How Accurate Are CGMs? Reliability Facts

Have you ever thought about having a blood sugar dashboard? Continuous Glucose Monitors (CGM) are dashboard-like devices that let you know your glucose level 24/7 with almost no action. Given that CGMs are becoming a staple in diabetes management, we often get questions about their accuracy.

Can we rely on them to make decisions about dosing insulin? Can we rely on them to alert us when working out or sleeping? Understanding how CGMs work, how reliable they are, and what indicates their readings is essential for safe usage. In this article, we will explore the science of CGM accuracy, types of metrics, variables that affect performance, and real-life tips to give you the most consistent data.

Metrics Used to Measure CGM Accuracy

The researchers employ several different measures to determine how closely CGMs reflect true blood glucose.

• Mean Absolute Relative Difference (MARD): This is expressed as a percentage to describe the relative error of the CGM compared to the laboratory reference. MARD values of <10% are deemed excellent. Continuing with the example of the Dexcom G7, the MARD is 8.2%, excellent accuracy with significant reliability in “real-world” scenarios.

• Clarke Error Grid Analysis: This tool breaks down CGM readings into zones of clinical risk. Zones A and B (minimal or no risk) are clinically safe, and zones C-E are potential harm from incorrect readings.

• Consensus Error Grid: This is a modification of the Clarke error grid that evaluates how errors affect treatment decisions in contemporary diabetes care delivery.

Recognizing these metrics enables users to make informed decisions and requires healthcare providers to make decisions about interpreting CGM readings.

How Do CGMs Measure Glucose?

CGMs measure glucose using a small filament wire in the body, which is inserted just beneath the skin, where it senses glucose in the interstitial fluid (ISF) and not in the blood.

The Role of Interstitial Fluid

Glucose in the ISF is around 5 – 15 minutes behind blood glucose as a result of the time required to diffuse, which is a normal and typical response, especially with rapidly changing glucose events such as meals or exercise.

Enzymatic Reaction Inside the Sensor

Most CGMs measure glucose through enzymatic reactions with glucose oxidase. When glucose meets the binding sites of this enzyme an electrical current is generated. The CGM will transform that signal into a digestible reading for the user displayed by a receiver, smartphone, or connected insulin pump.

Frequency of Monitoring

Modern CGMs monitor glucose at least every 5 minutes — that’s 288 readings every 24 hours! This continuous stream of data allows a full picture of glucose trends over time, as well as the ability to detect trends, patterns, and ways to proactively help manage the glucose levels.

Built-In Algorithms and Calibration

Newer CGMs often come pre-calibrated and will have built-in algorithms to ensure accuracy without needing to dual calibrate with a fingerstick. Our best practice for calibration is to calibrate twice daily and always dual calibrate when glucose changes so fast or slowly that the glucose levels are not accurately reflected by the CGM’s previously established algorithms. Algorithms may also keep data consistent by controlling for a time lag between a user typing of food down and the time of CGM glucose, or controlling for timing lag with changes in environmental impact and how that could affect readings.

Integration with Smart Devices

Most SCGs are capable of apps or programming connected with a mobile app, or an insulin pump. These capabilities can include trend graphing displays for the user as well as a (Best)/ (worst) algorithm alert, as well as predictive alerts to warn the user about trending shapes that would not be able to be represented by a 5 minute timer before glucose levels become bad or dangerously high or dangerously low.

Factors That Affect CGM Accuracy

CGMs are notable for their accuracy, however, there are multiple elements that can affect the value of the readings: 

Sensor Placement Site

Placement is important. The abdominal area or the upper arm is a reliable area where there is stable interstitial fluid flow. Areas of dense scar tissue, heavy area of movement or where it might be compressed like lying on it during sleep should be avoided. 

Hydration and Body Temperature

Dehydration to voluntary/involuntary fasting can reduce ISF volume which impairs readings. Likewise, physiological values like elevated body temperature from fever or cold exposure can affect readings as these result in altered enzyme activity.

Sensor Age and Wear

As a sensor ages whether it be in days (7-14 days of wear typically), the enzymes on the filament become degraded, decreasing detection ability in relation to sensitivity. During this time adhesion glue can become compromised affecting the maternal assessment of detecting glucose for accuracy.

Rapid Glucose Changes

CGMs have limitations when it comes to fast moves of glucose levels. After meal glucose spikes, extended exercise, insulin moves, etc. CGMs may show a delay in relating blood glucose. This is referred to as “physiological lag” as noted so there will not be accuracy in the short term but is not a limitation of CGM long term trend tracking.

Medications and Interferences

Some medications like acetaminophen or vitamin C can affect readings detected by older CGMs but many new CGMs like Dexcom G6/G7 and FreeStyle Libre 2/3 have processing settings specifically aimed at bicarbonate to filter out irrelevant reading and to track glucose levels more accurately.

Tips to Maximize CGM Accuracy

Follow these best practices to get the best, most accurate results from your CGM:

Choose Correct Insertion Sites

Use the correct recommended sites and rotate placement to avoid irritation or scarring. Do not use sites that are irritated, bruised, or have been used recently.

Let the Sensor Warm Up

Do not count on the first reading after insertion. The CGM has a 1-2 hour warm-up period to get the sensor ready and allow for stabilization and synchronization of the device and your body.

Avoid Compression Lows

Pressure on the CGM during sleep compresses ISF flow. This can cause the CGM to report false lows and is more common when the CGM is placed on the arm. Consider sleeping on your back or change sides frequently during sleep.

Stay Hydrated and Nourished

Hydration helps to keep the proper volume of ISF to help with accurate readings. Eating a balanced diet, with healthy carbs helps to maintain a more stable glucose trend over time, ultimately improving the CGM sensor quality and performance.

Keep Sensor Secure

Sensors can also loosen from sweat, friction, or water. Consider using overlays or patches when you are working out, showering, or swimming to help keep the sensor snug.

Ways CGMs Measure Glucose

The technologies that underlie CGMs continue to progress rapidly, enabling opportunities that go beyond traditional electrochemical sensors.

Electrochemical Detection

The most widely characterized method. Enzymes that catalyze glucose react in order to create an electrical signal (electron transfer); which then is transformed into a glucose reading.

Optical Sensing (Emerging Tech)

Some newer CGMs are experimenting with light-based sensing, which explores the potential of infrared light to analyze glucose levels in interstitial fluid using optical detection through the skin — a pathway to eventually enabling non-invasive glucose monitoring.

Transdermal Monitoring

Still in progress. Techniques for measuring glucose through the skin without a needle are in patch form and goal to enable pain-free glucose level monitoring.

Microneedle Arrays

Similar to the “sliver” from traditional finger-stick blood glucose measurements, microneedles are ultra-thin needles that don’t penetrate very far (just into the interstitial fluid (ISF)) and cause significantly less discomfort while being part of many of the next-gen CGMs.

Integrated Closed-Loop Systems

Some CGMs integrated with closed loop (automated delivery) systems (aka “artificial pancreas”) that automatically adjust insulin based on CGM readings in real-time, provide tighter control over glucose levels with less input from the user.

CGM vs Traditional Glucose Meters: Key Comparison

Feature	CGMs	Traditional Glucometers
Measurement	Interstitial Fluid	Capillary Blood
Data Frequency	Continuous (every 5 min)	Manual (4–6x/day)
Trend Analysis	Yes	No
Alerts	Yes (High/Low/Predicted)	No
Invasiveness	Minimally invasive	Invasive (multiple finger pricks)
Accuracy	Slight lag, trending data	Real-time, single-point accuracy

While glucometers offer immediate accuracy for a single point in time, CGMs provide continuous data, overall better management, and proactive alerts and are therefore much better for long term management.

Are CGMs Reliable Enough for Medical Decisions?

In most cases, yes. CGMs have become accurate enough where you can usually use them for dosing decisions, especially the FDA-approved versions. A MARD of less than 10% means the average distance from the lab value is pretty small, and often insignificant clinically.

Doctors and endocrinologists trust CGM readings more than ever to:

• Tweak insulin therapy
• Monitor nocturnal glucose excursions
• Understand glucose spikes related to meals
• Change a treatment plan based on trends

That being said, fingersticks may be needed from time to time, such as when glucose is changing rapidly or when there appears to be a discrepancy between symptoms and the reading.

Frequently Asked Questions (FAQs)

Q1. Can I rely on my CGM for insulin dosing?

Yes. The aforementioned devices such as Dexcom G6/G7 & Libre 2/3 may be used for insulin dosing without the need for a fingerstick glucose check. The physician’s directions must always be followed.

Q2. Why do CGM readings differ from fingersticks?

Because CGMs sample the interstitial fluid, not blood, the glucose will show a slight variance. CGM readings vary slightly when glucose levels change rapidly; this would be the time when interstitial fluid would demonstrate a natural lag time of about 5 to 15 minutes after an abrupt change in glucose level.

Q3. How often should I replace my CGM sensor?

Depending on the CGM system you use, the sensor will be changed every 7 to 14 days. This should not exceed the suggested days for replacement, as the accuracy may decrease and may further cause skin irritation or infection if used for longer than suggested period.

Q4. Can compression affect readings?

Yes. Compression on the sensor like when sleeping on it will cause temporary false lows on the CGM. This happens more often when the sensor is placed on the back of the arm.

Q5. Do medications affect CGM accuracy?

Older models could be impacted by medications such as acetaminophen. Many of the most common CGMs are engineered to resist interference from most normal medications.

Conclusion

How accurate are CGMs? Very — if they are used correctly. Through a combination of factory calibration and algorithmic enhancements, today’s CGMs are both precise and reliable, making them a major advantage in the world of diabetes management.

Maybe you still use fingersticks occasionally as a backup, but CGMs provide better insight into your glucose patterns — giving you the health information you need to make informed decisions, minimize complications, and gain more confidence in your health and vitality.

Still questioning whether you’re ready for a CGM? At Healify Now, we know there is still some education needed to understand how CGMs fit into your health journey. Whether you are managing diabetes, keeping track of glucose for weight and energy, or simply investigating your metabolic health, we are here to support you.

Book your FREE consultation today and take charge of your health!

Evelyn Brown