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Continuous glucose monitoring (CGM) devices may overestimate blood glucose levels in healthy adults, potentially leading to unnecessary dietary changes and confusion among users, according to a recent study published in The American Journal of Clinical Nutrition.

Researchers at the University of Bath in Bath, England, evaluated glycemic responses in healthy volunteers with normal body mass index using two methods: A CGM device (Abbott Freestyle Libre 2) and the standard finger-prick test for capillary blood glucose.

Participants — nine women and six men with an average age of 34 years — wore a CGM device, and capillary samples were collected every 15 minutes over a 2-hour period. During this time, they consumed 50 g of glucose from different carbohydrate sources.

After consuming a smoothie, the CGM overestimated the glycemic index (GI) by 30%, recording a moderate GI of 69 compared with a low GI of 53 using conventional testing. Similarly, whole fruits were classified by the CGM as moderate to high GI foods, while finger-prick tests indicated a low GI.

“This could lead users to wrongly believe that fruit causes harmful glycemic spikes,” said senior author Javier Gonzalez, PhD, professor of nutrition and metabolism at the University of Bath, in an interview with Medscape’s Portuguese edition.

“The study concluded that CGMs overestimate blood glucose levels compared to capillary testing — and that this overestimation varies depending on the type of food or drink. In other words, the inaccuracy is inconsistent, suggesting that CGMs are likely not valid tools for determining a food’s glycemic index.”

Wearables in Medicine

“Medical wearables are devices that can be worn on the body — such as smartwatches, fitness bands, oximeters, or patches — and are capable of monitoring and collecting health data from the user,” explained Erika Campana, MD, PhD, associate professor of cardiology at the State University of Rio de Janeiro, Rio de Janeiro, Brazil, and vice president of the Hypertension Department of the Brazilian Society of Cardiology, speaking with Medscape’s Portuguese edition.

She noted that the most commonly used health-related wearables today include:

• Smartwatches: Monitor heart rate, physical activity, and sleep quality and may offer ECG features.
• Fitness bands and rings: Primarily used to track steps, calories burned, and sleep patterns; popular among those aiming to improve fitness and general well-being.
• Glucose monitors: Mainly used by individuals with diabetes to monitor blood glucose levels in real time.
• Patches and health sensors: Applied to the skin, these devices continuously monitor vital signs such as body temperature and heart rate.
• Smart earbuds: Some advanced models can monitor heart rate and physical activity while the user listens to music or takes calls.

In terms of accuracy, home-use devices may produce results that differ from those obtained using standard clinical or hospital equipment. “Many self-monitoring wearables are designed to deliver reliable results, but their precision may vary,” Campana said. “For example, blood pressure monitors embedded in smartphones, wristbands, or rings are not yet fully validated, and their readings can differ significantly from those of traditional monitors.”

While not as precise as clinical-grade equipment, validated home-use wearables remain valuable tools for continuous health monitoring — provided they are approved by health regulatory agencies.

Smartwatches and Heart Health

One example of wearable technology in health monitoring was presented by Zhanlin Chen, MD, student from Northwestern University Feinberg School of Medicine, Chicago, at the American College of Cardiology (ACC) Scientific Session 2025. Published in the Journal of the American Heart Association, the study proposed a new metric for evaluating cardiovascular fitness using smartwatch data.

Researchers found that dividing a person’s average daily heart rate by their total number of steps — referred to as the Daily Heart Rate Per Step (DHRPS) — provided a more reliable measure of cardiovascular fitness than heart rate or step count alone.

The study analyzed data from nearly 7000 adults in the United States and found that individuals with a high DHRPS were more likely to have conditions such as type 2 diabetes, heart failure, and hypertension. The researchers suggested that the metric could assist in the early detection of cardiac issues and be integrated into wearable devices to support cardiovascular health monitoring. However, they emphasized that further prospective studies are needed to validate its clinical utility.

Challenges of CGM Devices

Denise Franco, endocrinologist specializing in diabetes technology and member of the Brazilian Society of Diabetes, noted that CGM devices are subject to specific regulatory scrutiny by the US Food and Drug Administration before being approved for market use.

She explained that conditions such as severe anemia, kidney dysfunction, or edema may affect CGM accuracy by altering the composition of interstitial fluid, which forms the basis for CGM readings. Additionally, substances such as acetaminophen, ascorbic acid, mannitol, and sorbitol can interfere with sensor readings, often leading to false-positive hyperglycemia alerts.

Another challenge associated with CGM devices is delayed readings, especially during rapid fluctuations in blood glucose levels due to eating or physical activity. “Patients often feel anxious during the adjustment period because they don’t yet understand how to interpret the device graphs,” Franco noted. “Clinicians should be mindful of this to help support adherence.”

Maximizing Potential

Campana emphasized the importance of proper training for healthcare professionals in using and interpreting CGM data. “Without adequate training, the full potential of these technologies may not be realized,” she said.

Cost is another barrier. However, Franco noted that growing competition among sensor manufacturers may help drive prices down.

Interoperability — the ability of devices to communicate with other health systems and platforms — is also critical. “A lack of standardization can hinder effective data integration,” Campana added.

She also stressed the importance of data privacy and security: “Health data collection and storage raise serious privacy concerns. It’s vital to ensure that user data is protected from unauthorized access.”

Calibration is another key issue. Poorly calibrated devices can produce inaccurate results, compromising their usefulness. “Imprecise data can lead to incorrect health conclusions and poor clinical decisions,” Campana warned.

She added that personalization — considering factors such as age, weight, and physical condition — can improve accuracy and patient engagement. “Calibration isn’t universally accurate for all users, particularly older adults or elite athletes. Regular updates or manual adjustments may be required,” she explained.

Trusting Wearable Devices

“No measurement is perfect,” Gonzalez emphasized. “The acceptable level of inaccuracy depends on the intended use and goals. Our data suggest the CGM tested in the study is likely not accurate enough for classifying food glycemic indexes — but it may still be useful for understanding general dietary trends.”

Improving device performance, he said, requires a better understanding of the physiological basis of inaccuracy. He advised users to consider a device’s limitations and to confirm worrisome readings using additional methods, such as finger-prick tests.

A 2024 review evaluated the potential of wearables to provide accessible and reliable physiological data. It showed that these devices can be effective for early detection, diagnosis, and management of both acute and chronic diseases through longitudinal monitoring. Moreover, wearables could ease pressure on healthcare systems by preventing severe clinical events, reducing hospital readmissions and emergency room visits, and improving recovery outcomes.

The authors believe that cardiac rehabilitation and metabolic disease management, including diabetes, may be the next areas to benefit from wearable health monitoring. These tools offer personalized care and continuous tracking of glycemia and other parameters.

Wearables are becoming increasingly popular for their ability to deliver real-time health data and enhance health management. With advancements in sensor accuracy, artificial intelligence, machine learning, and connectivity, “these innovations promise to make health monitoring more accessible, efficient, and integrated into daily life — supporting a more preventive and personalized approach to medicine,” Campana concluded.

This story was translated from Medscape’s Portuguese edition using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.