Managing blood glucose levels is a central aspect of diabetes care. Precise and reliable glucose measurements are essential for individuals and healthcare providers to make informed decisions about diet, physical activity, and medication. The ability to monitor glucose accurately helps prevent both dangerously high and low blood sugar.
How Glucose Monitoring Systems Work
Two primary methods exist for personal glucose monitoring: finger-stick blood glucose meters (BGMs) and continuous glucose monitors (CGMs). Finger-stick meters obtain a small blood sample, which is then applied to a test strip. The meter then displays a numerical reading within seconds, offering a snapshot of the glucose level at that specific moment.
Continuous glucose monitors operate differently, measuring glucose levels from the interstitial fluid. A small sensor is inserted just under the skin and remains in place for several days to weeks. This sensor continuously measures glucose in the interstitial fluid, sending data wirelessly. The fundamental difference lies in the source of the fluid measured: BGMs use capillary blood, while CGMs use interstitial fluid.
Factors Influencing Measurement Accuracy
The accuracy of both finger-stick meters and continuous glucose monitors can be influenced by various factors. For finger-stick meters, user technique plays a significant role. Improper handwashing, insufficient blood samples, or using expired or damaged test strips can lead to inaccurate readings. Hematocrit levels can also affect BGM accuracy, as can extreme altitudes.
Continuous glucose monitors, while offering continuous data, also have specific factors that can impact their accuracy. Sensor placement is important. Hydration status can influence the concentration of glucose in interstitial fluid, affecting CGM accuracy. Certain medications, such as high doses of acetaminophen, and supplements like vitamin C, can interfere with sensor readings. Pressure on the sensor, known as a compression artifact, can temporarily lower readings.
Additionally, there is an inherent physiological lag time, typically ranging from 5 to 15 minutes, between blood glucose levels and interstitial fluid glucose levels, meaning CGM readings may not reflect real-time blood glucose, especially during rapid changes. This lag is a natural result of glucose moving from the bloodstream into the interstitial fluid.
Interpreting Readings and Practical Use
When considering which method is more accurate, it is important to understand their distinct applications and limitations. Finger-stick meters provide a direct, real-time measurement of blood glucose, which is often considered the gold standard for immediate, precise values, particularly for making insulin dosing decisions. However, they only offer a single snapshot and do not show trends or patterns over time.
Continuous glucose monitors, conversely, excel at providing a comprehensive picture of glucose trends throughout the day and night. They offer insights into how food, exercise, and other factors influence glucose levels, and many systems include alerts for high or low readings. This continuous data helps users and healthcare providers identify patterns and make proactive adjustments to management plans. Due to the physiological lag, CGM readings might differ from finger-stick results, especially when glucose levels are changing rapidly, such as after meals or during exercise. In such situations, or if symptoms do not align with CGM readings, a finger-stick test is often recommended to confirm the glucose level.
Understanding Accuracy Standards
The accuracy of glucose monitoring devices is rigorously evaluated against established standards. For continuous glucose monitors, Mean Absolute Relative Difference (MARD) is a common metric used to quantify accuracy. MARD represents the average percentage difference between a CGM reading and a reference blood glucose measurement, with lower MARD values indicating greater accuracy. Current CGM devices typically have MARD values ranging from approximately 7.9% to 9.7%, depending on the model.
For finger-stick blood glucose meters, the International Organization for Standardization (ISO) sets performance criteria, specifically ISO 15197. The ISO 15197:2013 standard requires that for glucose levels below 100 mg/dL, 95% of readings must be within 15 mg/dL of a reference laboratory value, and for levels at or above 100 mg/dL, 95% of readings must be within 15% of the reference. These standards ensure that devices provide sufficiently reliable results for daily diabetes management.