A continuous glucose monitor, or CGM, is a small wearable device that tracks your glucose levels around the clock. Instead of pricking your finger for a single snapshot, a CGM takes a reading every few minutes and sends it to your phone or a handheld receiver, giving you a real-time picture of how your glucose rises, falls, and responds to food, exercise, and sleep. Originally designed for people with diabetes, these devices have become increasingly popular as consumer health tools.
How a CGM Measures Glucose
A CGM doesn’t measure glucose in your blood directly. It measures glucose in interstitial fluid, the thin layer of liquid that surrounds your cells just beneath the skin. This fluid picks up substances that leak out of nearby blood capillaries, including glucose. A tiny sensor filament sits in this fluid and continuously detects how much glucose is present.
Because glucose travels through your bloodstream first and then seeps into interstitial fluid, there’s a built-in delay. CGM readings typically lag behind a standard blood glucose reading by about 10 minutes. That gap matters most when your glucose is changing quickly, like right after a meal or during intense exercise. When levels are relatively stable, the CGM reading and a fingerstick result will be very close.
The Three Main Components
Every CGM system has three parts that work together:
- Sensor. A small, flexible filament inserted just under the skin using a one-press applicator. The needle retracts immediately after insertion, leaving only the filament behind. Depending on the model, a single sensor lasts anywhere from 10 to 15 days before you replace it. One implantable option lasts several months.
- Transmitter. A lightweight piece that snaps onto the sensor patch. It processes the raw signal from the filament and sends a glucose reading wirelessly every five minutes using Bluetooth. Some newer models combine the transmitter and sensor into a single disposable unit.
- Display device. This can be a dedicated handheld receiver or, more commonly now, a smartphone app. It shows your current glucose number, a trend arrow indicating whether you’re rising or falling, and a graph of your readings over the past several hours. It also sends alerts if your glucose drops too low or climbs too high.
Accuracy and Calibration
Modern CGMs are remarkably accurate. Accuracy is measured using a metric called mean absolute relative difference, which represents how far the CGM reading strays from a lab blood glucose value on average. The two most widely used models, the Dexcom G7 and the Abbott FreeStyle Libre 3, have scores of 8.2% and 7.9% respectively. In practical terms, if your actual blood glucose is 100 mg/dL, the CGM reading will typically land somewhere between about 92 and 108.
Most current CGMs are factory calibrated, meaning they’re ready to go out of the box with no fingerstick blood tests needed for setup or daily maintenance. Older models required you to enter a fingerstick value once or twice a day to keep the sensor accurate. That requirement has largely disappeared, though you may still want to confirm with a fingerstick if a reading doesn’t match how you feel.
What You See on the Screen
The real power of a CGM isn’t any single number. It’s the continuous trend line. Over the course of a day, you can see exactly how a bowl of oatmeal, a stressful meeting, or a 30-minute walk affected your glucose. That pattern recognition is something fingerstick testing simply can’t provide, since each prick only captures one isolated moment.
Clinicians and the American Diabetes Association use a metric called Time in Range to evaluate CGM data. For most nonpregnant adults with diabetes, the goal is to spend more than 70% of the day with glucose between 70 and 180 mg/dL. That 70% target corresponds roughly to an A1C of about 7%. For older adults with more complex health conditions, a more relaxed goal of 50% Time in Range is considered appropriate. A 10- to 14-day window of CGM data, with the sensor worn at least 70% of the time, gives a reliable picture of overall glucose control.
Benefits for People With Diabetes
CGMs have changed daily life for people managing both type 1 and type 2 diabetes. The most immediate benefit is catching low blood sugar episodes early. A CGM can alert you when glucose is dropping fast, sometimes 20 or 30 minutes before you’d feel symptoms like shakiness or confusion. For people on insulin, that early warning can prevent a dangerous low.
On the other side, seeing post-meal glucose spikes in real time helps you learn which foods, portions, or timing choices push your numbers higher than expected. Studies consistently show that people using CGMs achieve lower A1C levels and experience fewer episodes of both high and low blood sugar. They also report greater confidence in managing their condition day to day. These benefits hold across different insulin regimens, whether someone uses an insulin pump, multiple daily injections, or a simpler treatment plan.
When paired with an insulin pump, some CGMs can automatically adjust insulin delivery based on predicted glucose levels. This combination, sometimes called a closed-loop or automated insulin delivery system, reduces much of the minute-to-minute decision-making that makes diabetes management exhausting.
CGMs for People Without Diabetes
A growing number of companies now market CGMs directly to consumers interested in “metabolic health” or optimizing their diet, even without a diabetes diagnosis. The FDA has approved some devices for over-the-counter sale without a prescription. However, that approval was based on the devices being safe and reasonably accurate, not on proof that they improve health outcomes for people without diabetes.
Experts at Johns Hopkins note that all the clinical guidance on how to interpret and act on CGM data was developed for people with diabetes. In people with normal glucose regulation, the significance of day-to-day glucose fluctuations is unclear. No major clinical trials have shown that monitoring or adjusting glucose patterns in non-diabetic individuals leads to better long-term health. There’s also a risk of tunnel vision: glucose is just one piece of metabolic health, and focusing too heavily on it can lead you to ignore equally important factors like cholesterol, triglycerides, and blood pressure.
That said, wearing a CGM for a week or two can be an educational experience. Many people are genuinely surprised to see how different meals affect their glucose, and that short-term awareness can motivate lasting changes in eating habits. The question is whether continuous, ongoing monitoring adds value beyond that initial learning period for someone whose body regulates glucose normally.
Cost and Insurance Coverage
Without insurance, CGM supplies typically run between $160 and $500 per month, depending on the brand and where you purchase them. That cost covers replacement sensors (every 10 to 15 days) and, for some systems, a transmitter that lasts about three months.
Insurance coverage has improved significantly in recent years. Medicare now covers CGMs for eligible beneficiaries and has dropped the previous requirement of testing your blood sugar with fingersticks four times daily to qualify. Most private insurers also cover CGMs for people with type 1 diabetes, and coverage for type 2 diabetes has expanded steadily. If you’re prescribed a CGM, checking with your insurance plan or the manufacturer’s patient assistance program can significantly reduce your out-of-pocket cost.
What It Feels Like to Wear One
The sensor patch is roughly the size of two stacked coins and sticks to your skin with adhesive, typically on the back of your upper arm or your abdomen. Insertion takes a few seconds using a spring-loaded applicator. Most people describe a brief pinch, and many say they feel nothing at all. Once in place, you can shower, swim, and exercise with the sensor on, though adhesive loosening can be an issue for some people, especially in humid weather or during heavy sweating.
After applying a new sensor, there’s a brief warm-up period (usually 30 to 60 minutes, depending on the model) before readings begin. From that point on, data flows continuously to your phone or receiver without any action required from you. Notifications for high and low glucose can be customized, and most apps let you share your data with a family member or healthcare provider in real time.