5 to 12 minutes depending on the system.
The algorithm considers your current glucose level, where your glucose is predicted to go, and how much insulin is already active in your body. This cycle of measure, calculate, and adjust runs automatically day and night. The first commercially available system of this type was the Medtronic MiniMed 670G, released in 2016. Since then, newer systems have adopted Bluetooth Low Energy for wireless communication between the sensor, algorithm, and pump, improving battery life and reliability. Clinical trials and real-world data show these integrated systems improve both glucose control and quality of life, particularly for people with type 1 diabetes, by reducing the constant mental burden of managing insulin doses manually.