What Is a Continuous Glucose Monitor (CGM) and How Does It Work?
Learn what CGM devices like FreeStyle Libre and Dexcom do, how people with diabetes use them, and how to read the data.
At a Glance
Talk to your doctor about whether a CGM is right for you
~20sDownload the companion app before applying the sensor
~19sQuick Tip
Make sure your phone's Bluetooth is turned on. The sensor communicates with your phone wirelessly.
Apply the sensor to your skin
~25sWarning
Apply sensors only to the body sites approved for your specific device. Using unapproved sites can give inaccurate readings.
Read your glucose level and trend arrow
~21sReview your glucose history and patterns
~17sYou Did It!
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A continuous glucose monitor, or CGM, is a small medical device that tracks your blood sugar levels throughout the day and night — without you having to prick your finger every few hours. It is one of the most significant advances in diabetes management in recent years.
How it works: A tiny sensor is applied to your skin (usually the back of the upper arm or the abdomen). The sensor has a very thin filament that rests beneath the skin and measures glucose in the fluid between your cells. It sends readings to a receiver device or directly to your smartphone, usually every one to five minutes.
The two most widely used CGMs in the United States are FreeStyle Libre (made by Abbott) and Dexcom (made by Dexcom Inc.). Both have smartphone apps that display your current glucose level, a trend arrow showing whether it is rising or falling, and a graph of the past several hours.
The trend arrow is especially important. A number of 140 mg/dL with a straight horizontal arrow means your sugar is steady. The same number with a rapidly rising arrow means it could be much higher in 15 minutes and you should act sooner. This real-time trend information is something finger sticks cannot give you.
CGMs require a prescription and are covered by Medicare Part B for people with diabetes who meet certain criteria. Many private insurance plans also cover them. Your doctor or endocrinologist sets up the prescription and trains you to apply the sensors correctly.
The sensors usually last 10–14 days before you replace them. They are generally water-resistant but check your specific device's guidelines.
Quick Tip: The Dexcom G7 and FreeStyle Libre 3 both send alerts to your phone if your glucose goes too high or too low — even while you are sleeping.
Important disclaimer: This guide is for informational purposes only. CGM devices are medical tools that require a prescription. They do not replace professional medical supervision. Always follow your doctor's instructions for managing your diabetes and interpreting your glucose readings.
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