How does a continuous glucose monitor work? A CGM uses a hair-thin sensor wire inserted just under the skin to measure glucose in interstitial fluid. The wire is coated with the enzyme glucose oxidase, which reacts with glucose and generates a tiny electrical current. A transmitter sits on top of the skin and sends that reading to a phone or receiver by Bluetooth every 1 to 5 minutes.
The Three Physical Parts of a CGM
Every modern CGM has three components, though newer systems bundle them into one disposable patch:
| Part | What It Does | Location |
|---|---|---|
| Sensor wire | Sits in interstitial fluid, hosts the glucose oxidase reaction | Just under the skin, 3-5 mm deep |
| Transmitter | Converts the electrical signal and sends it by Bluetooth | On the skin surface, attached to the sensor |
| Receiver or app | Displays readings, trend arrows, and alerts | Dedicated receiver, smartphone, or smartwatch |
Older Dexcom G5 and G6 systems had a reusable transmitter that clicked onto a new sensor each wear cycle. Newer Dexcom G7, Stelo, and FreeStyle Libre 3 put the sensor and transmitter into a single all-in-one disposable applicator.
The Chemistry: Glucose Oxidase in Interstitial Fluid
The sensor wire is thinner than a human hair and is coated with the enzyme glucose oxidase (GOx). When glucose molecules from interstitial fluid reach the enzyme, the following reaction occurs:
Glucose + Oxygen -> Gluconolactone + Hydrogen peroxide
The hydrogen peroxide is oxidized at a nearby electrode, releasing electrons. The resulting electrical current is proportional to glucose concentration. A thin polymer membrane on the outside of the sensor filters out larger molecules and tunes the permeability so the reaction stays linear across the physiologic glucose range.
Because the sensor measures interstitial fluid instead of blood, there is a natural 5 to 15 minute lag during rapid glucose changes, such as after meals or insulin doses. This lag is built into CGM algorithms and is one reason readings may differ from a fingerstick during a spike or drop.
Calibration: Factory-Calibrated vs User-Calibrated
Early CGMs required users to perform two fingerstick blood glucose checks per day and enter them into the device to calibrate the sensor. Modern systems like Dexcom G7, FreeStyle Libre 2 and 3, and Dexcom Stelo are factory-calibrated, meaning the manufacturer uses in-house reference data to pre-set the algorithm. You insert the sensor, wait the warm-up period (30 minutes to 2 hours depending on the brand), and begin reading.
Confirmatory fingersticks are still recommended in specific situations: when symptoms do not match the sensor reading, during the first day of a new sensor, or before acting on a low reading.
Bluetooth Transmission and Apps
The transmitter broadcasts every minute to a nearby compatible phone or receiver over Bluetooth Low Energy. Apps like Dexcom G7, Dexcom Clarity, FreeStyle LibreLink, and Libre View display the current reading, a trend arrow, and a 24-hour graph. Data sync to the cloud so that clinicians and family members can view trends remotely using follower apps such as Dexcom Follow or Libre LinkUp.
If Bluetooth drops, most sensors continue recording internally and backfill data when the connection returns.
What the Trend Arrows Mean
Trend arrows are the feature that separates CGM from a fingerstick. They tell you the rate of change:
- Steady horizontal arrow: glucose changing less than 1 mg/dL per minute.
- Single diagonal: changing 1-2 mg/dL per minute.
- Double arrow: changing more than 2-3 mg/dL per minute.
Arrows help you anticipate a spike or drop before it happens. For example, a reading of 110 mg/dL with a double-down arrow warns you that 80 mg/dL is imminent.
Accuracy: What Is MARD?
CGM accuracy is reported using mean absolute relative difference (MARD), the average percent difference between sensor readings and lab blood samples. Lower is better. Modern systems report:
- Dexcom G7: about 8.2 percent adult MARD.
- FreeStyle Libre 3: about 7.9 percent MARD in labeled conditions.
- Dexcom Stelo: about 10 percent MARD (non-insulin users).
The FDA considers a MARD under 10 percent excellent and generally accepts MARD under 15 percent for non-adjunctive use (dosing insulin without a fingerstick). For background on glucose testing, see the A1C levels hub.
How Long Does a Sensor Last?
Wear time depends on the brand:
| Sensor | Wear Time | Warm-Up |
|---|---|---|
| Dexcom G7 | 10 days plus 12-hour grace period | 30 minutes |
| Dexcom Stelo | 15 days | 30 minutes |
| FreeStyle Libre 3 | 14 days | 60 minutes |
| Medtronic Guardian 4 | 7 days | 2 hours |
You remove the old sensor, apply a new one, and the system continues tracking.
Who Uses CGM and Why It Matters
CGMs were originally designed for type 1 diabetes and insulin-dependent type 2 diabetes. Over the past decade, use has expanded to non-insulin type 2 diabetes, gestational diabetes, prediabetes, and general metabolic health. According to the FDA, several CGMs are now cleared for over-the-counter use by adults without diabetes, including Dexcom Stelo and Abbott Lingo.
For context on prediabetes detection, see the prediabetes 101 hub.
The Bottom Line
How does a continuous glucose monitor work? A tiny enzyme-coated wire sits in interstitial fluid, converts glucose into an electrical current, and your phone receives a new reading every 1 to 5 minutes by Bluetooth. The technology is now accurate enough to use without routine fingerstick calibration and is quickly becoming standard tool for prediabetes, type 2 diabetes, and general metabolic awareness.
Medical disclaimer: This article is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your physician or qualified healthcare provider with any questions about a medical condition or medication.
