Imagine knowing your blood sugar is dropping while you’re asleep-before you even feel dizzy or sweaty. That’s not science fiction. It’s what a continuous glucose monitor does every night for millions of people with diabetes. Unlike fingerstick tests that give you a single snapshot, CGMs track your glucose levels 24/7, showing not just where you are, but where you’re headed. This isn’t just convenience-it’s a life-saving tool that’s changing how diabetes is managed.
How a CGM Actually Works
A CGM doesn’t measure blood sugar directly. Instead, it reads glucose in the fluid between your cells-called interstitial fluid. A tiny sensor, about the size of a coin, is inserted just under your skin, usually on your belly or arm. Inside that sensor is a filament coated with glucose oxidase, an enzyme that reacts with glucose and creates a small electric signal. That signal gets converted into a glucose reading.
The sensor sends that data wirelessly to a receiver or your smartphone every 1 to 5 minutes. Modern devices like the Dexcom G7 is a real-time CGM that measures glucose every 5 minutes and requires twice-daily calibration. Also known as Dexcom Continuous Glucose Monitor, it was FDA-cleared in 2022 and has a mean absolute relative difference (MARD) of 9.1% compared to lab blood tests. The FreeStyle Libre 3 is a flash CGM that measures glucose every minute, requires no calibration, and transmits data via NFC. Also known as Abbott Libre 3, it received FDA approval in July 2022 with a MARD of 7.9%. And then there’s the Eversense E3 is an implantable CGM with a sensor that lasts up to 180 days and sends alerts via vibration. Also known as Eversense Continuous Glucose Monitoring System, it requires minor surgery for insertion and was approved by the FDA in August 2022.
Here’s the catch: there’s a lag. Glucose in your interstitial fluid doesn’t change as fast as glucose in your blood. During a rapid drop-like after a workout or insulin dose-it can take 5 to 20 minutes for the CGM to catch up. That’s why you’re still told to check with a fingerstick if you feel symptoms that don’t match your CGM reading.
Real-Time vs. Flash Monitoring: What’s the Difference?
Not all CGMs are created equal. There are two main types: real-time and flash.
Real-time CGMs like Dexcom G7 and Eversense E3 send data automatically. They beep or vibrate when your glucose is going too high or too low-even if you’re not looking at your phone. Some can even pause insulin delivery in a hybrid closed-loop system. These are ideal for people who need constant alerts, especially overnight.
Flash monitors like FreeStyle Libre 3 don’t send data unless you scan them with your phone or reader. No alarms, no automatic updates. But they’re cheaper, easier to use, and don’t require fingerstick confirmations anymore. You get a full picture when you choose to check.
Here’s how they stack up:
| Device | Measurement Frequency | Sensor Life | Calibration Needed? | Alerts | Monthly Cost (US, no insurance) |
|---|---|---|---|---|---|
| Dexcom G7 | Every 5 minutes | 10 days | Yes, twice daily | Yes, customizable | $399 |
| FreeStyle Libre 3 | Every 1 minute | 14 days | No | No automatic alerts | $110 |
| Eversense E3 | Every 5 minutes | 180 days | No | Vibration only | $650 (includes implant) |
The FreeStyle Libre 3 is the most affordable and easiest to use. The Dexcom G7 is the most reliable for alerts and integration with insulin pumps. Eversense is for people who hate changing sensors every two weeks-but you need a doctor to put it in and take it out.
Who Benefits Most From a CGM?
If you have type 1 diabetes, a CGM isn’t optional anymore. The American Diabetes Association and the American Association of Clinical Endocrinologists both say everyone with type 1 should use one. Why? Because studies show people using CGMs reduce severe low blood sugar episodes by 31% and lower their A1C by 0.5% to 0.8% on average.
But it’s not just for type 1. People with type 2 diabetes who take multiple insulin injections daily also benefit. If you’re on insulin and still getting unpredictable highs or lows, a CGM gives you the data to fix it. One user on Reddit said, “My Libre 3 alarm saved me from a 32 mg/dL hypo at 3 AM last week-I’d have been dead without it.” That’s not rare. It’s common.
Even non-diabetics are starting to use CGMs for wellness. Athletes track how food affects their energy. People with prediabetes use them to see how lifestyle changes impact glucose spikes. But insurance won’t cover CGMs for non-diabetics-yet. Right now, that’s out-of-pocket only.
The Real Game-Changer: Trend Arrows
Most people think the number on the screen is the most important thing. It’s not. The trend arrow is.
That little up, down, or sideways arrow tells you if your glucose is rising fast, falling fast, or staying steady. A number of 85 mg/dL might sound fine-but if the arrow is pointing down sharply, you’re headed for a low in 20 minutes. That’s the difference between a normal night and an emergency.
Dr. Anne Peters, a leading diabetes expert, says, “The trend arrow showing how quickly glucose levels are changing is arguably more important than the current number itself.” That’s why CGMs aren’t just monitors-they’re predictive tools.
Problems and Pitfalls
CGMs aren’t perfect. Skin irritation is the #1 complaint, especially with Dexcom’s adhesive. Some users solve it with Skin Tac wipes or medical tape like Opsite Flexifix. Others just switch to Libre, which has a gentler adhesive.
False alarms happen too. One user reported their Dexcom showed 50 mg/dL while they were cycling-when their fingerstick was 110. That’s because during exercise, blood flow changes and the lag gets worse. The device wasn’t wrong-it just didn’t catch up yet.
And then there’s acetaminophen (Tylenol). Most CGMs over-read glucose when you take it. If you’re on pain meds and your CGM spikes, don’t panic-check with a fingerstick.
Bluetooth interference is another issue. Some users report connection drops with Dexcom G7, especially in crowded areas or near microwaves. It’s rare, but it happens.
Cost and Insurance: What You Really Pay
Without insurance, a CGM can cost $300 to $400 a month. That’s just the sensors. The transmitter or receiver might cost extra. But insurance changes everything.
In the U.S., Medicare and most private insurers cover CGMs for people on insulin. Dexcom and Medtronic are usually covered; FreeStyle Libre was only added in 2023. In the UK, the NHS gives FreeStyle Libre 2 to over half a million people-free. In Australia, the PBS subsidizes CGMs for eligible type 1 patients, but out-of-pocket costs still run $50-$100 per month.
For people without coverage, the Libre 3 is the most affordable option. At $110 a month for three sensors, it’s less than half the price of Dexcom. And since it doesn’t need calibration or fingersticks, it’s simpler to use.
What’s Next for CGMs?
The next wave is even smarter. Dexcom’s G7 already cut warm-up time from two hours to 30 minutes. Abbott’s Libre 3 Plus now tracks insulin doses right on the app. And companies are testing non-invasive sensors-think a patch that reads glucose through your skin without a needle.
Apple is rumored to be working on a glucose-sensing smartwatch. If it happens, it could make CGMs mainstream overnight. But for now, the best tech is already here.
Hybrid closed-loop systems-like Tandem’s Control-IQ-are getting closer to an artificial pancreas. These systems use CGM data to automatically adjust insulin doses, day and night. In trials, users spent 70% more time in their target glucose range.
One 2023 study in the New England Journal of Medicine followed 15,000 CGM users for a year. They had 24% fewer hospital visits because of diabetes-related emergencies. That’s not just better health-it’s lower costs, less stress, and more freedom.
Getting Started
If you’re thinking about a CGM, talk to your doctor first. You’ll need a prescription. Most clinics offer training on how to insert the sensor, interpret the arrows, and respond to alerts. The first week is the hardest-most people feel overwhelmed by the data. But within 3 to 5 days, it becomes second nature.
Use the apps. Dexcom Clarity and LibreView turn numbers into charts you can share with your care team. Join online communities like the 45,000-member CGM Users Facebook group. People there share tips, troubleshoot problems, and celebrate wins.
Don’t expect perfection. CGMs aren’t lab tests. They’re tools to help you make better decisions. Learn the lag. Trust the trend. And when in doubt-check with a fingerstick.
Can CGMs replace fingerstick tests completely?
For most people using modern CGMs like FreeStyle Libre 3 or Dexcom G7, fingersticks are no longer required for daily decisions. But you should still check with a fingerstick if your symptoms don’t match your CGM reading, if you’re having rapid glucose changes (like after exercise), or if you suspect the device is wrong. CGMs measure interstitial fluid, not blood, and there’s always a small delay.
Are CGMs covered by insurance?
In the U.S., most insurance plans cover CGMs if you have type 1 diabetes and use insulin. Some plans now cover them for type 2 diabetes on insulin too. Medicare began covering Libre devices in 2023. In countries like the UK and Australia, government programs subsidize or fully cover CGMs for eligible patients. Always check with your insurer-coverage varies by plan and device.
Can I use a CGM if I don’t have diabetes?
Yes, but you’ll pay out of pocket. Many people without diabetes use CGMs to understand how food, sleep, and stress affect their glucose. Athletes, people with prediabetes, and those managing metabolic health find value in the data. However, insurance won’t pay for it unless you have a diagnosed condition like diabetes or prediabetes with insulin use.
How accurate are CGMs compared to blood tests?
Modern CGMs are very accurate. The FreeStyle Libre 3 has a MARD (mean absolute relative difference) of 7.9%, meaning its readings are, on average, within 7.9% of a lab blood test. Dexcom G7 is at 9.1%. For comparison, traditional glucometers have a MARD of 10-15%. CGMs aren’t perfect, but they’re reliable enough for daily decisions.
How long does a CGM sensor last?
Sensor life varies by device. Dexcom G7 lasts 10 days, FreeStyle Libre 3 lasts 14 days, and Eversense E3 lasts up to 180 days. Most sensors are designed to be worn continuously until they expire. Some users extend wear beyond the recommended time, but accuracy drops after the labeled duration, and skin irritation increases.
Do CGMs work during exercise?
Yes, but with a caveat. During intense exercise, blood flow changes can cause a lag between blood glucose and interstitial fluid glucose. This means CGMs may show a drop when you’re actually stable-or miss a rapid rise. Many athletes use CGMs to track trends over time rather than rely on single readings during activity. Always verify with a fingerstick if you feel symptoms.
Can CGMs help prevent nighttime lows?
Absolutely. Nighttime hypoglycemia is one of the biggest dangers for people with diabetes. CGMs with alarms can wake you up before your glucose drops dangerously low. Studies show users reduce nighttime lows by up to 40%. Many users say their CGM has saved their life during sleep. That’s why real-time CGMs with customizable alerts are so valuable.
Final Thoughts
A CGM isn’t just a gadget. It’s a window into your body’s rhythm. It tells you how your food, sleep, stress, and insulin interact-not just in the moment, but over hours and days. For people with diabetes, it’s the closest thing to having a personal health coach living in your pocket.
The technology has come a long way since 2006. Today’s devices are smaller, smarter, and more affordable. The data they give you isn’t just numbers-it’s control, peace of mind, and freedom.
If you’re managing diabetes, especially with insulin, a CGM isn’t a luxury. It’s a necessity. And if you’re curious about your glucose patterns-even if you don’t have diabetes-it’s worth exploring. The body speaks in trends. CGMs help you listen.