Your Sensor Glucose and Blood Glucose do not match?

Your Sensor Glucose and Blood Glucose do not match?

17 January 2022

CGM Sensors


Continuous Glucose Monitoring (CGM) Sensors are a useful medical device to monitor your blood sugar levels. CGMs measure your blood sugar continuously and for longer periods. CGMs are small and implantable sensors. They use an electrochemical sensor to measure the glucose in the interstitial fluid. Their task is to monitor your blood sugar level every 2 minutes without repeated finger pricking. Your glucose readings are received wirelessly on your smartphone via a mobile application every 2 or 5 minutes depending on the CGM model you are using.

However, blood sugar sensors require additional care and protection. Initially, it is essential how and where you insert a CGM sensor. Most CGM sensors are inserted on the arm, between the elbow and the shoulder, or in the abdomen, on the left or right side of the belly button. It is recommended you insert a CGM sensor on a site with most subcutaneous fat. Once inserted, the sensor needs to be protected and checked regularly. Below you will find valuable tips for using CGM sensors and why you may not expect your readings to match with your BGM (Blood Glucose Meter).



for better accuracy and longer sensor life


  • Choose insertion site according to the recommended ares specified in the user manual.
  • Clean the insertion sitethoroughly with alcohol pads/wipes for better adhesion.
  • Remove any hair on the insertion site. 
  • Insert sensor on sites with the most subcutaneous fat. CGM sensors need at least 5 mm subcutaneous fat for best performance and accuracy.
  • We recommend BMI (Body Mass Index) of at least 18.5 for best accuracy and less chance of sensor failure - calculate yours here.


15-15 RULE

and why your SG and BG won't always match?


Защо има разлики между сензор и глюкомер?

SG is your Sensor Glucose from a CGM, while BG is your Blood Glucose measured by a BGM. For instance, after we eat, blood sugar begins to rise approximately 15 minutes after food is ingested and reaches its peak around an hour later. Postprandial glucose (your glucose after a meal) depends on several factors, such as the type of food consumed, your metabolism, your BMI, etc.


When you measure your blood sugar with a glucose meter, an increase shall be seen 15 minutes after a meal. When you use a sensor, there is an additional "delay" of around 15 minutes until the glucose "reaches" the sensor. You should expect an increase in your sensor glucose readings around 30 minutes after a meal. The same pattern applies to the decrease in blood glucose.


When you compare a sensor reading to a glucometer reading, you should not always expect the same values.


Simply put, BGM and CGM measure different values:

  • Glucose meters measure glucose from a blood sample. BGMs are usually 15 minutes ahead from Sensor readings.
  • Sensors measure glucose in the interstitial fluid (it is important to have enough subcutaneous fat on the insertion site).


Blood sugar is like a roller coaster.


You may be wondering why your Blood Glucose (BG) and Sensor Glucose (SG) values do not match. To help you better understand, we will briefly explain why it is normal and why to expect it.


Glucose is the main source of energy of our body. We obtain it from the food we eat. When food, especially carbohydrates, is digested, it is converted into glucose. Glucose is absorbed from the digestive tract into the bloodstream. Only then does glucose move from the bloodstream to the fluid that surrounds all cells in the body. This fluid is called the interstitial fluid. Once in the interstitial fluid, glucose moves to the cells, where it can be used for energy.


When you prick your finger, you measure your blood glucose level. This type of measurement is Blood Glucose (BG). A CGM sensor measures glucose levels in the interstitial fluid (the fluid that surrounds our cells). This type of measurement is called Sensor Glucose (SG).


No match in readings between your BGM and CGM occurs because the information about your glucose levels has not yet reached to the interstitial fluid (SG). BGM and CGM measure glucose levels in two different areas. Glucose always moves between these two areas. Your Blood Glucose (BG) and Sensor Glucose (SG) will rarely be exactly the same. This is normal and you should expect it. Follow the calibration tips below for best performance.


Note that the information provided by the sensor is intended to add up, but not replace the blood glucose readings from a glucometer. Before treating, always check BG values with a glucose meter.



the most important part of achieving better accuracy.


This section applies to sensors which require calibration. Calibration is the step where mistakes are most common. Poor calibration can lead to sensor inaccuracy.


  1. CALIBRATE your CGM sensor when:
    • your blood sugar is stable (not rising/falling);
    • before a meal;
    • at least 2 hours after your last meal.
  2. CALIBRATE sensor from a clean finger.
  3. CALIBRATION NEEDED alert does not stop the monitoring function. Postpone calibration until you reach steady, non-fluctuating levels.
  4. DO NOT CALIBRATE sensor if your sugar levels have not been stable for at least 15 minutes. Postpone calibration until you reach steady levels.
  5. DO NOT CALIBRATE sensor if your sugar levels are out of the range 4 - 10 mmol/L.
  6. DO NOT CALIBRATE more than once a day.


Sensor calibration is a recommended feature with a sole purpose - safety. CGM systems which do not require calibration may be dangerous for your health as you may forget to compare your BG and SG readings.


Despite the differences you can observe between BG and SG readings, CGMs are a useful tool to monitor and control blood sugar levels. CGM readings provide broader and more detailed information about your glucose levels without pricking your finger every 2 minutes. Sensors are a great indicator of your glucose trends and levels. Glucose alarms come in handy, and they can be personalized (Glucose Alerts), as well as the alerts for quickly increasing or decreasing glucose levels (Rapid Rise/Fall).



CGM sensors should be used with caution.


To protect your CGM, you must

  • not lean on the sensor.
  • not apply pressure to the sensor*.
  • not wear tight clothing; this may impair sensor performance.
  • be careful not to hit external objects and objects, such as doors, furniture, etc.


*We do not recommend inserting a sensor on the side you sleep. During sleep, you may unconsciously apply pressure on the sensor or obstruct the signal without your knowledge. If you often sleep on your right side, we recommend inserting a sensor on your left side.




Always confirm your Sensor Readings with a Glucose Meter before treatment.


CGM Sensors Medtrum TouchCare A7+


Our team tested the Medtrum TouchCare A7+ (in some countries Slim) CGM System to study the accuracy and performance of the system. We will continue to inform you about the most effective and efficient methods of using CGM sensors. Medtrum TouchCare is a certified product in the European Union and is reimbursed in several countries across Europe. Medtrum TouchCare sensors have been approved for reimbursement by the National Health Insurance Fund in Bulgaria at the end of 2021.

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