A continuous glucose monitor (CGM) is a small wearable device that measures glucose levels throughout the day and night, without the repeated finger pricks that traditional testing requires. Instead of a single reading at one moment, a CGM shows a continuous stream of glucose values, along with the direction and speed they are changing. This guide explains how a CGM works, how it attaches to the body, and how it compares to a glucometer and finger-prick testing.
How Does a CGM Work?
A CGM works by measuring glucose in the interstitial fluid, the fluid that surrounds the cells just beneath the skin, rather than directly in the blood. A tiny, flexible sensor filament sits under the skin and reacts with glucose in the interstitial fluid, producing a small electrical signal that rises and falls with glucose levels. A transmitter attached to the sensor sends these readings wirelessly, usually over Bluetooth, to a smartphone app or a dedicated receiver.
The device takes a reading every one to five minutes, which adds up to hundreds of measurements a day. The app or receiver turns those readings into a live glucose value, a trend arrow showing whether glucose is rising, falling, or steady, and a graph of the hours and days behind it. This continuous picture is what separates a CGM from a single-point test: it shows not just where glucose is, but where it is heading.
Two details are worth understanding. First, because a CGM measures interstitial fluid rather than blood, its readings lag real-time blood glucose by a few minutes, which matters most when glucose is changing quickly. Second, most modern CGMs are factory-calibrated and require no finger-prick calibration, though some older or specific systems still require occasional finger-prick checks. A sensor is typically worn for 10 to 15 days before being replaced.
How Does a CGM Attach to the Body?
A CGM attaches using an applicator that places the sensor filament just under the skin in a quick, nearly painless step, while an adhesive patch holds the sensor and transmitter in place on the surface. Depending on the system, the sensor is worn on the back of the upper arm, the abdomen, or the upper buttock. The patch is designed to stay attached through showering, exercise, and daily activity for the full wear period, and the insertion filament is thin and flexible enough that most users do not feel it once it is in place.
CGM vs Glucometer: What Is the Difference?
The difference between a CGM and a glucometer comes down to continuous versus single-point measurement. A glucometer, also called a blood glucose meter (BGM), measures glucose from a single drop of blood, usually obtained via a finger prick, providing a precise reading at that moment. A CGM measures glucose continuously via a sensor placed under the skin, showing the current level and the trend over time. A glucometer answers “what is my glucose right now”; a CGM answers that plus “where has it been and where is it going.”
CGM vs glucometer (BGM) at a glance:
CGM | Glucometer (BGM) | |
Measures from | Interstitial fluid, under the skin | A drop of blood, usually a finger prick |
Frequency | Continuous, every few minutes | One reading at a time, on demand |
Shows trends | Yes, with direction and history | No, single value only |
Finger pricks | Few or none | Every reading |
Best for | Ongoing monitoring and patterns | A precise single check or confirmation |
Neither replaces the other entirely. Many people use a CGM for continuous monitoring and keep a glucometer for occasional confirmation, and some regulated situations still call for a blood glucose reading.
CGM vs Finger Prick: Do You Still Need to Prick Your Finger?
A finger prick is the method a glucometer uses, so CGM versus finger prick is really the same comparison from the user’s point of view: a CGM greatly reduces, and often eliminates, the routine finger pricks that traditional testing requires. Instead of pricking a finger several times a day, a CGM user glances at an app. That said, some CGMs still ask for an occasional finger-prick reading for calibration, and a finger prick remains a useful confirmation when a CGM reading does not match how a person feels, or when glucose is changing very fast and the interstitial-fluid lag matters.
Adjunctive vs Non-Adjunctive CGM: What Is the Difference?
The difference between an adjunctive and a non-adjunctive CGM is whether its readings can be used on their own to make treatment decisions. An adjunctive CGM is intended to supplement finger-prick testing, so a confirmatory blood glucose reading is still required before acting on a value, for example, before dosing insulin. A non-adjunctive CGM is cleared to be used on its own for treatment decisions, without a confirmatory finger prick.
This distinction reflects how the device is regulated and how much its readings can be relied on alone. Most current mainstream CGMs are non-adjunctive, which is part of why they have replaced routine finger-prick testing for so many users. The category of highly accurate CGMs cleared for standalone use is sometimes referred to by regulators as integrated CGM, or iCGM.
Dexcom vs FreeStyle Libre: How Do the Leading CGMs Compare?
Dexcom and the FreeStyle Libre, made by Abbott, are two of the most widely used CGM families, and they work on the same underlying principle while differing in some details. Dexcom systems stream glucose readings continuously to a phone or receiver with configurable high and low alarms. The FreeStyle Libre began as an intermittently scanned system, where the user scanned the sensor to see a reading, and its newer generations now stream continuously like Dexcom. Sensor wear time and exact features vary by generation, and both families now include non-adjunctive systems cleared for treatment decisions without a finger prick.
For an individual, the better choice usually comes down to specific features, sensor wear time, alarm behavior, app experience, and insurance coverage, rather than differences in core technology, which is very similar between them.
Frequently Asked Questions
How does a CGM measure glucose?
A CGM measures glucose in the interstitial fluid beneath the skin using a small sensor filament. The sensor produces an electrical signal that varies with glucose levels, and a transmitter sends readings wirelessly to an app or receiver every few minutes.
Does a CGM measure blood sugar directly?
Not directly. A CGM measures glucose in the interstitial fluid around the cells, not in the blood itself. Because of this, readings lag blood glucose by a few minutes, which is most noticeable when glucose is changing quickly.
How long does a CGM sensor last?
Most CGM sensors are worn for ten to fifteen days before being replaced, depending on the system. The sensor stays attached with an adhesive patch during daily activities, showering, and exercise throughout the full wear period.
Is a CGM better than a finger prick?
A CGM and a finger prick serve different purposes. A CGM gives continuous data and trends without repeated pricks, which many people find more convenient and informative, while a finger prick gives a precise single reading useful for calibration or confirmation.
Continuous glucose monitors have changed diabetes management by turning a single daily number into a continuous, trend-rich picture of glucose. Behind every CGM is a software layer that turns raw sensor signals into the readings, alerts, and trends people rely on. Sequenex builds that software for the companies developing connected medical devices, including CGM and biosensor systems.
