Continuous glucose monitors (CGMs) have fundamentally altered the landscape of diabetes management over the past two decades. By providing real-time, dynamic data about blood sugar levels, these devices allow individuals with type 1 diabetes to make immediate, informed decisions regarding insulin dosing, diet, and exercise. However, a recent landmark study from Bangor University in the UK has brought a critical technical limitation of this health technology to light. According to the research, while CGMs are exceptional at tracking day-to-day glucose fluctuations, they cannot accurately quantify the most extreme glucose events. Understanding this limitation is essential for both patients relying on these devices and the healthcare professionals interpreting the data.
Understand the Mechanics Behind Continuous Glucose Monitors
To comprehend why continuous glucose monitors struggle with extreme readings, it is necessary to look at how the underlying health technology actually functions. Most modern CGMs utilize a tiny, flexible sensor inserted just beneath the skin. This sensor contains an enzyme—typically glucose oxidase—that reacts with interstitial glucose. The reaction generates a small electrical current, which the device’s transmitter converts into a glucose reading displayed on a smartphone or dedicated receiver.
Like all scientific instruments, these sensors have a finite measurement range. For the vast majority of commercially available continuous glucose monitors, this operational range is capped between 40 mg/dL and 400 mg/dL (2.2 to 22.2 mmol/L). When glucose levels fall below 40 mg/dL or rise above 400 mg/dL, the electrochemical reaction reaches a point of saturation or depletion. At this juncture, the sensor can no longer generate a proportional electrical current. Consequently, the device stops providing a numerical value and instead displays a generic “LOW” or “HIGH” indicator. The device knows the threshold has been breached, but it cannot calculate exactly how far beyond that threshold the glucose level has traveled.
Schedule a free consultation to learn more about how clinical professionals interpret CGM data in practice.
Analyze the Data Behind the Measurement Limits
The assertion that continuous glucose monitors cap out at extreme levels is not entirely new; it is printed in device manuals. However, until now, the real-world impact of this limitation had never been systematically investigated on a large scale. Researchers at Bangor University sought to fill this gap in diabetes research by conducting the largest investigation of this issue to date.
Publishing their findings in the journal Diabetic Medicine, the research team analyzed an astonishing 47 million individual glucose readings. This massive dataset was drawn from 948 individuals living with type 1 diabetes. The sheer volume of data provided a high-resolution picture of how often patients encounter these measurement limits during their daily lives.
The results were striking. The study found that between 94% and 100% of participants experienced at least one reading that hit the sensor’s upper or lower measurement limit during the monitoring period. Furthermore, the data revealed that when extreme hyperglycemia occurred, the glucose level often remained at the upper measurement limit for an hour or longer. This means that for extended periods, clinicians and patients are completely blind to the true severity of the blood sugar spike. A reading displayed simply as “HIGH” could represent a glucose concentration of 401 mg/dL, 500 mg/dL, or even 800 mg/dL, but the health technology cannot differentiate between these vastly different clinical scenarios.
Why Standard Diabetes Metrics Remain Reliable
Despite the loss of data during these extreme events, the Bangor University study yielded a highly reassuring finding for the broader field of diabetes research. The researchers discovered that these measurement limits do not substantially distort the standard summary metrics that doctors rely on to evaluate long-term diabetes control.
Metrics such as mean glucose levels, Time in Range (TIR), the Glucose Management Indicator (GMI), glucose variability, and standard deviation all remain robust and accurate. Because extreme events—while dangerous—make up a relatively small percentage of the total time over a weeks-long or months-long evaluation period, their “censoring” by the device does not skew the overall mathematical averages. Patients and doctors can continue to trust TIR and GMI as reliable benchmarks for assessing baseline diabetes management.
The Danger of Censored Data in Extreme Hyperglycemia
While long-term averages survive the censoring effect, the immediate clinical danger of losing severity data during an extreme event cannot be understated. When a patient sees a “HIGH” reading on their continuous glucose monitor, they must decide how much corrective insulin to administer. Insulin dosing for high blood sugar is typically calculated using a correction factor, which dictates how many points a single unit of insulin will drop the blood sugar.
If a patient assumes their blood sugar is only slightly above 400 mg/dL when it is actually nearing 600 mg/dL, they will administer an insufficient dose of insulin. This delay in appropriate correction can prolong the time the patient spends in a state of extreme hyperglycemia, increasing the risk of acute complications such as diabetic ketoacidosis (DKA), a potentially life-threatening condition. The absence of precise quantification removes a critical layer of safety and precision from acute diabetes management.
Identify Which Patient Demographics Face the Highest Risk
The diabetes research from Bangor University also identified specific demographic patterns regarding who is most likely to encounter these measurement-limit readings. According to the data, censored readings were significantly more common in two specific groups: younger people and individuals with higher baseline HbA1c levels.
Younger patients with type 1 diabetes often experience more volatile blood sugar fluctuations due to hormonal variations, unpredictable physical activity levels, and difficulties in precise carbohydrate counting. Similarly, individuals with elevated HbA1c levels—indicating poorer long-term glycemic control—are inherently more prone to experiencing marked hyperglycemia. For these populations, the limitations of continuous glucose monitors are not merely a theoretical data issue; they represent a frequent, practical barrier to optimal health management. Recognizing that these groups are most vulnerable allows healthcare providers to tailor their educational efforts and monitoring strategies accordingly.
Share your experiences in the comments below if you have encountered measurement limits on your CGM device.
Evaluate the Future of Health Technology in Diabetes Care
Dr. John Mulley, the lead researcher from Bangor University’s School of Environmental and Natural Sciences, emphasizes that these findings are not a criticism of continuous glucose monitors. On the contrary, CGMs are widely recognized as one of the most important advances in diabetes care. Instead, the study serves as a crucial reminder that all scientific instruments have boundaries, and understanding those boundaries is vital for interpreting the data correctly.
Moving forward, the UK health technology sector has a clear opportunity to improve the software ecosystems surrounding CGM devices. The study’s authors suggest that future CGM software updates should routinely track, quantify, and report how often sensor measurement limits are reached, and exactly how long the sensor remains capped at those limits. By integrating this metadata into clinical reports, software could alert endocrinologists and patients to the fact that severe, unquantified events are occurring, even if the standard TIR and GMI metrics look acceptable.
Beyond software algorithms, this diabetes research highlights the ongoing need for hardware innovation. Developing sensors with wider dynamic ranges that can accurately quantify glucose up to 600 mg/dL or higher without sacrificing accuracy in the normal range remains a significant engineering challenge. However, as health technology continues to evolve, addressing the “ceiling effect” will be critical for the next generation of diabetes management tools.
Apply These Findings to Daily Diabetes Management
For individuals living with type 1 diabetes and the clinicians supporting them, the immediate practical takeaway from this Bangor University study is the importance of maintaining a healthy skepticism when a CGM displays a “HIGH” or “LOW” reading. While the device is accurately signaling an emergency, it is not providing the full picture needed for precise corrective dosing.
In these specific instances, reverting to traditional fingerstick blood glucose meters can provide a more accurate quantification of the extreme event, as some traditional meters can measure up to 600 mg/dL. Furthermore, patients should be educated to treat prolonged “HIGH” readings with a high degree of caution, recognizing that the actual glucose level may be significantly higher than the 400 mg/dL cap, and acting aggressively to bring the levels down safely while checking for ketones.
Continuous glucose monitors represent a massive leap forward in patient autonomy and health outcomes. By acknowledging and adapting to their specific limitations, patients and providers can use this health technology more safely and effectively.
Explore our related articles for further reading on the latest advancements in diabetes management and medical devices.
Have questions? Write to us! We welcome your inquiries about navigating the complexities of diabetes technology.
