What Continuous Blood Sugar Monitoring Is and How It Works
Continuous blood sugar monitoring (also called continuous glucose monitoring or CGM) is a technology that tracks your blood sugar levels throughout the day and night. Unlike traditional finger-stick blood tests that give you a reading at one moment in time, a CGM system provides readings every few minutes, creating a detailed picture of how your blood sugar changes over 24 hours.
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A CGM system has three main parts. First is a small sensor, about the size of a postage stamp, that you wear on your skin. Most commonly, people wear it on the back of their arm or abdomen. The sensor has a thin filament that sits just under your skin and measures glucose in the fluid surrounding your cells. Second is a transmitter, a small device that clips or adheres to the sensor and sends the glucose readings wirelessly to a receiver. Third is either a dedicated receiver device or your smartphone, which displays your current glucose level and shows trends—whether your blood sugar is rising, falling, or staying steady.
The sensor needs to be replaced every 7 to 14 days, depending on the brand. Most CGM systems are water-resistant, meaning you can shower or swim while wearing them. The sensor doesn't hurt to insert; most people describe only mild discomfort, similar to applying a sticker. Once in place, many users forget it's there.
For people with type 1 diabetes, type 2 diabetes, or prediabetes, a CGM provides information that wasn't previously available without frequent doctor visits. You can see patterns—for example, that your blood sugar always spikes after breakfast, or that evening walks help bring it down. This real-time information can help you and your healthcare team make decisions about diet, exercise, and medication adjustments.
Practical takeaway: A CGM is a wearable device that continuously measures your blood sugar using a small sensor under your skin and sends readings to a receiver or phone. It gives you detailed information about how your blood sugar changes throughout the day and night.
Types of Continuous Glucose Monitoring Systems Available
Several CGM systems are currently available in the United States, each with different features, costs, and requirements. The major systems are FreeStyle Libre, Dexcom, Medtronic Guardian, and Eversense. Each has been approved by the FDA for use, and each works in slightly different ways.
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The FreeStyle Libre system (made by Abbott) uses a sensor worn on the back of your arm. You scan the sensor with a reader device or your smartphone to get glucose readings. The sensor lasts 14 days. One advantage is that you don't need fingerstick calibrations—the system is calibrated at the factory. Many people find this convenient. The system is often among the more affordable CGM options and may be available without a prescription in some settings.
Dexcom systems (G6, G7, and newer models) use a sensor that you can wear on your abdomen or arm. The system sends readings automatically to a receiver or smartphone every 5 minutes. Dexcom systems require fingerstick calibrations with some models, though newer versions reduce this requirement. The Dexcom sensor lasts 10 to 14 days depending on the model. One feature many users appreciate is the ability to share readings with family members or caregivers through a smartphone app.
Medtronic Guardian is designed to work with insulin pump systems, though it can also be used on its own. The sensor is worn on the abdomen and sends readings every 5 minutes. Medtronic Guardian requires calibrations and can alert you if your blood sugar is predicted to go too high or too low. The sensor lasts 7 days.
Eversense is implanted under the skin on the back of your arm through a small procedure. It lasts up to 90 days, so you replace it much less often than other systems. You wear a transmitter on top of the implant. Because it requires a medical procedure to insert and remove, Eversense has different considerations than the other options.
Each system has different features regarding alarm alerts, sharing capabilities, smartphone compatibility, and how you calibrate the system. Your healthcare provider can discuss which system might fit your needs and lifestyle best.
Practical takeaway: Four main CGM systems are available: FreeStyle Libre, Dexcom, Medtronic Guardian, and Eversense. They differ in wear location, how often you scan or receive readings, how long the sensor lasts, and what features they offer. Talking with your healthcare provider can help you understand which options might work for your situation.
What Information a CGM Shows You
A CGM provides several types of information that help you understand your blood sugar patterns. The most basic information is your current glucose level, typically displayed as a number. Most systems show readings in milligrams per deciliter (mg/dL) in the United States, though some people use millimoles per liter (mmol/L) in other countries.
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Beyond the current number, a CGM shows you a glucose trend arrow. This arrow indicates whether your blood sugar is rising quickly, rising slowly, steady, falling slowly, or falling quickly. This trend information is powerful because it tells you the direction your blood sugar is heading, not just where it is right now. If you see your glucose rising steadily after a meal, you know to monitor it. If you see it falling, you might eat a small snack to prevent it from dropping too low.
Over time, a CGM creates a detailed log of your readings. You can look at a graph showing your blood sugar over the past few hours, past day, past week, or past month. This historical view reveals patterns you might not notice day-to-day. For example, you might see that your blood sugar is consistently high between 3 and 5 a.m., or that it tends to drop after you do a particular type of exercise. These patterns help you and your healthcare team understand what triggers your blood sugar changes.
Most CGM systems calculate statistics called "time in range" or TIR. This tells you what percentage of your readings fell within your target blood sugar range over a chosen time period. For example, a report might show you were in your target range 70% of the time over the past two weeks. Many healthcare providers use this metric to understand how well overall blood sugar control is working.
Advanced features on some systems include glucose variability measurements, which show how much your blood sugar swings up and down. They also include predicted highs and lows—the system can warn you that your blood sugar is predicted to drop below your target range in the next 15 to 30 minutes, even if it's still in range right now.
Practical takeaway: A CGM shows you your current glucose number, trend arrows showing which direction it's heading, historical patterns over hours or weeks, time-in-range statistics, and predictive alerts. This detailed information helps you see what affects your blood sugar and make real-time decisions about food, activity, and medication.
Cost, Insurance, and Access Considerations
The cost of CGM systems varies widely depending on the brand, your insurance coverage, and your location. The sensor itself typically costs between $30 and $100 per sensor when purchased without insurance. Since most sensors last 7 to 14 days and need to be replaced regularly, the annual cost of sensors alone can range from $1,500 to $5,000 per year. Some systems also require a separate receiver device, which can cost $300 to $500 initially.
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Insurance coverage for CGM systems has expanded significantly in recent years. Many private insurance plans cover CGM for people with type 1 diabetes. Coverage for type 2 diabetes and prediabetes varies by insurance plan and may depend on whether you use insulin. Medicare covers CGM for people with diabetes who use insulin, though the coverage details can vary by part of the Medicare program you're enrolled in. Medicaid coverage also varies by state.
If you have insurance that covers CGM, your out-of-pocket cost may be just a copay per sensor, or you might have a deductible to meet first. It's important to contact your insurance company directly to understand your specific coverage, as policies vary considerably. You can ask: Does your plan cover CGM? If so, for what conditions? What is the copay or coinsurance percentage? Is there a deductible? Do you need prior authorization from your doctor?
For people without insurance or whose