Continuous Glucose Monitors (CGMs) have become a widely adopted technology, offering real-time insight into glucose levels without the need for constant fingersticks. This modern approach to blood sugar tracking provides a more complete picture of daily glucose patterns than traditional spot checks. For individuals managing diabetes, a primary concern is the reliability of the number displayed on the receiver or smartphone. Understanding how these devices measure glucose and the factors that can temporarily affect their performance is important for trusting the readings. The accuracy of a CGM is not only a technical specification but also involves physiological principles and external variables that influence the displayed value.
Defining CGM Accuracy: The MARD Score
The accuracy of a Continuous Glucose Monitor is quantified using a technical metric called Mean Absolute Relative Difference (MARD). This figure represents the average percentage difference between the glucose reading provided by the CGM and a reference value, typically a laboratory-grade blood glucose analyzer. A lower MARD score signifies a higher level of accuracy, indicating the CGM readings are closely aligned with the true blood glucose concentration.
MARD is calculated by taking numerous paired measurements—one from the CGM and one from the lab reference—and then averaging the absolute relative differences between them. For current commercial CGM devices, the typical MARD range is approximately 8 to 10%, which is a significant improvement from earlier models. This metric is a fundamental standard that allows users and healthcare providers to compare the performance claims of different monitoring systems. While MARD provides a statistical measure of overall accuracy, individual readings may still deviate from a fingerstick at any given moment.
Physiological Differences Between Interstitial and Capillary Readings
A frequent source of confusion is the difference observed between a CGM reading and a finger-stick blood glucose meter reading, even when the CGM is functioning correctly. This discrepancy is rooted in the fact that the two devices measure glucose from different bodily fluids. Traditional meters measure glucose in capillary blood, which is a direct measure of the glucose circulating in the bloodstream.
A CGM, however, uses a tiny sensor inserted under the skin to measure glucose in the interstitial fluid, which is the fluid surrounding the body’s cells. Glucose must travel from the blood vessels into the interstitial space through a process of diffusion. This physical distance and movement create a natural delay, known as the “lag time,” before changes in blood glucose are reflected in the interstitial fluid.
This lag time is typically reported to be between 5 and 15 minutes, depending on the individual and the device. The difference between the two readings is most noticeable when glucose levels are changing rapidly, such as after consuming a meal or during intense physical exercise. For example, if blood glucose is rising quickly after a carbohydrate-heavy meal, the finger-stick reading will display the spike first, while the CGM reading will briefly lag behind. This physiological difference means that a CGM is most accurate in reflecting trends and patterns over time, rather than providing an exact real-time snapshot like a capillary blood test.
External Factors That Compromise CGM Accuracy
Beyond the natural physiological lag time, several external factors can temporarily disrupt the electrochemical process within the sensor and compromise its reading accuracy. One common issue is “compression low,” which occurs when sustained pressure is applied directly to the sensor site, often while sleeping. This pressure reduces blood flow and interstitial fluid circulation, leading the sensor to report a falsely low glucose reading until the pressure is relieved.
Environmental Factors
Dehydration is another environmental factor that can affect the concentration of glucose in the interstitial fluid, potentially leading to inaccurate sensor readings. Extreme fluctuations in ambient temperature can also impact the sensor’s performance, as the chemical reactions within the device are sensitive to thermal changes.
Chemical Interferences
Users must also be aware of specific chemical interferences from certain medications and supplements. High doses of acetaminophen, a common over-the-counter pain reliever, can interfere with the sensor’s enzyme, leading to falsely elevated glucose readings on some older or non-interferent-mitigated devices. Similarly, large quantities of Vitamin C (ascorbic acid) can also interfere with the sensor chemistry of some models, causing a temporary distortion of the reading. Understanding these non-physiological variables is important for interpreting a reading that appears unexpectedly high or low.
Practical Guidance: When Verification is Necessary
While Continuous Glucose Monitors are highly reliable for tracking glucose trends, there are specific situations where a traditional finger-stick test is necessary to verify the reading for safety and treatment decisions. Users should always perform a capillary blood test if the CGM reading does not align with their physical symptoms. For instance, if the device reports a normal glucose level but the individual feels symptoms of severe hypoglycemia, a finger-stick is required for confirmation before treating.
Verification is also recommended whenever the device alerts for an extreme high or low reading, especially if the reading is unexpected. During periods of rapid glucose change, such as immediately following a meal or intense exercise, a finger-stick provides the most current glucose value for immediate insulin dosing or carbohydrate intake decisions. Furthermore, most sensors require a “warm-up” period after insertion, and it is a common practice to verify the readings with a finger-stick during the first 12 to 24 hours of a new sensor’s use to ensure stability.