If someone on your care team has mentioned a CGM, or you have seen people wearing a small sensor on their arm, you may be wondering what it is and whether it is for you. You are not alone in asking. So, what is a CGM? It is a continuous glucose monitor: a small wearable device that checks your glucose around the clock and shows you the results on a phone or reader.
This guide walks through the basics in plain language. It is not about any one brand. It is about understanding the tool, so you can have a more confident conversation with your healthcare team.
What Is a CGM and What Does It Measure?
A traditional fingerstick meter measures glucose in a drop of blood at one moment in time. A CGM works differently. A tiny, flexible filament sits just under your skin and measures glucose in the interstitial fluid, the fluid that surrounds the cells in your tissue.
Most CGMs take a new reading every few minutes, day and night. Instead of a handful of snapshots, you get a steady stream of numbers that can be shown as a line on a graph. That line can help you and your care team see how your glucose moves through the day and night, not just where it is at a single moment.
Why CGM readings can lag a little
Glucose moves from your blood into interstitial fluid, and that takes some time. Because of this, a CGM reading may trail a few minutes behind your blood glucose. The gap usually matters most when your glucose is rising or falling quickly, such as after eating, during exercise, or when treating a low. This is one reason your device instructions and your care team may tell you when to double-check with a fingerstick.
How CGM Sensors, Readers, and Apps Work Together
Most CGM systems have a few basic parts. Designs differ by device, so always follow the instructions that come with yours.
- Sensor: A small device worn on the skin, often on the back of the upper arm or the belly, depending on the device. A thin filament goes just under the skin. Sensors are replaced on a schedule set by the manufacturer.
- Transmitter: Sends readings wirelessly. On some systems it is built into the sensor; on others it is a separate piece that clips on.
- Reader or app: Readings appear on a dedicated handheld reader, a smartphone app, or both. Some systems also connect with insulin pumps or smartwatches.
- Alerts: Many CGMs can sound or vibrate when glucose goes above or below levels you set with your care team, or when it is changing quickly.
- Data sharing: Many systems let you share data with your care team, and some let you share with a family member or caregiver. This can be helpful for parents, partners, and caregivers who help with day-to-day care.
Your screen will usually show your current glucose number, a graph of recent readings, and a trend arrow showing which way your glucose is heading. To learn what those arrows and reports mean, see our guide to understanding CGM trend arrows and reports.
CGM vs. Fingerstick Checks
A CGM and a fingerstick meter answer slightly different questions. A fingerstick tells you your blood glucose right now. A CGM shows you the bigger picture: the direction, the speed of change, and patterns over days and weeks.
| Fingerstick meter | CGM | |
|---|---|---|
| What it measures | Glucose in a drop of blood | Glucose in interstitial fluid under the skin |
| How often | Only when you check | Every few minutes, automatically |
| Shows direction of change | No | Yes, with trend arrows and graphs |
| Alerts for highs and lows | No | Yes, on many devices |
When fingersticks may still be needed
Even with a CGM, many people keep a meter on hand. Depending on your device and your care team's advice, a fingerstick may be recommended when:
- Your symptoms do not match the number on the screen.
- Your device instructions call for a check, such as during a sensor warm-up period or when a reading is not available.
- Your glucose is changing quickly and a decision needs to be made.
- Your sensor has fallen off, failed, or been damaged.
Some devices also require fingerstick calibration, while others do not. Your device guide and your care team are the best sources for when to check.
Who Uses a CGM?
CGMs are used by people with type 1 diabetes, by many people with type 2 diabetes, and in some other situations. The American Diabetes Association's Standards of Care recommend offering CGM to many people with diabetes who use insulin, and note that CGM may also be helpful for some people who do not.
That said, the right choice depends on you: your medications, your goals, your daily routine, and how you feel about wearing a device. Some people love the steady feedback. Others find the alerts or the constant data stressful at first. Both reactions are normal.
The best next step is simple: talk with your care team about whether a CGM fits you.
Talking With Your Care Team About a CGM
You do not need to know all the technical details to start the conversation. Here are a few questions you might bring to your next visit:
- Would a CGM be helpful for my type of diabetes and my current treatment?
- Which devices would work with my phone, my insulin pump (if I use one), or my routine?
- Will I still need fingerstick checks, and when?
- What alert levels should I set, and what should I do when an alert goes off?
- How will we look at my CGM data together, and how often?
- Is there a diabetes educator who can help me get started?
Diabetes self-management education and support (DSMES) programs can be a great place to learn hands-on skills like this. Ask your care team whether one is available to you. If you like to prepare in writing, the appointment prep worksheet in our free 7-Day Starter Pack can help you jot down questions ahead of time.
Questions to ask your insurance plan or pharmacy
Coverage for CGMs varies a lot between plans. A short phone call can save surprises later. You might ask:
- Is a CGM covered under my plan, and which devices are included?
- Is it covered as a pharmacy benefit or as durable medical equipment?
- Does my plan need prior authorization or specific paperwork from my doctor?
- How many sensors are covered each month, and what will my share of the cost be?
When to Talk to Your Care Team
Reach out to your healthcare team if you are curious about starting a CGM, if your CGM readings often do not match how you feel, if you get frequent low or high alerts, or if you have skin irritation where you wear the sensor. Never change your medication or insulin based on CGM data without guidance from your care team. If you have signs of a severe low, such as confusion or being unable to eat or drink safely, follow your emergency plan and get help right away.
A CGM is a tool, not a test you pass or fail. Used with support from your care team, it can give you a clearer picture of your days, so you can focus on living your life.
