How continuous glucose monitors work
A CGM reads glucose from the fluid under your skin every few minutes. Knowing what it measures explains why it sometimes disagrees with a fingerstick.
Kivorent News desk, , 2 min read
What to know
- A CGM measures glucose in the fluid between your cells, through a thin filament under the skin.
- Its readings trail blood glucose by several minutes, most noticeably when levels are changing quickly.
- The direction and speed of change are often more useful than the single number on the screen.
A continuous glucose monitor, or CGM, is a small sensor worn on the body, usually on the back of the arm, that reports glucose levels around the clock. It was developed for people with diabetes, and versions sold without a prescription have been available in the United States since 2024.
What is under the patch
When you apply a sensor, a thin, flexible filament a few millimeters long is placed just under the skin. It does not sit in a blood vessel. It sits in interstitial fluid, the liquid that surrounds your cells.
The filament is coated with an enzyme that reacts with glucose. The reaction produces a tiny electrical current, and more glucose produces more current. The sensor converts that current into a glucose estimate and sends it to your phone or a reader, typically every one to five minutes. A sensor is worn for about 10 to 15 days, depending on the product, and then replaced.
Why it lags behind a fingerstick
A fingerstick meter measures glucose in blood. A CGM measures it in interstitial fluid. Glucose reaches the blood first and takes several minutes to move into the surrounding fluid.
When your level is steady, the two agree closely. When it is rising after a meal or falling during exercise, the CGM runs behind. A difference between the two readings in those moments is expected and is not a sign of a broken sensor.
Reading the display
- The number is the current estimate.
- The arrow shows the direction and speed of change. A steep arrow tells you the number is about to be different.
- The graph shows the pattern over hours: how high a meal took you and how long it took to come back.
- Time in range, in many apps, reports the share of the day spent inside a target band.
Things that throw readings off
- The first day. Many sensors are less accurate for the first several hours after insertion.
- Pressure. Sleeping on the sensor can squeeze fluid away from it and produce a false low.
- Dehydration.
- Some medications and supplements. Each product's instructions list the ones that interfere with it.
Prescription and over-the-counter models
Prescription CGMs for diabetes management can include urgent low-glucose alarms and can connect to insulin pumps. Over-the-counter models are intended for adults who do not use insulin and generally leave those features out. Check which kind you have before relying on any alert.
What a CGM is not
A CGM estimates glucose. It does not diagnose diabetes or any other condition. If you use insulin or another glucose-lowering medication, follow your care team's instructions about when to confirm a reading with a fingerstick before acting on it.
New rules this year changed what noninvasive wearables can say about glucose. See what the FDA's 2026 wellness rules let your wearable tell you.
This article is general information about how technology works. It is not medical advice. Talk to a clinician about your own health.
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