A continuous glucose monitor, or CGM, is a small wearable device that tracks your blood sugar levels around the clock, taking a new reading every few minutes. Instead of pricking your finger several times a day, a CGM gives you a continuous stream of data you can check on your phone or a dedicated receiver, along with alerts when your levels are heading too high or too low.
How a CGM Works
A CGM system has three main parts. The first is a tiny sensor, about the size of a coin, that sits just under your skin, typically on your belly or the back of your upper arm. A small adhesive patch holds it in place. The second part is a transmitter attached to the sensor that wirelessly sends data to the third part: a smartphone app, insulin pump, or standalone receiver that displays your glucose readings.
The sensor doesn’t actually measure glucose in your blood. It measures glucose in the interstitial fluid, the thin layer of liquid that surrounds your cells just beneath the skin. Glucose travels from your bloodstream into this fluid, where nearby cells use it for energy. When your blood sugar is stable, the reading in interstitial fluid closely matches what a finger-prick test would show. But when glucose is changing quickly, like after a meal or during exercise, there’s a lag of roughly 5 to 25 minutes between what’s happening in your blood and what the sensor detects. A fast-absorbing, high-carb meal creates a steeper spike in blood glucose, which means the lag can be more noticeable in those moments.
This lag is worth understanding because it explains why your CGM reading might not perfectly match a finger-prick test at any given moment. Over the course of a day, though, the two track closely enough that CGMs have become the preferred monitoring tool for millions of people with diabetes.
Who Uses a CGM
CGMs were originally developed for people with type 1 diabetes, who need to monitor glucose closely to dose insulin throughout the day. They’re now widely used by people with type 2 diabetes as well, particularly those who take insulin injections or use an insulin pump. The CDC notes that CGMs can benefit anyone with diabetes, but they’re especially helpful for people who frequently test their blood sugar or have difficulty keeping levels in a healthy range.
Insurance coverage reflects this. Most private insurance plans, Medicare, and Medicaid cover CGMs for people with type 1 diabetes. For type 2 diabetes, coverage typically requires that you take daily insulin injections, use an insulin pump, or have a history of severe low blood sugar episodes. Medicare specifically requires that a CGM be prescribed to improve glucose control and that the user meets at least one qualifying criterion, such as being insulin-treated or having documented problematic hypoglycemia.
More recently, CGMs have become available to people without diabetes. In 2024, the FDA cleared the first over-the-counter CGM, the Dexcom Stelo, for adults 18 and older who don’t use insulin. It’s designed for people managing type 2 diabetes with oral medications or anyone who simply wants to see how diet and exercise affect their blood sugar in real time.
What a CGM Shows You
The most important metric 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. The American Diabetes Association recommends that most adults aim for at least 70% of the day in that range, which corresponds roughly to an A1C of about 7%. For older adults, the target is more relaxed at 50% or more, with an emphasis on limiting time spent in dangerously low territory to no more than about 15 minutes per day.
Beyond the numbers on screen, real-time CGMs offer programmable alerts that notify you when glucose crosses a threshold you’ve set, whether that’s a high limit after meals or a low limit overnight. Some systems go further with predictive alerts. These use algorithms to forecast where your glucose is heading and warn you before a dangerous low actually happens. One widely used alert triggers when the system predicts glucose will drop to 55 mg/dL or below within the next 20 minutes, giving you time to eat something and prevent a hypoglycemic episode. These predictive alerts are a significant safety feature, especially for people who experience lows without warning symptoms.
How Accurate Are CGMs
CGM accuracy is measured using something called MARD, or Mean Absolute Relative Difference, which tells you how far the sensor’s reading strays from a lab-quality blood glucose measurement on average. Lower is better. A head-to-head comparison of two leading sensors found that the FreeStyle Libre 3 had a MARD of 8.9% and the Dexcom G7 came in at 13.6% when compared to lab reference values. In practical terms, the FreeStyle Libre 3 placed 91.4% of its readings within 20 mg/dL of the reference, compared to 78.6% for the Dexcom G7.
Accuracy can vary depending on your glucose level. Both sensors performed best in the normal-to-high range (70 to 250 mg/dL) and less accurately at very low glucose levels. During the first 12 hours after insertion, both sensors showed similar accuracy, but the FreeStyle Libre 3 improved noticeably after that initial break-in period while the G7 remained relatively stable. This is worth knowing because it means the first few hours of a new sensor may be less reliable regardless of which brand you use.
Sensor Types and How Long They Last
Most CGM sensors are disposable patches you replace every 10 to 15 days. You insert them yourself using an applicator that pushes the thin filament under your skin in a quick, mostly painless motion. After the wear period, you peel off the old sensor and apply a new one.
There’s also a longer-lasting option. The Eversense E3 is an implantable CGM where a doctor places a small sensor under your skin in a brief office procedure. That sensor lasts up to 180 days, or about six months, before it needs to be replaced. You still wear a transmitter on the skin above the implant, but you avoid the cycle of swapping adhesive patches every couple of weeks. The Eversense E3 is FDA-approved for adults 18 and older with diabetes.
Real-Time vs. Intermittently Scanned CGMs
Not all CGMs work the same way. Real-time CGMs continuously push data to your phone or receiver and can send alerts without you doing anything. Intermittently scanned CGMs (sometimes called “flash” glucose monitors) store data on the sensor but only show it when you actively scan the sensor with your phone or reader. You still get a continuous record of your glucose, but you won’t receive automatic alerts unless you’re scanning regularly.
This distinction matters for outcomes. A meta-analysis of CGM studies in people with type 1 diabetes found that real-time CGMs increased Time in Range by about 5.6 percentage points more than intermittently scanned systems and also showed greater reductions in time spent in hypoglycemia. Overall, CGM users spent an average of 114 additional minutes per day in the target glucose range compared to those using traditional finger-prick monitoring, and experienced up to 72% less time in dangerously low glucose territory below 54 mg/dL. Average A1C dropped by 0.38 percentage points, a meaningful improvement that translates to lower risk of long-term complications.
What It’s Like to Wear One
The sensor patch is small and relatively discreet. Most people wear it on the back of the upper arm or on the abdomen. You can shower, swim, and exercise with it on, though adhesion can be a challenge during heavy sweating or extended water exposure. Many users add an extra adhesive overlay to keep the sensor secure.
The real adjustment is informational, not physical. Seeing your glucose fluctuate in real time can be eye-opening. You’ll notice how specific foods, stress, sleep, and exercise shift your levels in ways a few daily finger pricks never revealed. For many people, this visibility is the most valuable part of wearing a CGM, because it turns abstract numbers into patterns you can actually act on.