How Do Continuous Glucose Monitoring Devices Work and Reach the Market?

A continuous glucose monitor is a wearable sensor that measures glucose in interstitial fluid around the clock and streams values to a smartphone app. The category hit a regulatory milestone on March 5, 2024, when FDA cleared Dexcom's Stelo as the first over-the-counter continuous glucose monitor, per the agency's press release.

What does a CGM actually measure?

A CGM does not measure blood sugar directly. A small filament sensor inserted just under the skin sits in interstitial fluid, where glucose concentration tracks blood glucose with a lag of several minutes. An enzyme-based electrochemical reaction on the sensor generates a current proportional to glucose concentration, and the transmitter sends readings on a fixed schedule to a display device. For the Stelo system, FDA's clearance announcement states that the device presents blood glucose measurements and trends every 15 minutes in the accompanying app, and that users can wear each sensor up to 15 days before replacement.

The signal chain is where the regulatory risk lives. Sensor accuracy, calibration strategy, and the algorithm that smooths raw current into a glucose value all determine whether the system meets accuracy criteria for its category. Interstitial lag also means CGMs are not equivalents of blood glucose meters during rapid glucose changes, which is why labeling constrains who should use any given system.

What is an integrated CGM, and why does the category matter?

The integrated CGM, or iCGM, is a device category FDA created for systems designed to connect safely with other devices, such as insulin pumps and dosing algorithms. iCGM systems must meet special controls on accuracy and reliability so that downstream devices can rely on their signal. The Stelo clearance was for an iCGM, which means the category now spans both prescription-connected systems and consumer wearables.

The iCGM category itself was established through FDA's De Novo classification process, the pathway for novel devices of low-to-moderate risk that have no legally marketed predicate. Under the De Novo classification request framework, a granted request classifies the device into class I or II and creates a predicate that later 510(k) submissions can cite. That is the standard pattern in medtech: one novel device establishes a category, and followers enter through predicate-based 510(k) clearances.

Who is the over-the-counter version for?

The OTC indication is deliberately narrow. FDA's announcement states the Stelo system is intended for adults 18 and older who do not use insulin, including people managing diabetes with oral medications and people without diabetes who want to understand how diet and exercise affect glucose levels. The agency was explicit that the system is not for individuals with problematic hypoglycemia, because it is not designed to alert the user to that condition.

That boundary reflects what an OCGM can and cannot do. A wellness-oriented wearable without hypoglycemia alarms cannot substitute for a prescription CGM in insulin-using patients, where low-glucose alerts are a safety function. FDA's then-CDER-and-CDRH framing in the announcement emphasized access: the clearance lets individuals purchase a CGM without involving a health care provider, which the agency described as a step forward in health equity.

How do these devices get cleared?

Most CGM line extensions follow a short, predictable sequence.

1. Design inputs: accuracy targets, wear duration, and intended user population are fixed against the chosen category's controls.
2. Bench and clinical performance studies against a reference method, typically YSI laboratory glucose analysis.
3. Submission, either a 510(k) demonstrating substantial equivalence to a predicate or a De Novo request if the category is new.
4. FDA review and clearance with labeling that defines the indicated population.
5. Postmarket surveillance, including adverse event reporting through the manufacturer's obligations.

What should professional readers take away?

The CGM story is a case study in how device categories evolve. A prescription-only monitoring tool became a platform category, and the category then split into clinical and consumer branches, each with its own labeling. The numbers that define the branch points are in the FDA documents, not in marketing claims: 15-day wear, readings every 15 minutes, adults not on insulin, no problematic hypoglycemia. For anyone evaluating a new entrant, the clearance letter and indicated population are the first documents to read.

How do prescription and OTC monitors differ in practice?

The split now runs through the middle of the category. Prescription iCGMs are indicated for insulin-using patients and integrated with alarms and, in some systems, automated insulin delivery. OTC systems are indicated for adults not on insulin, and their labeling excludes people with problematic hypoglycemia because the hardware does not promise alerts for dangerous lows. The Stelo clearance language draws that line explicitly, which means the two branches are not substitutes.

The user interface follows the indication. A prescription CGM is a disease-management tool whose data feeds clinical decisions. A consumer CGM is a behavior-feedback device: it shows glucose responses to meals, exercise, and sleep so users can adjust lifestyle. Regulators treat the difference seriously because the populations differ in consequence, not just in intention.

For manufacturers, the branch point defines the evidence package. A consumer submission leans on usability and clear labeling for an untrained population; a prescription submission leans on clinical accuracy in the indicated disease population. Getting the population wrong is the most expensive mistake in the category, because labeling fixes who may buy the product at all.

What do the cleared specs actually say?

The cleared specifications tell more than marketing materials do.

Read as a set, the specs describe a wellness monitoring device with clinical-grade measurement at its core. The 15-minute cadence is slower than alarm-capable prescription systems, which stream continuously to support alerts, and that difference is one more expression of the same population boundary.

What should readers watch in this category?

Three developments define the near term. First, wear time and accuracy targets continue to move, and each clearance that extends duration resets the predicate conversation for followers. Second, the OTC branch is expanding indication by indication, and each new population is a separate regulatory decision rather than an extension of the last. Third, connectivity matters: iCGM classification exists precisely so other devices can build on the glucose signal, and interoperability obligations are part of what the category buys.

The documentary trail remains the fastest way to evaluate any claim in this market: the clearance letter states the population, the specs, and the limitations in FDA's own words. Anything not in that letter is marketing until shown otherwise, and the gap between the two is where most category confusion originates.

The category's trajectory is also a lesson in how regulation follows engineering. Sensors improved first, alarms and connectivity followed, and only after prescription systems matured did a consumer branch open with its own population and its own labeling. Each step left a documentary trail: a De Novo classification that created the iCGM category, predicate clearances that widened it, and an OTC clearance that defined its consumer edge. Readers who follow those documents, rather than launch announcements, will see the next branch point before the press release arrives.