Continuous Glucose Monitoring (CGM) sensors are finicky, expensive and we all know of how much of a process it is for Medtronic to replace faulty sensors. For me, changing Medtronic CGM sensors is a ritual I do not look forward for many reasons. Getting my sensor inserted and working correctly seems to be a hit-or-miss proposition.
But the delicate nature of sensors is due to the fact they are miniature electrodes that are affected by small changes in the environment around them including small movements or presence of interfering chemical substances.
Sensor Electrochemistry 101
To understand why CGM sensors are so delicate, we have to think of how the sensor electrode (part inserted into your body) interacts with our bodies. The interface between the sensor and your body fluids is what we Electrochemists call the Electrical Double Layer as shown in Figure 1. The solid surface is the surface of your sensor electrode while the bulk liquid is the interstitial fluid interacting with your sensor.
The Double layer interface between your sensor and interstitial fluid is very small and in the nano-meter range. Therefore, any small changes at the surface of the minuscule sensor electrode, interface or in the bloodstream will lead to unexpected changes in your BG readings.
Generally for a material to be used as an accurate sensor, it must meet two criteria.
- It must be selective. CGM sensors and BG test strips coated with a material that will only measure glucose and glucose only (in the case of Enlite and Guardian sensors it is an enzymatic material that specifically interacts with glucose coated on a very conductive gold substrate)
- It must be sensitive enough. The transmitter connected to your sensor powers the electrode inside your body at a certain potential so that glucose can be brought to the surface of the electrode. The transmitter will the record the change in electrical current resulting from glucose interacting with the surface of the electrode. In the case of the Medtronic Sensor the current reading is presented as ISIG readings. The ISIG is then converted by your pump through a series of calculations against a reference into a BG reading.
Greater Sensitivity means better ISIG and Stable Readings for Sensors
The sensitivity of the sensor is what mostly affects how stable the sensor is going to be during measurements. Ideally, all the Enlite or Guardian sensors you receive from Medtronic should have same sensitivity out of the box since they are assembled from identical materials. But in reality each sensor will have its own sensitivity since no two electrode surfaces can be the same.
Differing sensitivities is why one sensor will be flawless and another will be problematic. The higher the sensitivity, the more stable and accurate the sensor will be during its seven-day life.
When a sensor is working correctly there will be a good correlation between the ISIG and your BG reading (i.e a repeatable pattern where a higher BG reading will correspond to a higher ISIG reading and vise-versa).
Why are Low Readings more inaccurate for Sensors?
Because of how small in size sensor electrodes are, BG readings on the lower side are more prone to be less accurate and unstable since you are nearly at the lower sensitivity limit of the electrode.
You will notice at low BG that the ISIG readings are really low and your sensor readings might be quite different from your meter readings .
Sensitivity issues with current CGM sensor is one of the main reasons why researchers today are working towards larger, implantable, self-powered and more sensitive sensors.
Why Does My Sensor Flatline at Very High BG Readings?
Your sensor could flatline for two reasons:
- Either the sensor has reached it upper limit for sensitivity and it does not see any changes in glucose concentration.
- Either the electronics in you pump can’t handle the current readings from your sensor i.e. the analog-digital converter has reached its upper limit.
Life of the Sensor
As time goes by the sensors start to degrade and it might lose its selectivity and/or becomes less sensitive.
And that is why towards the end of the life of your sensor, readings are sometimes all over the place. Hence the requirement by Medtronic that you change your sensor every 7 days.
The selectivity of the sensor can also be affected if your sensor cannot make a distinction between glucose and another chemical substances in your bloodstream.
For example, Dexcom G5 readings are affected by the presence of acetaminophen. In this case if the sensor will most likely give a false high if acetaminophen is present.
Why a Quiet time of 2 hours for my Sensors?
Why is it that we have to wait for 2 hours before we perform the first calibration? This also has to do with the stability of the double layer interface. Since the glucose molecules have to travel from your bloodstream to the surface of the electrode, it takes time before the interface becomes stable. The electrode surface also has to be conditioned (more or less filling the nook and crannies of the surface electrode with glucose molecules). When this is all done the electrode readings will become more stable.
In the case of the Enlite and Guardian and the Dexcom G5 sensors, two hours is enough to stabilize the electrode (they are all coated with a similar enzymatic material selective to glucose).
It might take a little longer than two hours for a sensor to stabilize and that is why you might end up getting erratic readings during the first day.
And sometimes a sensor simply just doesn’t stabilize.
Read an indepth analysis of CGM sensor technology and emerging technologies here
Photo:By Larryisgood [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons