7 Factors That Affect Your CGM Accuracy: Sensor Placement, Medications, and More

For many people with diabetes, continuous glucose monitors (CGMs) are more than just a gadget—they’re a lifeline. When your CGM isn’t accurate, it can feel overwhelming and even dangerous. If you’ve ever wondered, “Why is my CGM not accurate?” or felt frustrated by wrong CGM readings, know that you’re not alone. In this post, we’ll explore 7 common factors that affect your CGM accuracy and share practical tips to help you get the most reliable readings possible.

Whether you’re new to CGMs or have been using them for a while, understanding these factors can empower you to troubleshoot and improve your device’s performance. Let’s dive in.

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1. Sensor Placement and Adhesion

The accuracy of your CGM begins at the moment you insert the sensor. Correct sensor placement is crucial because the sensor needs to be in an area rich with interstitial fluid to accurately reflect your blood sugar levels. For example, Dexcom sensors (such as the G6 and G7) are approved for use on the abdomen and the back of the upper arm—with the G7 specifically recommended for arm placement for optimal performance (and for children ages 2–6, the upper buttocks may be used). In contrast, Abbott’s FreeStyle Libre sensors are approved solely for application on the back of the upper arm, while Medtronic’s Guardian Connect sensors are approved for use either on the abdomen or the back of the upper arm, provided there is an adequate amount of subcutaneous fat. Further, if the sensor isn’t securely attached, you might experience dislodgement or inconsistent readings.

Practical Tips

• Follow the Guidelines: Stick to the recommended insertion sites provided by your CGM manufacturer. Common spots include the upper arm and abdomen. 
• Prep Your Skin: Wash and thoroughly dry the area before applying the sensor. Using skin barrier wipes or sprays can help ensure a clean surface for better adhesion.
• Enhance Adhesion: If you find your sensor frequently coming loose, consider using additional adhesive patches or overtapes designed for CGM devices.
• Rotate Sites: Regularly change your sensor location to avoid scar tissue build-up, which might affect sensor performance over time.

Finding the right spot and keeping the sensor in place during everyday activities can be challenging. Small adjustments can make a big difference in how accurately your CGM reflects your true glucose levels.

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‍2. Medication Interference

‍Some medications and supplements can interfere with CGM readings. These interferences are due to chemical interactions. Being aware of these potential issues and discussing them with your healthcare provider can help you manage your diabetes more effectively.

‍Practical Tips

• Know Your Meds: Some over-the-counter pain relievers, like acetaminophen (Tylenol), can affect certain CGM systems. High doses of acetaminophen can lead to falsely elevated readings due to the formation of acetaminophen glucuronide in the interstitial fluid.
• Watch for Manufacturer Specific Interferences:
  
  • Dexcom: Even though the Dexcom G6 has reduced sensitivity to acetaminophen, taking more than the maximum dose may still skew your readings. Additionally, the Dexcom G6 may be affected by hydroxyurea which can result in higher-than-actual glucose values.
  • Abbott’s FreeStyle Libre: Excessive intake of vitamin C (ascorbic acid), particularly doses above 500 mg per day, can interfere with sensor readings, potentially leading to missed alerts for low blood sugar. Similarly, aspirin or other salicylate-containing medications may cause the sensor to register lower glucose readings than the true value.
• Monitor Changes: If you start a new medication or supplement, track your CGM readings and consider confirming with a fingerstick test.
• Consult Your Healthcare Provider: Always discuss any concerns or changes in your medication and supplement regimen with your healthcare team.

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3. Everyday Products

‍Beyond medications, everyday products can also affect your CGM’s performance. Products like skin lotions, creams, or even certain cleaning chemicals might interfere with the sensor’s ability to measure glucose accurately.

‍Practical Tips

• Clean Your Skin: Always wash and thoroughly dry the insertion site before applying your sensor.
• Be Mindful of Products: Avoid applying creams, lotions, or alcohol-based sanitizers on or near the sensor, as these can affect adhesion and sensor function.
• Follow Manufacturer Guidelines: Your CGM provider may have a list of substances that could interfere with sensor performance. Keeping this list handy can help you make informed choices about the products you use.

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4. Environmental Factors

‍Temperature, humidity, and even altitude changes can affect the chemical reactions within your sensor, leading to potentially skewed readings. By planning ahead and being aware of how your environment may impact your CGM, you can take proactive steps to ensure more consistent readings.

‍Practical Tips

• Temperature Sensitivity: Avoid exposing your sensor to extreme temperatures above 40°C (104°F) or below 10°C (50°F), as excessive heat or cold can impact sensor accuracy. If you’re exercising outdoors in hot weather or spending time in freezing conditions, be aware that temperature fluctuations may affect readings.
• Watch the Humidity: High humidity levels above 85% can weaken the sensor’s adhesive, increasing the risk of it falling off and causing erratic readings. If you live in a humid climate or sweat heavily, consider using additional adhesive patches to keep the sensor secure.
• Plan for Travel: If you’re flying or traveling to high altitudes, note that changes in atmospheric pressure might temporarily affect your sensor’s performance. Consider extra monitoring during these times.

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5. Physical Activity and Sweat

‍Vigorous physical activity can cause your sensor to shift or be affected by sweat, leading to unexpected fluctuations in readings.

‍Practical Tips

• Secure the Sensor: If you plan to exercise, ensure your sensor is well secured. Using extra adhesive patches can help keep it in place during strenuous activities.
• Post-Exercise Verification: After intense workouts, consider confirming your CGM readings with a traditional fingerstick test to ensure accuracy.
• Track Your Patterns: Keeping a log of your physical activities and corresponding glucose readings can help you identify if exercise consistently impacts your sensor’s performance.

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6. Calibration and Sensor Warm-Up Period

‍Calibration and sensor warm-up are critical steps in ensuring your CGM provides reliable data. Most CGM devices require a short period to stabilize after insertion, during which the readings might not be fully accurate.

‍Practical Tips

• Allow for Warm-Up: Be patient and allow the recommended warm-up period (typically 0.5–2 hours) before relying on the sensor data for important decisions.
• Stick to Calibration Schedules: Some CGMs require periodic calibration with a fingerstick test. Following these schedules meticulously can help maintain consistent accuracy.
• Monitor and Adjust: If you notice ongoing inaccuracies, review your calibration process. Even slight deviations in timing or technique can lead to less reliable readings.

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7. Device Malfunctions and User Error

‍Sometimes, inaccuracies in your CGM readings are not due to external factors but rather device malfunctions or simple user errors. Issues such as sensor expiration, software glitches, or improper insertion can compromise your device’s accuracy.

‍Practical Tips

• Keep Track of Expiration Dates: Sensors have a finite lifespan. Using them beyond their recommended period increases the risk of faulty readings.
• Update Regularly: Ensure that your CGM’s firmware and any associated apps are updated. Manufacturers often release updates that improve accuracy and address glitches.
• Educate Yourself: Familiarize yourself with the manufacturer’s instructions and troubleshooting guides. Being informed can prevent many common mistakes.
• Seek Professional Help: If you suspect a malfunction or consistent user error, don’t hesitate to reach out to your healthcare provider or the device’s customer support.

Dealing with technology can be stressful—especially when it impacts your health. It’s okay to feel frustrated if your CGM isn’t performing as expected.

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Takeaways

‍For many people with diabetes, CGMs are a critical tool in daily life. When your CGM isn't accurate, it can feel like an extra burden in managing your health. By understanding the various factors that can lead to wrong CGM readings—including sensor placement, medication interference, everyday products, environmental conditions, physical activity, calibration issues, and device malfunctions—you’re better equipped to take control of your diabetes management.

• Review your sensor placement techniques and ensure your sensor is securely attached.
• Be aware of potential medication or supplement interference, especially the specific limitations noted for the specific device you are using.
• Prepare for environmental changes and physical activity by planning ahead and verifying your readings when necessary.
• Follow calibration guidelines carefully and keep your device’s software up-to-date.
• Don’t hesitate to seek professional advice if you suspect a device malfunction or persistent inaccuracies.

Your journey with diabetes is unique. Every small step you take towards better dealing with your device can help you achieve more reliable readings and better overall control. If you continue to face challenges, know that support is available—from healthcare professionals, diabetes communities, and the device manufacturers.

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