The promise of a smartwatch that can monitor blood sugar without a painful needle prick has captured immense public interest. For individuals managing diabetes, a non-invasive continuous glucose monitor (CGM) would be a life-changing convenience. This high level of demand has fueled a race among technology companies to develop a device that can accurately track glucose levels from the wrist. The central question remains whether these devices are currently reliable enough to be trusted for medical decisions.
How Non-Invasive Glucose Monitoring is Attempted
The challenge of non-invasive glucose monitoring stems from the need to measure a substance deep within the body without breaking the skin barrier. Unlike traditional methods that analyze blood or interstitial fluid, smartwatches must rely on indirect measurements taken from the skin’s surface. Researchers are exploring several scientific methods, often integrating sophisticated sensors into a wrist-worn form factor.
Optical sensing is one widely investigated approach, using light to analyze tissue components. This often employs near-infrared (NIR) spectroscopy, where light is shined into the tissue. Glucose molecules interact with this light, causing a measurable change in the absorption or scattering pattern that correlates with concentration.
Another method is bio-impedance spectroscopy, which involves applying a small electrical current to the skin. The electrical impedance, or resistance, of the body’s tissues changes based on the concentration of substances like glucose. By sweeping across radio frequencies, this technique attempts to measure subtle differences in the tissue’s electrical properties related to glucose levels. Both optical and electrical methods are complicated because the signal must pass through multiple layers of skin, fat, and muscle, introducing significant background noise and variability.
The Standard for Medical-Grade Glucose Accuracy
For a glucose monitoring device to be considered medically reliable, it must meet a high standard of accuracy established by regulatory bodies. The gold standard metric for evaluating the performance of a continuous glucose monitor (CGM) is the Mean Absolute Relative Difference, or MARD score. MARD represents the average percentage difference between the device’s reading and a precise reference measurement, such as a laboratory blood draw.
Medical-grade continuous glucose monitors typically achieve MARD scores of 9% to 10% or lower. A lower MARD percentage indicates a smaller average error and closer agreement with actual blood glucose levels, necessary for effective diabetes management. Approved CGMs measure glucose in the interstitial fluid (the fluid surrounding cells), which naturally lags slightly behind blood glucose concentration, especially during rapid changes.
Furthermore, a device intended for medical use must undergo a rigorous review process and receive clearance from regulatory bodies like the U.S. Food and Drug Administration (FDA). This clearance signifies that the device has been clinically validated to be safe and effective for its intended purpose. Without this authorization, the device’s measurements cannot be relied upon for making decisions about medication dosage, such as insulin, where an inaccurate reading could have life-threatening consequences.
Current Consumer Watch Reliability for Glucose Tracking
The direct answer to the question of watch accuracy is that no major, widely available consumer smartwatch currently possesses the necessary regulatory clearance or proven clinical reliability to measure blood glucose independently. Companies like Apple, Samsung, and Google have not released a feature that performs non-invasive glucose monitoring, and the FDA has issued clear warnings against using unauthorized wearables for this purpose. The FDA emphasizes that no smartwatch or smart ring has been authorized or approved to measure or estimate blood glucose values on its own.
Relying on unvalidated devices risks inaccurate readings and significant errors in patient care. For someone with diabetes, an incorrect glucose reading could result in taking the wrong dose of insulin or other medication, potentially causing dangerously low blood glucose levels, leading to confusion, coma, or death. While some smartwatches display data from an FDA-authorized CGM, this is merely a display function and does not mean the watch performs the measurement itself.
The primary reasons for the lack of accuracy in wrist-worn measurements are the interference from other biological factors and the indirect nature of the measurement. Variables such as movement, changes in skin temperature, sweat, and variations in skin thickness from person to person can all corrupt the subtle optical or electrical signals being measured. These external factors make it extremely difficult for algorithms to consistently isolate the signal that is purely attributable to glucose concentration. The technology remains promising but is not a replacement for traditional blood glucose meters or approved continuous glucose monitors.