Continuous glucose monitors (CGMs), explained

How a CGM works

A CGM has a small sensor with a thin, flexible filament that sits just under the skin. It measures glucose in the fluid around your cells (called interstitial fluid) every few minutes. A transmitter sends the readings to a phone app, a reader or, for some people, an insulin pump.⁴

Most CGM sensors are worn on the back of the upper arm or on the belly and are replaced every one to two weeks, depending on the brand. There are also implantable sensors. These are placed under the skin of the upper arm during a short in-office procedure and last for months rather than days. Capital Diabetes & Endocrine Associates lists placement of the Eversense implantable CGM at its Silver Spring office.

CGM versus fingerstick checks

A fingerstick meter measures glucose in a drop of blood at one moment. A CGM tracks glucose all day and night and shows which way and how fast it is moving, using trend arrows.

Because the CGM reads fluid around the cells rather than blood, its numbers can trail a few minutes behind a fingerstick, especially when glucose is rising or falling fast, such as after a meal or during exercise.⁴ Most modern CGMs don't need routine fingerstick calibration. Still, it's wise to check with a meter if:

• your symptoms don't match the CGM reading,
• the sensor is new and readings seem off, or
• your device's instructions tell you to confirm before treating.

Who can benefit from a CGM?

CGMs started as a tool mainly for people with type 1 diabetes. That has changed. In its Standards of Care in Diabetes—2026, the American Diabetes Association recommends offering CGM at diagnosis or at any time afterward to people with diabetes who may benefit, and not only to people who use insulin.¹ The ADA also notes that CGM is beneficial and recommended for people at high risk of low blood sugar.²

People who often find CGM helpful include those who:

• take insulin, whether multiple daily injections or a pump,
• have frequent lows, or lows they don't feel,
• want to see how food, activity, stress and sleep affect their glucose,
• find fingersticks hard to keep up with.

Insurance rules for CGM coverage vary by plan and may differ from guideline advice. The office team can help you understand what your plan needs.

Reading your CGM report

Many CGM apps create a standard one-page summary called an ambulatory glucose profile (AGP). It stacks many days on top of each other so patterns stand out. Key parts include:³

• Time in range (usually 70–180 mg/dL), with a common goal of more than 70% for many adults,
• time below range, with a common goal of less than 4% below 70 mg/dL,
• glucose management indicator (GMI), an estimate of A1C from CGM data,
• glucose variability, which shows how much readings swing.

Read more in Understanding your A1C and time in range.

Alerts: helpful, not overwhelming

Most CGMs can alert you when glucose is low, high, or changing quickly. Alerts can prevent dangerous lows, especially overnight. Too many alerts, though, can lead to "alarm fatigue." It's reasonable to work with your care team to set alert levels that keep you safe without constant buzzing. In general, low alerts deserve the most attention.

CGMs and insulin pumps

Some insulin pumps can connect to a CGM and adjust insulin automatically. These are called automated insulin delivery (AID) or "hybrid closed-loop" systems. The ADA Standards describe AID systems as improving glucose levels without increasing hypoglycemia, and the 2026 update removed several older requirements for starting pumps and AID in type 1 diabetes.²¹ Capital Diabetes & Endocrine Associates states that it works with all insulin pumps and continuous glucose monitors. You still play an active role: AID systems need meal announcements and regular sensor and infusion set changes.

Everyday tips

• Skin care: Clean, dry skin helps sensors stick. Rotate sites. If you notice redness or itching, let your care team know; barrier products or a different adhesive may help.
• Medicines: Some medicines and supplements can affect certain sensors. Check your device's instructions and tell your clinician what you take.
• Imaging: Many sensors must be removed before MRI, CT scans or some X-rays. Check the instructions or ask before your appointment.
• Sharing: Most apps can share data with your clinic and with family members you choose. This can make visits faster and more useful.

What about over-the-counter CGMs?

Some CGMs are now sold without a prescription. They may suit people who are curious about patterns, but they are not designed for everyone, and some are not meant for people who use insulin or are at risk for lows. If you have diabetes, talk with your clinician before relying on one to make treatment decisions.

This article is general education and does not endorse any brand. Device features and coverage change often, so check the current instructions for your device and plan.