Just had this conversation the other day with my CDE. The sensor (part that is inserted) is coated with an enzyme and it actually measures electrical activity in the interstitial fluid. There is an algorithm that converts this info to a glucose measurement. The initial calibration tell it how current conditions relate to current bg values.
The recalibrations are to check that conditions haven't changed due to the body's reaction to the sensor or other factors.
I haven't searched yet, but you might want to google continuous glucose monitor enzyme, or electrical activity, or.....
thanks. I know very well that we calibrate all Glucometers against a solution of know glucose contents, but this is not done every 12 hrs. Does anyone know of a reason why we have to calibrate CGM every 12 hrs? This shouldnt be necessary- i think, if the interstitial fluid is a proxy for blood glucose and if an algorithm will seamlessly marry the 2, then we shouldnt calibrate that often... just thinking out loud. thanks anyway
Adding to above, "interstitial fluid" is blood that has made its way into the body's tissues (arteries>capillaries>body tissues). The glucose content of interstitial fluid is apparently not as up to date as glucose in bloodstream, thus causing the "lag" time associated with CGM.
Sometimes the calibrations are small adjustments and sometimes they are large adjustments. Certain sensor systems will continue working without a recalibration where as others will just stop until you put in a value. Occasionally, the enzymes can go bad (aka bad sensors) or it can be inserted somewhere other than the interstitial fluid. All of these are factors that can't be accounted for without the whole finger stick. Whenever they do get around to finding a better sensor for glucose that last for longer durations, we will be that much closer to having a closed loop system or an artificial pancreas.
More to note is that certain chemical compounds, such as acetaminophen, cause the readings to become skewed.
In the case of acetaminophen, the drug works in part by retarding electrical impulses to your brain to help reduce pain. A byproduct of this process is that the interstitial fluid becomes more electrically resistant in your entire body. Because the CGM measures electrical resistance, your BG readings on your CGM may skyrocket when popping a Tylenol.
Also my CDE tells me the sensor will begin to "drift" without calibrations, and given enough time, the numbers will no longer correlate to the actual blood glusoce readings. She claims that without the calibrations, the "drift" will become so large that repeated calibrations will not be able to bring the sensor back into a correct reading and it would finally "fail."
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