A continuous glucose monitoring system (CGMS) is a wearable device that tracks your blood sugar levels around the clock, taking a new reading every few minutes. Unlike traditional fingerstick tests that give you a single snapshot, a CGM shows you where your glucose is, where it’s heading, and how fast it’s changing. These systems are used primarily by people with diabetes, though their use has expanded in recent years.
How a CGM Works
Every CGM has three core parts: a tiny sensor inserted just under the skin, a transmitter that sits on top of the sensor, and a display device (either a dedicated receiver or a smartphone app). The sensor measures glucose not in your blood directly, but in the fluid between your cells, called interstitial fluid. Glucose moves from your capillaries into this fluid through simple diffusion, so the reading lags slightly behind what’s actually in your bloodstream.
That lag time averages about 5 to 10 minutes under normal conditions, though studies have reported anywhere from 0 to 45 minutes depending on how rapidly glucose is changing. When your blood sugar is relatively stable, the CGM reading closely matches a fingerstick. When it’s rising or falling quickly, such as right after a meal or during exercise, the lag becomes more noticeable. This is why some situations still call for a confirmatory fingerstick.
What the Numbers Mean
CGMs generate a continuous stream of data, but a few standardized metrics help make sense of it all. The most important is Time in Range (TIR), which measures the percentage of the day your glucose stays between 70 and 180 mg/dL. The American Diabetes Association recommends a TIR goal of greater than 70% for most nonpregnant adults, meaning roughly 17 out of 24 hours in that target zone.
Two other metrics track the danger zones. Time Below Range (TBR) captures how often glucose drops too low. The goal is to spend less than 4% of the day below 70 mg/dL and less than 1% below 54 mg/dL, which represents more serious hypoglycemia. Time Above Range (TAR) tracks the opposite problem, with separate categories for moderate highs (181 to 250 mg/dL) and severe highs (above 250 mg/dL).
Together, these metrics give a much fuller picture than a single HbA1c lab test, which only reflects an average over two to three months. A person with an “acceptable” HbA1c could still be swinging between dangerous lows and highs throughout the day, and only a CGM would reveal that pattern.
Clinical Benefits
People who use CGMs consistently see meaningful improvements in blood sugar control. Studies in people with type 2 diabetes on insulin therapy have documented HbA1c reductions ranging from 0.8% to 1.6%, which is a significant drop. One study found an average HbA1c decrease from 8.6% to 7.5% after just three months of CGM use, alongside a 10% improvement in Time in Range with no increase in hypoglycemia episodes.
Much of this improvement comes from the real-time feedback loop. When you can see your glucose climbing after a specific meal or dropping during a workout, you learn to adjust your behavior and insulin dosing in ways that quarterly lab results never could. The trend arrows, which show whether glucose is stable, rising, or falling, are particularly useful for making in-the-moment decisions about food, activity, and medication timing.
Alerts and Safety Features
CGMs can sound alarms when glucose crosses thresholds you or your doctor set for highs and lows. Most systems also offer a rate-of-change alert that warns you when glucose is dropping or rising quickly. Some devices include a predictive low alert that estimates where your glucose is heading and warns you before you actually reach a dangerous level.
One critical safety feature is the urgent low alarm, which on some systems is factory-set at 55 mg/dL and cannot be turned off or adjusted. This serves as a last line of defense against severe hypoglycemia, especially overnight when you might not notice symptoms.
Calibration: Older vs. Modern Devices
Earlier CGM systems required fingerstick calibrations every 12 hours. You’d prick your finger, enter the blood glucose reading into the device, and the system would use that value to keep its sensor accurate. This was a significant hassle and a barrier to consistent use.
Modern CGMs are factory-calibrated, meaning the accuracy information is programmed into the sensor during manufacturing. Abbott’s FreeStyle Libre became the first factory-calibrated system approved by the FDA in 2017, followed by the Dexcom G6 in 2018. These devices work straight out of the box with no routine fingersticks needed, which has made CGM far more practical for everyday use.
Types of CGM Devices
Most CGMs are disposable sensors worn on the skin, typically on the back of the upper arm or the abdomen. The Dexcom G7, for example, combines its sensor and transmitter into a single unit that lasts 10 days. Other popular systems like the FreeStyle Libre line use a similar adhesive patch design with wear periods of 14 days.
For people who want a longer-term option, the Senseonics Eversense E3 is an implantable CGM. A doctor places a small sensor under the skin in a brief procedure, and it stays there for up to 180 days (six months) before needing replacement. A removable smart transmitter worn on top of the skin communicates with the implanted sensor. The trade-off is that the transmitter must be removed before MRI scans, and certain medical procedures like lithotripsy, diathermy, and electrocautery near the sensor site are either not recommended or could damage the device.
Who Can Get One
CGMs were originally prescribed almost exclusively for people with type 1 diabetes, but coverage has expanded substantially. Medicare now covers CGMs for people with diabetes who take insulin or have a history of problematic low blood sugar episodes, as long as their doctor prescribes the device. Most private insurers follow similar guidelines, though specific coverage criteria vary by plan.
The cost without insurance can be significant, running several hundred dollars per month for sensors and transmitters. With insurance coverage, out-of-pocket costs drop considerably, and some manufacturers offer savings programs. The expanding evidence base showing improved outcomes has steadily pushed more insurers to broaden their coverage criteria beyond just insulin-dependent patients.