Continuous Glucose Monitor (CGM) readings can sometimes differ from traditional fingerstick tests, which can be frustrating. This article explores common reasons why CGM readings might vary from other glucose measurement methods.
Understanding CGM Accuracy
Continuous Glucose Monitors measure glucose levels in the interstitial fluid, which is the fluid found in the spaces around your cells, rather than directly in your bloodstream. Glucose first enters the bloodstream and then diffuses into the interstitial fluid. This process results in a physiological lag time, meaning there can be a delay between your blood glucose level and the interstitial glucose level measured by the CGM. This lag time typically ranges from 5 to 15 minutes, and can sometimes be up to 20 minutes, particularly during periods of rapid glucose changes, such as after meals or during exercise.
Traditional blood glucose meters (BGMs) provide a snapshot of glucose levels in capillary blood at a single point in time. Due to the inherent difference in measurement sites and the physiological lag, some variance between CGM and BGM readings is expected and considered normal. The U.S. Food and Drug Administration (FDA) requires these devices to fall within an acceptable range of accuracy, generally within 15% of the true laboratory value at least 95% of the time.
Factors Influencing Readings
Several factors can lead to discrepancies in CGM readings, affecting their accuracy. These include issues related to the sensor itself, various physiological conditions, and environmental influences. Understanding these elements can help in interpreting readings more effectively.
Sensor-related issues are a common source of inaccurate readings. Improper sensor insertion, such as placement in areas with excessive fatty tissue or muscle, or in sites with scar tissue, can affect accuracy. Physical pressure on the sensor, known as “compression,” for instance, from sleeping directly on the sensor, can temporarily cause falsely low readings. Furthermore, sensor expiration, manufacturing defects, or physical damage can compromise the device’s ability to provide reliable data.
Physiological factors also play a role in CGM accuracy. During rapid changes in glucose levels, such as sharp increases after a meal or rapid declines during exercise, the physiological lag time between blood and interstitial fluid glucose becomes more pronounced, leading to greater differences between CGM and BGM readings. Dehydration can concentrate interstitial fluid, potentially affecting CGM readings and sometimes causing them to appear falsely low. Illness or significant body temperature fluctuations can also temporarily impact glucose metabolism and, consequently, CGM accuracy. Certain medications, such as acetaminophen (paracetamol), can interfere with some CGM sensors, leading to falsely elevated glucose readings for several hours after ingestion.
Environmental factors, although less common, can also influence sensor performance. Extreme ambient temperatures may potentially affect the chemical reactions within the sensor, leading to less accurate readings. For CGM systems that require calibration, incorrect or mistimed calibration can introduce inaccuracies. Calibrating when glucose levels are rapidly changing or are very low can lead to less reliable readings.
Strategies for Better Accuracy
Optimizing CGM accuracy involves adopting several practical strategies related to sensor application, personal habits, and device usage. Following manufacturer guidelines and understanding how your body interacts with the device can significantly improve the reliability of readings.
Proper sensor application is fundamental for accurate readings. Users should carefully follow the manufacturer’s instructions for insertion technique and rotate sensor sites with each new application. This practice helps prevent scar tissue buildup and ensures the sensor is placed in an area with adequate interstitial fluid. Cleaning the insertion site thoroughly with an alcohol wipe and allowing it to dry completely before application is also important for proper adhesion and function.
Maintaining adequate hydration is another important step. Staying well-hydrated helps ensure consistent interstitial fluid levels, which supports more accurate CGM readings. Allowing the full warm-up period specified by the manufacturer, typically between 60 minutes to two hours, before relying on the readings is also important. This allows the sensor to hydrate and equilibrate with the surrounding tissue.
To avoid compression-related inaccuracies, users should be mindful of pressure on the sensor site, especially during sleep. Adjusting sleeping positions can help prevent temporary false readings. Strategic use of a blood glucose meter (BGM) for comparison is also recommended. It is appropriate to compare CGM readings with a fingerstick BGM when symptoms do not match the CGM reading, during rapid glucose changes, or before making treatment decisions like insulin dosing. Always verify sensor expiration dates before use, as expired sensors may provide unreliable data.
When to Seek Guidance
In certain situations, relying solely on CGM readings is not advisable, and seeking guidance from a healthcare professional or the device manufacturer is important. If CGM readings consistently differ significantly from BGM readings without a clear cause, or if symptoms of high or low blood sugar do not align with the CGM reading, perform a fingerstick BGM check. In these cases, medical decisions, especially insulin dosing, should be based on the confirmed BGM reading.
Sensor malfunctions, such as constant “low” or “high” readings, no readings, or persistent error messages, indicate a need for intervention. Contact the CGM device manufacturer’s support for assistance or to report a faulty sensor. Consulting a healthcare professional is advisable for persistent discrepancies or when symptoms do not match readings.