Continuous glucose monitor
A continuous glucose monitor (CGM) is a wearable medical device designed to automatically measure and display blood glucose levels in real time by detecting glucose concentrations in the interstitial fluid beneath the skin, offering frequent updates—typically every 1 to 15 minutes—to support effective diabetes management throughout the day and night.[1][2][3] CGMs consist of three main components: a small sensor filament inserted subcutaneously, usually in the abdomen or arm, which measures interstitial glucose; a transmitter attached to the sensor that wirelessly sends data; and a receiver, dedicated display device, or smartphone application that shows current glucose values, trends, and alerts for highs or lows.[1][4] Unlike traditional fingerstick blood glucose meters, which provide only intermittent snapshots, CGMs generate approximately 288 readings per day, enabling users to identify patterns in glucose fluctuations and adjust insulin, diet, or activity accordingly.[4][3] Approved for use in people with type 1 and type 2 diabetes, as well as during pregnancy including gestational diabetes, and for other insulin-dependent individuals, CGMs have demonstrated significant clinical benefits, including improved hemoglobin A1c levels, reduced incidence of severe hypoglycemia, and better time in target glucose range.[2][4][5] Popular systems include the Dexcom G6 and G7, which require no fingerstick calibrations and are suitable for ages 2 and older; the FreeStyle Libre series, featuring factory-calibrated sensors lasting up to 15 days;[6] and integrated pump-CGM hybrids like the Medtronic MiniMed and Tandem t:slim, which enable automated insulin delivery.[4] As of 2025, advancements in CGM technology, including 15-day sensor options, enhanced accuracy during hypoglycemia, and integration with artificial intelligence for predictive analytics, continue to expand access and efficacy across diverse patient populations.[7][8][9] Despite these advantages, CGMs are not without limitations; sensors must be replaced every 7 to 15 days for most models, potential skin irritation or adhesion issues can occur, and while costs have decreased with insurance coverage expansions, equitable access remains a challenge for underserved communities.[1][2][6] Ongoing research and advocacy efforts focus on broadening reimbursement policies and developing longer-wear implantable options to further democratize this transformative tool in diabetes care.[2][7]Definition and principles
Overview of CGM
A continuous glucose monitor (CGM) is a wearable medical device designed to measure glucose concentrations in the interstitial fluid surrounding cells, providing automated readings typically every 1 to 15 minutes for real-time or retrospective analysis of glucose levels.[10] This technology allows for frequent, minimally invasive monitoring of glycemia without the need for repeated manual interventions.[3] The primary purpose of CGM is to support diabetes management by monitoring trends in glucose levels, enabling users to identify patterns and take proactive steps to avoid episodes of hypoglycemia (low blood sugar) or hyperglycemia (high blood sugar).[11] Studies have shown that CGM use can reduce time spent in hypoglycemic and hyperglycemic ranges, thereby improving overall glycemic control and reducing associated health risks.[12] In contrast to traditional fingerstick blood glucose monitoring, which involves discrete capillary blood samples obtained via lancet pricks, CGM derives measurements from interstitial fluid rather than blood, offering a continuous stream of data that captures fluctuations throughout the day and night.[13] The basic workflow of a CGM system begins with the subcutaneous insertion of a sensor, followed by ongoing data collection; this information is then wirelessly transmitted to a dedicated receiver, smartphone, or other display device, where users access current readings, customizable alerts for out-of-range values, and graphical trend analyses.[10]Glucose measurement mechanisms
Continuous glucose monitors (CGMs) primarily measure glucose levels in interstitial fluid rather than blood, as the sensor is typically implanted subcutaneously. Glucose diffuses from the bloodstream into the interstitial space through capillary walls, a process governed by concentration gradients and Fick's laws of diffusion. This diffusion introduces a physiological lag time of approximately 5-10 minutes between blood glucose changes and corresponding interstitial glucose levels, which can affect the timeliness of readings during rapid glucose fluctuations.[14][15] The core detection mechanism in most CGMs is enzymatic electrochemical sensing, relying on the glucose oxidase (GOx) enzyme immobilized on a working electrode. Glucose in the interstitial fluid reacts with GOx in the presence of oxygen to produce gluconolactone and hydrogen peroxide (H₂O₂), as described by the reaction:\ce{glucose + O2 + H2O ->[GOx] gluconolactone + H2O2}.
The H₂O₂ is then electrochemically oxidized at the electrode surface (typically at +0.6 V vs. Ag/AgCl), generating electrons that produce a measurable current proportional to the glucose concentration. In some designs, electron mediators facilitate direct electron transfer from GOx to the electrode, reducing interference from oxygen limitations.[16][17] This current output follows the steady-state diffusion-limited equation for amperometric sensors: I = n F A \frac{D C}{\delta}, where I is the measured current, n is the number of electrons transferred per glucose molecule (typically 2 for H₂O₂ oxidation), F is the Faraday constant (96,485 C/mol), A is the electrode surface area, D is the diffusion coefficient of glucose or H₂O₂, C is the glucose concentration, and \delta is the thickness of the diffusion layer near the electrode. This relationship ensures the sensor signal scales linearly with glucose levels under controlled conditions. Alternative measurement methods, though less common in commercial CGMs, include optical approaches such as fluorescence-based sensing, where glucose-binding dyes change emission properties, or spectroscopic techniques like near-infrared absorption to detect glucose-specific spectral shifts noninvasively. Amperometric methods without enzymes, using direct electrocatalytic oxidation of glucose on nanostructured electrodes, are also under investigation to improve stability and eliminate enzyme degradation.[18][16]