The Accuracy of CGMs
Key Takeaways
CGMs have changed diabetes management by eliminating the need for frequent finger pricks and providing continuous glucose monitoring.
CGMs and glucometers measure glucose differently, with CGMs tracking interstitial fluid (ISF) glucose, leading to a slight delay known as "the lag."
CGMs also offer real-time alerts for glucose spikes and drops, helping users take proactive action.
The Mean Absolute Relative Difference (MARD) is a key metric for CGM accuracy, with lower values indicating higher precision.
CGM accuracy can be affected by multiple factors, including sensor placement, skin temperature, physical activity, environmental conditions, and device errors.
Accurate CGM readings are especially crucial for diabetes management, as they help users adjust insulin doses, prevent complications, and make informed health decisions.
For individuals with diabetes, monitoring blood sugar levels regularly is extremely important to reduce the risk of serious health issues.
Continuous Glucose Monitors (CGMs) have changed diabetes management in numerous ways. For one, they have eliminated the painful finger pricks that were how blood sugar was traditionally tested using a glucometer. A glucometer uses a drop of blood obtained by these finger pricks, which is then placed on a test strip and inserted into the glucometer. The glucometer will then display your blood sugar level.
On the other hand, CGMs are wearable devices that check blood sugar continuously. CGMs use a small wearable sensor, which is usually applied on the abdomen or the arm. The sensor measures blood sugar on an ongoing basis. The reading (data) is transmitted from the sensor to the smartphone or a handheld reader.
Your CGM reading won’t always match your glucometer reading
Glucometers and CGM sensor readings often vary because they measure glucose in different areas of the body. While glucometers take readings directly from capillary blood, CGMs measure glucose levels in the interstitial fluid (ISF). The ISF is that thin layer of fluid surrounding the cells beneath the skin. Since glucose enters the bloodstream first before diffusing into the ISF, sensor readings may lag behind blood glucose levels by a few minutes. This delay is known as "the lag."
Establishing a Baseline for CGMs
Since CGMs have a slight delay, you can establish a baseline by calibrating the CGM. Some CGMs require occasional calibration with a glucometer, while some (such as the Dexcom G6 and FreeStyle Libre) do not require calibration since the calibration was already performed in the factory. You can also compare the glucometer and CGM readings during a stable period (e.g., before meals).
How CGMs Help Alert to Spikes
Unlike glucometers, CGMs offer real-time alerts for glucose spikes and drops in blood glucose:
Hyperglycemia Alerts. CGMs can notify users when glucose rises too high, so users can take corrective action before the blood glucose reaches dangerous levels.
Hypoglycemia Warnings. Low blood sugar alerts help prevent severe hypoglycemia, reducing the risk of complications.
Recommended Reading: Glucometer Use vs. CGM
Accuracy of Continuous Glucose Monitors
The Mean Absolute Difference (MARD)
The accuracy of CGMs varies among the different models available and the manufacturers. One metric used for assessing the accuracy of CGMs is the Mean Absolute Relative Difference (MARD). The MARD measures the average difference between a CGM’s test result and the reference blood glucose measurements at normal to high levels.
A lower MARD value means higher accuracy. However, the accuract
How Accuracy May be Affected
1. Sensor Placement and Adhesion
The accuracy of the CGM begins with how you place the sensor – that is, they must be placed in an area rich in interstitial fluid; and they must be securely attached. It is recommended to keep the skin clean and dry for better adhesion. Some products such as lotions or skin creams can possibly affect how a CGM sensor functions.
However, different CGM devices are approved to be sited for different areas of the body. For example, the Abbott FreeStyle Libre sensor is approved for placement for the back of the upper arm. Most commonly, CGMs are sited either over the abdomen or the upper arm. Remember to follow the manufacturer’s guidelines – if the manufacturer provides a list of substances that interfere with accuracy, try to steer clear of those.
2. Skin Temperature and Activity
Changes in skin temperature, hydration or heavy sweating may cause sensor displacement or even interfere with the sensor’s accuracy. It might help to use adhesive patches to keep sensors in place during physical activities.
3. The Environment
Studies have shown that changes in humidity, temperature and altitude can affect the CGM readings. Refer to the manufacturer’s guidelines on the extremes of temperature to avoid that may impact sensor accuracy. In cases of high humidity, you may use additional patches to keep the sensor in place – just in case you sweat heavily.
4. Device Errors
However, inaccuracies in CGM readings may possibly be due to an expired sensor (tip: always check the sensor expiry before purchasing), or software issues. Always update your CGM firmware or app. If necessary, contact the device support hotline (if indicated) or seek help from your healthcare provider.
Why is CGM Accuracy Important?
CGM accuracy is extremely important to effectively manage diabetes. Accurate monitoring of blood glucose readings in real time has helped many users make informed decisions about their health. For example:
A highly accurate CGM can alert users to dangerously low (hypoglycemia) or high (hyperglycemia) glucose levels in real-time. Unfortunately, inaccurate readings could lead to delayed treatment or unnecessary interventions, increasing health risks.
People using insulin therapy or glucose-lowering medications rely on precise glucose data to adjust their dosages. Meanwhile, incorrect readings may lead to under- or overdosing, which can have serious consequences.
Yet, it can be overwhelming to make sense of the rich data from your CGM, especially when you are new to it. We encourage you to have a discussion with your healthcare provider before you start monitoring so that you get the appropriate support and guidance throughout the process – including ensuring better CGM reading accuracy.