Why Is My CGM So Off: 7 Reasons For Inaccurate Readings

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2026-03-26T08:00:06+08:00

Wondering why is my cgm so off? Discover 7 common reasons for inaccurate CGM readings and learn how to improve sensor accuracy for better health tracking!

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When you excitedly put on a continuous glucose monitor only to find its readings completely different from your finger stick results, you might feel anxious and ask: "why is my cgm so off?"

Do not worry, as this does not necessarily mean your device is broken.

As an experienced researcher who has helped patients with daily monitoring in the CGM field for 11 years, I often hear this confusion.

In actual user feedback, many people go through this stage when using a CGM for the first time.

Actually, a CGM measures glucose in the interstitial fluid (ISF) rather than directly in the blood.

This causes a normal physiological delay of 5 to 15 minutes between the two.

According to the American Diabetes Association (ADA) standards, a device with a MARD (Mean Absolute Relative Difference) value within 10% is considered highly accurate.

To reduce the trouble caused by this variance, brands like SIBIONICS use advanced factory calibration-free technology.

This greatly improves the initial stability and overall accuracy of the readings.

Understanding The Gap: CGM VS Finger Sticks

When many people first hear about a CGM, they usually have a question. How does it exactly work?

Why are there differences in readings? The key to understanding the accuracy of cgm vs finger sticks lies in knowing how sugar travels in the human body.

After we eat, glucose first enters the blood circulation. It then slowly seeps and diffuses into the interstitial fluid (ISF) around the cells.

Since the CGM sensor probe is implanted in the subcutaneous ISF, it will inevitably capture blood sugar changes later than a finger stick.

This time difference is medically known as physiological delay or lag time.

The data difference between the two will reach its peak especially when blood sugar fluctuates rapidly, such as right after a big meal or intense exercise.

The medical community usually uses the MARD value as the gold standard to scientifically measure the accuracy of a cgm device.

A lower MARD value means the gap between the device reading and actual venous or finger stick blood is smaller.

7 Common Reasons Why Is My CGM So Off

If you find that my cgm is way off, it usually comes down to the following 7 common physiological or operational reasons.

#1 The First 24-Hour "Settling In" Period

When you just implant the sensor under your skin, the tiny physical trauma will trigger a mild local immune response.

During this stage, the ISF around the sensor may experience temporary metabolic changes.

This can cause the sensor readings to be slightly lower or fluctuate during the first 24 hours.

This is a normal physiological adaptation process that usually takes 24 hours to self-adjust.

As a general guiding principle, we normally consider 48 hours as the upper threshold for evaluating sensor stability.

#2 Compression Lows (Crucial For Side Sleepers)

Many users are often awakened in the middle of the night by sharp low glucose alarms, only to test their finger stick and find everything is normal.

This is a typical case of "compression lows."

You might wonder why is my cgm reading low at night? This is largely because the body compresses the sensor during sleep.

Pressure can block the normal flow of local ISF, causing the glucose around the sensor to deplete and triggering a false low glucose alarm.

Regarding CGM placement for side sleepers, we recommend positioning the sensor on the back of the upper arm to reduce such false alarms.

#3 Severe Dehydration

Why is my CGM so off?

Water is an important carrier for transporting glucose in our body. The volume and mobility of ISF are directly affected by your body's hydration status.

When you are severely dehydrated, the concentration of the ISF changes and its mobility worsens.

This not only slows down the glucose diffusion rate but can also lead to abnormal sensor readings.

Therefore, staying well hydrated is crucial for maintaining CGM accuracy.

#4 The Lag Time (Delay Effect)

As mentioned earlier, physiological delay is an objective phenomenon that cannot be completely eliminated.

When you wonder why is my cgm reading higher than finger stick, it is usually because your blood sugar is dropping rapidly.

At this time, the glucose concentration in the blood has already decreased, but the concentration in the ISF has not yet dropped simultaneously.

This creates a time gap, causing the CGM to show higher data than the current finger stick value, and vice versa.

#5 Interfering Substances (Vitamin C & Medications)

Certain daily supplements and medications can interfere with the electrochemical reaction of the sensor.

For instance, taking over 1000mg of vitamin C daily or high doses of acetaminophen may artificially push up the sensor readings.

However, not all CGM devices will be affected in the same way.

SIBIONICS CGM uses innovative anti-interference technology that effectively resists interference from ibuprofen, acetaminophen, and hydroxyurea.

#6 Rapid Glucose Fluctuations

After eating high glycemic index (GI) foods or doing high-intensity interval training (HIIT), your blood sugar will sharply rise or fall like a roller coaster.

During this extreme fluctuation period, simply comparing specific numbers makes little sense.

At this point, the Trend Arrows on the CGM screen reflect your true physical condition better than specific numbers.

They can tell you how fast and in what direction your blood sugar is changing.

#7 Improper Placement Or Adhesion Issues

The installation position and the firmness of the sensor adhesion directly determine the signal quality.

If you install the cgm device in areas with dense muscle tissue or scar tissue, it will seriously affect the ISF penetration efficiency.

Moreover, if waterproofing and fixing are not done well in areas prone to sweating or friction, the glue may loosen and peel off.

The sensor probe may undergo tiny shifts, which will immediately cause data distortion or signal loss.

How To Improve The Accuracy Of Your CGM Device?

To get the best performance from your device, you can follow these scientific steps every time you wear a new sensor.

First is skin preparation. Thoroughly clean the skin with warm water and non-oily soap, then disinfect it with an alcohol pad to ensure the adhesive area is dry and oil-free.

Second, stay hydrated. Drink enough water every day to ensure good mobility of your interstitial fluid.

Furthermore, picking a reasonable placement is equally crucial. Please try to avoid areas that are frequently pressed and skin with scars.

Finally, give the new sensor enough time to stabilize. Be patient with slight reading deviations during the first 48 hours after implantation.

SIBIONICS CGM: Redefining Accuracy And Comfort

On the journey to pursue high accuracy and a comfortable experience, choosing the right device is essential.

Through long-term technical research and development, SIBIONICS has achieved an excellent MARD.

According to clinical data, its MARD value for children and adolescents (ages 3-17) is 8.7%, and for adults (ages 18 and above) is 8.83%. This indicates that its readings are highly reliable.

If you are considering upgrading your health monitoring gear, you might want to look into the latest SIBIONICS GS3. Here is a core parameter comparison between GS3 and GS1:

Feature | SIBIONICS GS3 | SIBIONICS GS1
Weight | 1.5g (Ultra-thin design) | 3.84g
Water Resistance | IP38 (High-performance waterproof) | IP28
Wear Duration | 14 Days | 14 Days
Calibration Method | Factory calibration-free | Factory calibration-free

Users in need can get the latest generation product through official channels via a cgm buy online search to enjoy up to 14 days of 24/7 continuous and accurate data monitoring.

When To Trust Your Finger Stick Over Your CGM?

Although modern CGM technology is very mature, you still need to rely on traditional finger stick tests in certain situations for medical safety.

If you feel obvious palpitations, sweating, or trembling, but your CGM screen shows your blood sugar is normal, you must immediately test with a finger stick meter.

Always remember that when symptoms do not match, trust your body's feelings and the finger stick results.

In addition, during extreme fluctuation periods right after a large meal or intense exercise, a finger stick can better reflect your real-time status.

The main purpose of using a CGM is to discover blood sugar patterns during daily activities. It should not be used as the sole basis for diagnosing diabetes.

Any adjustments to medical plans should be consulted with your professional doctor.

Verdict: Trust The Trends, Not Just The Numbers

In conclusion, the core value of continuous glucose monitoring lies in helping us observe long-term fluctuation trends.

It is not meant to keep us endlessly tangled in a few millimoles of difference from a finger stick.

After understanding the physiological delay and device operation mechanism, you will find that those tiny deviations are actually within scientific expectations.

Choosing an advanced device with a high waterproof rating (IP38) and a low MARD value like the SIBIONICS GS3 can greatly reduce the mental stress caused by abnormal readings.

This allows you to manage your healthy life more easily.

FAQ: Common Questions About CGM Inaccuracy

Why Is My CGM Always Lower Than My Finger Stick?

This is usually related to the physiological delay and the initial immune response of the sensor.

Since the CGM measures interstitial fluid (ISF), the sugar concentration in the ISF is often lower than that in venous or finger stick blood during the first 24 hours after sensor implantation.

Can I Calibrate My CGM When The Reading Deviation Is Large?

For modern systems like SIBIONICS CGM, they use advanced factory calibration-free technology.

This means the device is precisely set at the factory and usually does not require manual finger stick calibration intervention from the user to maintain accurate operation.

What Should I Do If The Sensor Reading Stays "Way Off"?

If the reading still has a huge and unreasonable deviation from the finger stick after a 48-hour adaptation period, we recommend checking if the wear position has shifted or loosened first.

If the physical condition is intact but the data remains abnormal, please contact official customer service directly.

References

[1] American Diabetes Association Professional Practice Committee. (2025). Standards of Care in Diabetes—2025. Diabetes Care , 48(Supplement_1), S1-S352. https://doi.org/10.2337/dc25-S001;
[2] Cengiz, E., & Tamborlane, W. V. (2009). A tale of two compartments: interstitial versus blood glucose monitoring. Diabetes Technology & Therapeutics , 11(S1), S-11-S-16.https://doi.org/10.1089/dia.2009.0002
[3] Mensh, B. D., et al. (2013). Susceptibility of interstitial continuous glucose monitor performance to sleeping position. Journal of Diabetes Science and Technology , 7(4), 863-870.https://doi.org/10.1177/193229681300700408

Disclaimer

This article is for educational purposes only and does not replace medical advice.

The information provided is for education and knowledge sharing and does not constitute professional medical advice or diagnosis.

If your symptoms do not match your CGM readings, please seek professional medical guidance.

Author Information

This article was written by the SIBIONICS Professional Health Content Team.

The author has years of research experience in CGM and diabetes management, helping users optimize their device experience through science-based practices.

Last Updated

March 26, 2026