Continuous glucose monitors, or CGMs, have changed diabetes management by showing glucose trends throughout the day and night rather than relying only on occasional fingerstick readings. These small wearable devices measure glucose in interstitial fluid, send readings to a receiver or smartphone, and help people with diabetes, clinicians, and caregivers make more informed decisions about food, exercise, medication, and safety.

TLDR: A CGM tracks glucose continuously and provides trend arrows, alerts, and reports that can support better diabetes management. Accuracy is generally strong in modern systems, but readings can lag behind blood glucose and may be affected by compression, sensor placement, or rapid glucose changes. Insurance coverage depends on diagnosis, insulin use, medical necessity, and plan rules. Leading devices include Dexcom G7, FreeStyle Libre 3, Medtronic Guardian, and Eversense E3, each with different strengths.

How Continuous Glucose Monitors Work

A CGM uses a small sensor inserted just under the skin, usually on the abdomen, upper arm, or another approved site depending on the device. The sensor measures glucose in the interstitial fluid, which surrounds the body’s cells. Because this fluid is not blood, CGM values may differ slightly from fingerstick blood glucose readings, especially when glucose is rising or falling quickly.

Most CGM systems include three parts:

  • Sensor: A small filament worn under the skin that measures glucose levels.
  • Transmitter: A component that sends readings to a display device, sometimes built into the sensor.
  • Receiver or app: A smartphone app, reader, insulin pump, or receiver that displays glucose data, trend arrows, alerts, and reports.

The biggest advantage of CGM technology is visibility. Instead of seeing a single number at one moment, users can see where glucose has been, where it is now, and where it appears to be heading. This creates a fuller picture of patterns such as overnight lows, post-meal spikes, exercise-related drops, or early morning increases.

CGM Accuracy Explained

CGM accuracy is commonly measured using MARD, which stands for mean absolute relative difference. A lower MARD generally means the CGM readings are closer to reference glucose values. Many modern CGMs have MARD values near or below 10%, which is considered strong performance for daily diabetes management.

However, no CGM is perfect. Several factors can affect accuracy:

  • Physiological lag: Interstitial glucose may trail blood glucose by several minutes, especially after meals, insulin dosing, or exercise.
  • Compression lows: Pressure on the sensor during sleep can cause falsely low readings.
  • Sensor warm-up: Readings may be unavailable or less stable during the first hours after insertion.
  • Placement issues: Sensors placed outside approved sites or on irritated skin may perform less reliably.
  • Medication interference: Some substances, including high doses of acetaminophen with certain older systems, may interfere with readings.

Many CGMs are approved for non-adjunctive use, meaning treatment decisions can often be made using CGM readings without a confirmatory fingerstick. Still, fingerstick checks may be recommended when symptoms do not match the CGM value, when readings seem inaccurate, or during rapid glucose changes.

Key CGM Features to Compare

When comparing CGM devices, accuracy is important, but it is not the only factor. The best device for one person may not be the best for another. Clinicians and users often consider the following features:

  • Wear time: Sensors may last 10, 14, 15, or even 180 days depending on the system.
  • Calibration: Some systems require no routine fingerstick calibration, while others may require periodic calibration.
  • Alerts: Real-time high and low glucose alerts can help reduce risk, especially overnight.
  • Data sharing: Caregivers, parents, or clinicians may be able to follow glucose data remotely.
  • App quality: Ease of use, report clarity, and phone compatibility can heavily influence satisfaction.
  • Pump integration: Some CGMs connect with insulin pumps for automated insulin delivery.
  • Cost and coverage: Out-of-pocket costs can vary widely depending on insurance and pharmacy benefits.

Best CGM Devices Compared

Dexcom G7

Dexcom G7 is one of the most widely recognized real-time CGMs. It features a compact all-in-one sensor and transmitter, a short warm-up period, and strong app-based data sharing. It provides customizable alerts, trend arrows, and compatibility with certain diabetes management platforms and insulin pump systems.

Dexcom G7 is often favored by people who want real-time alerts, caregiver sharing, and a polished app experience. Its 10-day wear time is shorter than some competitors, but its accuracy, convenience, and ecosystem integration make it a leading choice.

FreeStyle Libre 3

FreeStyle Libre 3 is a small CGM from Abbott that provides real-time glucose readings sent automatically to a smartphone. Earlier Libre models required scanning, but Libre 3 offers continuous data transmission along with optional alarms. Its small size and generally lower cash price make it attractive to many users.

Libre 3 is commonly chosen by people looking for a discreet device with strong accuracy and simple day-to-day use. It may be especially appealing when insurance coverage is limited or when affordability is a major concern.

Medtronic Guardian Sensor

Medtronic Guardian sensors are often used by people who also use Medtronic insulin pumps. Their main advantage is integration with Medtronic’s automated insulin delivery systems, which can adjust insulin delivery based on CGM readings.

For users already committed to the Medtronic pump ecosystem, Guardian sensors may be the most practical option. However, some versions have required calibrations, and the overall experience may depend heavily on the specific pump and sensor generation being used.

Eversense E3

Eversense E3 is different from most CGMs because its sensor is implanted under the skin by a healthcare professional and can last up to 180 days. A removable smart transmitter sits over the sensor and sends readings to an app. The transmitter can be taken off and replaced without removing the implanted sensor.

Eversense may suit people who dislike frequent sensor insertions or want a long-term implanted option. It does require an in-office insertion and removal procedure, which may be a drawback for those seeking the simplest setup.

Insurance Coverage for CGMs

Insurance coverage for CGMs has expanded significantly, but rules still vary by plan, diagnosis, and medical necessity. Coverage may be available through either the pharmacy benefit or the durable medical equipment benefit. Pharmacy coverage is often more convenient because supplies can be filled at a retail or mail-order pharmacy.

Many insurance plans consider coverage when a person has:

  • Type 1 diabetes, especially when using insulin.
  • Type 2 diabetes treated with multiple daily insulin injections or an insulin pump.
  • A history of severe hypoglycemia or hypoglycemia unawareness.
  • Documented need for frequent glucose monitoring.
  • A clinician’s prescription and supporting medical records.

Medicare coverage has also broadened over time. In many cases, Medicare may cover CGMs for beneficiaries who use insulin or have problematic hypoglycemia, provided documentation requirements are met. Private insurers may follow similar standards, but prior authorization is common.

Common reasons for denial include missing documentation, lack of proof of insulin use, use of a non-preferred brand, or coverage under a different benefit category. Appeals may succeed when clinicians provide chart notes, glucose logs, hypoglycemia history, prescription details, and a clear explanation of medical necessity.

How Much Does a CGM Cost?

CGM costs vary based on device type, insurance plan, pharmacy pricing, and manufacturer savings programs. Without insurance, users may pay for sensors monthly, and some systems also require transmitters or receivers. Costs can range from relatively affordable pharmacy prices for certain Libre systems to higher monthly expenses for advanced real-time CGMs.

With insurance, out-of-pocket costs may include copays, coinsurance, deductibles, or supply limits. Some plans cover only preferred brands. Others require users to obtain supplies from a specific pharmacy or medical equipment supplier. Because coverage rules differ, the most accurate cost estimate usually comes from the insurer, pharmacy, or supplier after a prescription is submitted.

Benefits and Limitations

The benefits of CGMs are substantial. They can help reduce severe lows, improve time in range, reveal hidden glucose patterns, and support more personalized treatment adjustments. For caregivers of children, older adults, or people at risk of hypoglycemia, remote alerts may add reassurance.

At the same time, CGMs can produce data overload. Frequent alerts may cause alarm fatigue, and users may feel anxious when watching glucose shifts in real time. Skin irritation, sensor failures, app interruptions, and insurance paperwork can also create frustration.

Choosing the Right CGM

The right CGM depends on medical needs, lifestyle, device compatibility, and coverage. Someone using an automated insulin delivery system may prioritize pump integration. A person seeking affordability may prefer a lower-cost pharmacy CGM. A caregiver may value remote monitoring and loud alerts, while another person may prefer a smaller sensor with fewer interruptions.

Healthcare professionals typically evaluate glucose goals, hypoglycemia risk, insulin regimen, skin sensitivity, technology comfort, and insurance rules before recommending a device. The best CGM is not always the most advanced one; it is the one that fits consistently into daily life and supports safer glucose management.

FAQ

Are CGMs more accurate than fingerstick meters?

CGMs provide continuous trend information, while fingerstick meters measure blood glucose at a single moment. Modern CGMs are highly accurate, but fingersticks may still be useful when symptoms do not match the CGM reading or glucose is changing rapidly.

Does a CGM hurt?

Most users report only mild discomfort during sensor insertion. After placement, the sensor is usually not noticeable during normal daily activities.

Can people with type 2 diabetes use a CGM?

Yes. CGMs are increasingly used by people with type 2 diabetes, especially those using insulin or experiencing hypoglycemia. Coverage depends on the insurance plan and medical documentation.

How long does a CGM sensor last?

Wear time depends on the device. Common wear periods include 10 days, 14 days, 15 days, or up to 180 days for implantable systems such as Eversense E3.

Is a prescription required for a CGM?

In most cases, a prescription is required. Insurance coverage also usually requires documentation from a healthcare professional.

Can a CGM be worn during exercise or swimming?

Many CGMs are water-resistant and suitable for exercise, but limits vary by device. Users should follow the manufacturer’s guidance for swimming depth, duration, and adhesive care.

Which CGM is best overall?

There is no single best CGM for everyone. Dexcom G7 is strong for alerts and sharing, FreeStyle Libre 3 is small and often cost-effective, Medtronic Guardian fits Medtronic pump users, and Eversense E3 offers long sensor life.