A continuous glucose monitor (CGM) is a wearable sensor that measures glucose in interstitial fluid — the fluid between skin cells — every few minutes and streams the readings to a phone or reader. The FDA cleared the first over-the-counter CGM, Dexcom's Stelo, on March 5, 2024 for adults 18 and older who do not use insulin (regulatory, FDA).
How does the sensor actually measure glucose?
The measurement happens on a filament roughly the width of a hair, inserted just under the skin of the upper arm. The filament carries an enzyme that reacts with glucose in the interstitial fluid, and that reaction produces a small electrical current proportional to the glucose concentration. The sensor's electronics convert the current into glucose values on a fixed cadence.
The process, in order:
- The filament sits in interstitial fluid, where glucose diffuses from blood capillaries.
- The enzyme reaction on the filament surface generates an electrical current.
- The transmitter digitizes the current and sends readings wirelessly to a phone or receiver.
- The app converts the signal into glucose values and trend arrows using factory calibration.
The key physical fact is the lag: interstitial glucose trails blood glucose by roughly a quarter hour, which is why a CGM shows trends rather than instant blood values. That trade-off buys continuity — dozens of readings per day where fingersticks give a handful of instants, per a peer-reviewed review in Pharmacy and Therapeutics (Funtanilla et al., 2019).
What is the difference between a CGM and a fingerstick meter?
A fingerstick meter measures capillary blood glucose at one moment: apply blood to a strip, get a number. A CGM measures a different compartment, interstitial fluid, continuously. The clinical difference is pattern visibility. The Pharmacy and Therapeutics review describes CGM as a newer method for assessing glucose levels on a regular basis without the need for perpetual finger-sticks and myriad supplies, with an important role in assessing treatment efficacy and safety.
For people using insulin, that visibility is operational: trend arrows feed dosing decisions. For people not on insulin — the population the FDA addressed in 2024 — the value is educational, showing how specific meals, exercise, and sleep move glucose over hours. Same hardware, very different clinical stakes.
The hardware itself is deliberately disposable. A sensor is worn for a wear period and replaced, because the enzyme reaction and the skin interface both age; the reusable part is the transmitter and the phone software. That split — consumable sensor, durable electronics, software in the middle — is the business model of modern CGM, and it is why the category sustains subscription pricing that fingerstick strips never could.
It is also why accuracy is a system property rather than a sensor property. The number on the phone is the output of a chain: enzyme reaction, electrochemical measurement, factory calibration, and an app's smoothing algorithm. A weak link anywhere in that chain degrades the reading, which is why regulators clear the system as a unit rather than approving components separately.
Who can buy one, per the FDA record?
The March 5, 2024 clearance changed the access model. The FDA's announcement states the Dexcom Stelo system is intended for anyone 18 and older who does not use insulin — including people treating diabetes with oral medications and people without diabetes who want to understand how diet and exercise affect blood sugar. No clinician involvement is required to purchase.
The agency drew one boundary in the same clearance: the system is not for individuals with problematic hypoglycemia, because it is not designed to alert the user to that potentially dangerous condition. FDA device-center director Jeff Shuren framed the clearance as expanding access, noting it allows individuals to purchase a CGM "without the involvement of a health care provider," as the FDA announcement states. The boundary matters for buyers: over-the-counter wellness hardware is not a clinical alarm system.
What does integrated CGM actually mean?
The FDA's clearance language describes the Stelo system as an integrated CGM, or iCGM — a regulatory category rather than a marketing label. The category exists because not every continuous sensor is equally accurate or reliable, and the designation carries performance expectations a device must meet before it can be cleared. It is the difference between a regulated medical device and a generic wellness gadget that estimates trends.
That category is also what made over-the-counter sale possible. An iCGM clearance with a defined intended population gave the FDA a basis to allow purchase without a prescription for adults not using insulin, because the device's claims, accuracy envelope, and excluded uses were all fixed in the clearance record. The same logic explains the boundary the agency drew: a system not designed to alert on hypoglycemia is cleared for awareness, not protection, and the label says so.
For buyers comparing devices, the practical checklist comes straight from the record: check that the device is cleared as an iCGM, read the intended-user statement, and check the exclusions before relying on any single reading. The clearance record is public, and it is more precise than any product page.
One more distinction from the clearance language is worth carrying: the FDA describes the intended user as someone who does not use insulin. That is not a demographic detail — it is the line between a wellness information device and a clinical management tool. Crossing it, for a buyer with an insulin regimen, means using a tool outside the population it was cleared for, whatever the marketing implies.
What did CGMs change in diabetes care?
The scale of the problem is documented in the review: more than 9 percent of the U.S. population — 30.3 million people — had diabetes according to the CDC's 2017 statistics report cited by Funtanilla and colleagues. Against that base, replacing perpetual fingersticks with a wearable stream changed daily management for insulin users first, then expanded outward.
The market followed the clinical logic. Prescription CGMs became standard companions for type 1 diabetes care, then extended to insulin-using type 2 patients, and the 2024 over-the-counter clearance opened the wellness market entirely. Each step widened the user base while narrowing the medical claim — a regulatory pattern that repeats across consumer health hardware.
What can't a CGM do?
The record is specific about limits. Over-the-counter units are not designed to alert on low blood sugar, so they cannot replace clinical monitoring where hypoglycemia risk exists. Readings lag blood by the interstitial delay, so rapid changes appear later than a fingerstick would show them. And a glucose curve is data, not a diagnosis — the FDA's clearance language consistently pairs access expansion with explicit statements of what the system is not intended for. Buyers who read only the marketing and not the label are exactly the people the caveats were written for.
