A Continuous Glucose Monitor (CGM) is a compact device that tracks glucose levels throughout the day and night. Unlike traditional fingerstick tests, which offer only a snapshot, a CGM provides a continuous picture of glucose trends. This valuable information helps individuals manage diabetes or gain deeper insight into their metabolic health. Accurate readings are fundamental for making informed decisions about diet, activity, and medication adjustments to maintain stable glucose levels.
How CGM Accuracy is Measured
The accuracy of continuous glucose monitoring systems is primarily assessed using Mean Absolute Relative Difference (MARD). MARD quantifies the average percentage difference between CGM readings and highly precise laboratory blood glucose measurements. A lower MARD percentage indicates higher accuracy.
While an overall MARD value is often reported, accuracy can vary depending on the glucose range (low, normal, or high). A sensor’s accuracy might also fluctuate over its wear time, sometimes showing minor differences during initial use.
Factors Affecting CGM Readings
Several real-world factors can influence the accuracy and reliability of CGM readings. Physiological conditions play a role, as rapid fluctuations in blood glucose (e.g., after a meal or intense exercise) can cause a temporary lag between blood glucose and interstitial fluid glucose readings. Hydration levels can also impact how accurately the sensor reflects glucose concentrations.
Certain medications and supplements may interfere with CGM sensor performance. For instance, high doses of acetaminophen or vitamin C can lead to falsely elevated readings, while salicylic acid (aspirin) might cause falsely low readings. Users should consult their device’s manufacturer or healthcare provider about potential drug interferences.
Sensor-related factors are also important. Improper insertion technique or suboptimal sensor placement can affect readings, as the sensor requires sufficient interstitial fluid. Pressure on the sensor site (e.g., from sleeping on it) can temporarily restrict blood flow, leading to falsely low readings. The sensor’s warm-up period, performance towards the end of its wear life, and extreme environmental temperatures can also influence consistency.
Comparing Leading CGM Systems
Leading continuous glucose monitoring systems offer distinct features and accuracy profiles.
The Dexcom G7 boasts an overall MARD of 8.2% for adults when worn on the upper arm. It provides real-time glucose readings every five minutes to a compatible receiver or smartphone app. The G7 features a rapid 30-minute warm-up time and is designed as an all-in-one sensor and transmitter for simpler application. It offers customizable alerts, including an “urgent low soon” alert, and has a 10-day wear time, with recent approval for 15 days.
The FreeStyle Libre 3 system is one of the smallest and thinnest sensors available. It demonstrates an overall MARD of 7.8% (7.5% for adults, slightly higher for pediatric users). This system delivers continuous, real-time glucose readings every minute directly to a smartphone, eliminating the need for manual scanning or fingerstick calibrations. The FreeStyle Libre 3 has a 14-day wear time, with a newer Plus version extending to 15 days, and is noted for its affordability.
The Medtronic Guardian 4 system integrates closely with Medtronic insulin pumps, such as the MiniMed 780G, for automated insulin delivery adjustments. Its reported MARD is 8.7%. This system offers real-time glucose readings and predictive alerts for high and low glucose levels, often up to an hour in advance. The Guardian 4 generally does not require routine fingerstick calibrations, though a manual check may be needed if symptoms do not match readings or if the pump requests it. The sensor has a wear time of up to seven days.
Choosing the Right CGM for Your Needs
Selecting the appropriate CGM system involves considering individual lifestyle and preferences. An active lifestyle, including water sports or frequent travel, might influence the choice of a waterproof and durable sensor. Preferences for real-time alerts, seamless smartphone integration, or ease of sensor application also play a significant role in user satisfaction. Some individuals may prioritize a smaller sensor size for discretion or specific wear locations for comfort.
Consulting with a healthcare provider, such as a doctor or diabetes educator, is recommended. They can offer personalized recommendations based on an individual’s medical history, glucose management goals, and existing treatment regimens. Practical considerations like cost and insurance coverage are also important factors influencing accessibility and long-term use.