The small, round device you’ve seen on someone’s arm is a continuous glucose monitor, or CGM. It’s a sensor about the size of two stacked coins that tracks blood sugar levels around the clock, sending readings to a smartphone or separate receiver without any finger pricks. Millions of people with diabetes wear one, and they’ve become one of the most visible pieces of medical technology in everyday life.
How a CGM Works
A CGM system has three parts: a tiny sensor filament that sits just under the skin, a transmitter on top of the skin that sends data wirelessly, and an app or receiver that displays the readings. The sensor doesn’t measure blood directly. Instead, it reads glucose levels in the thin layer of fluid between your cells, called interstitial fluid. Glucose naturally diffuses from your blood vessels into this fluid, so the readings closely mirror what a traditional blood test would show.
The sensor checks glucose every one to five minutes, 24 hours a day. That constant stream of data gets sent to a phone or insulin pump in real time, giving the wearer a live graph of where their blood sugar is, where it’s been, and where it’s trending. Most systems also fire off alerts if levels are dropping too low or climbing too high, which is especially valuable overnight when a person wouldn’t otherwise notice.
What It Looks Like Up Close
The two most common CGMs right now are the FreeStyle Libre 3 and the Dexcom G7. Both are small, flat, and disc-shaped. The Libre 3 is about 21 millimeters across and under 3 millimeters tall, roughly the diameter of a U.S. quarter. The Dexcom G7 is slightly larger at about 24 by 27 millimeters, still smaller than a bottle cap. Both stick to the skin with a medical adhesive patch and sit flush against the arm.
People wear the Libre 3 for up to 14 days before replacing it, while the Dexcom G7 lasts up to 10 days. Applying a new sensor takes about a minute: you press an applicator against your skin, a spring-loaded mechanism inserts a hair-thin flexible filament just under the surface, and any insertion needle retracts immediately. No needle stays in the arm. After a short warm-up period (30 minutes for the Dexcom G7, 60 minutes for the Libre 3), the sensor starts reporting glucose values.
Accuracy of Modern Sensors
CGMs aren’t quite as precise as a laboratory blood draw, but they’ve gotten remarkably close. Accuracy is measured by something called mean absolute relative difference, which tells you how far off, on average, a sensor’s reading is from a lab reference. Both the Libre 3 and Dexcom G7 score around 9 to 12 percent by this measure, depending on the comparison method. In practical terms, if your actual blood sugar is 150 mg/dL, the sensor might read anywhere from about 135 to 165. That’s accurate enough for day-to-day decisions about food, exercise, and insulin dosing.
Readings can be slightly less accurate in the first 12 hours after a new sensor is applied, as the body adjusts to the filament. After that initial period, accuracy improves and remains stable for the rest of the sensor’s life.
What “Time in Range” Means
One of the biggest advantages of wearing a CGM is a metric called Time in Range. This is the percentage of the day your glucose stays between 70 and 180 mg/dL, the window most clinicians consider healthy for someone with diabetes. A traditional blood test at the doctor’s office gives you one snapshot. Time in Range gives you the full picture: how your body responded to that pasta dinner, that morning run, or that stressful meeting.
This metric has become increasingly important because it correlates with long-term complications. Higher Time in Range is linked to lower risks of eye, kidney, and cardiovascular problems. It also isn’t thrown off by certain blood conditions that can skew traditional lab tests, making it a more reliable indicator for some people.
Who Wears One
CGMs were originally designed for people with type 1 diabetes, but their use has expanded significantly. Current guidelines from the American Diabetes Association recommend offering CGM to people with diabetes broadly, including those with type 2 diabetes and those who are pregnant. Early initiation, even at the time of diagnosis, is encouraged depending on a person’s needs and preferences.
The devices are particularly useful for anyone taking insulin or other medications that can cause blood sugar to drop dangerously low. A CGM can catch a downward trend and alert the wearer before they feel symptoms. For people with type 2 diabetes managed through diet or non-insulin medications, a CGM can reveal which foods and habits spike their blood sugar, turning abstract advice into concrete, personalized data.
Could It Be an Insulin Pump Instead?
Not every device on someone’s arm is a glucose sensor. Some people wear a tubeless insulin pump called an Omnipod, which looks like a slightly larger, oval-shaped pod. It’s about 1.5 by 2 inches and holds up to 200 units of insulin, delivering it continuously through a tiny cannula under the skin. The pod is waterproof and gets replaced every three days. Some people wear both a CGM and an insulin pump at the same time, sometimes on the same arm, and the two devices can communicate to adjust insulin delivery automatically.
The quick way to tell the difference: a CGM is smaller and flatter (coin-sized), while an insulin pump pod is noticeably thicker and more oval-shaped. Both attach with adhesive patches and are designed to be worn during showers, exercise, and sleep.
Living With a Sensor on Your Arm
Wearing a CGM is mostly unnoticeable after the first few minutes. The filament under the skin is flexible and thin enough that most people forget it’s there. The adhesive patch keeps it secure through sweating, swimming, and sleeping, though some people use extra adhesive covers for added security during intense exercise or humid weather.
The most common annoyance is skin irritation from the adhesive, which some people manage with barrier wipes or patches applied underneath. Clothing occasionally catches on the sensor, though its low profile makes this rare. For many wearers, the trade-off is straightforward: a small patch on the arm replaces multiple daily finger pricks and provides a level of insight into their blood sugar that simply wasn’t possible a decade ago.