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Continuous Glucose Monitoring
Troubleshooting Common Problems

by Neesha Ramchandani, PNP, CDE

With the DexCom system, the sensor is covered by an adhesive patch, so you will not be able to see whether there is blood at the site. If the DexCom sensor site is tender to touch, it is possible that there is or was some internal bleeding around the sensor. If the sensor glucose readings are accurate, you can leave the sensor in; if they are not, or if you are having difficulty with data transmission, you should change the sensor.

How do I calibrate the monitoring system?
The glucose level in interstitial fluid is not identical to the level in the bloodstream. To get the most accurate readings from your CGM, you need to calibrate it by checking your blood glucose level using a blood sample from a fingertip at certain time intervals and entering those readings into the system.

The DexCom SEVEN needs fingerstick calibration 2 hours after you activate (or start) the system and every 12 hours thereafter. Medtronic’s CGM systems need fingerstick calibrations 2 hours after activating the system (this time may be shortened if you insert the sensor early), 6 hours after that, and then every 12 hours thereafter. Except for the initial calibration, both the DexCom and Medtronic CGM systems can be calibrated earlier than is required (for example, if your next calibration is due at 1:00 AM, the system will let you calibrate it at bedtime even though it is not 1:00 AM, and it resets the clock so that the next fingerstick calibration is due 12 hours after the one you just put in). Failure to calibrate any CGM system at or before the requested time will result in lost data. Sometimes a little advance planning is needed to make sure your CGM system does not beep requesting a calibration in the middle of a meeting or class or in the middle of the night.

What is the best time of day to insert a new sensor?
Ideally, a new sensor should be inserted when your blood glucose level will be relatively stable for the next few hours; this ensures that the sensor calibrates correctly. The most common time to have stable blood glucose levels for several hours is overnight, while you’re sleeping.

It sometimes helps to insert the sensor several hours before you want to start using it. For example, you can insert it before you go to sleep, when your blood glucose is more likely to be stable, then start the calibration process the following morning. Doing this may also shorten the amount of time it takes for the sensor to calibrate once you start the calibration process. Be advised that if you do insert the sensor several hours before you intend to start using it, you must attach the transmitter to the sensor at the time of insertion. Failure to do so may expose vital parts of the sensor to air and moisture, causing the sensor to malfunction.

What’s causing my inaccurate readings?
The number one reason for inaccuracy in sensor glucose readings is that the CGM system was calibrated when your blood glucose level was changing rapidly. Only the DexCom SEVEN sensor can be calibrated during times of glucose fluctuation. It is also important to remember that sensor glucose readings are supposed to be within approximately 20% of fingerstick blood glucose values; they are not expected to be identical. CGM is most helpful for identifying trends in your glucose levels and for alerting you when your glucose is out of range or when it is going to be out of range within the next half hour. It is not meant to replace fingerstick blood glucose monitoring.

Other factors that may affect sensor accuracy include bleeding at the site, use of acetaminophen (Tylenol), vitamin C supplementation, and high levels of uric acid in the body (as occurs in people with gout). Both acetaminophen and vitamin C can give falsely elevated sensor glucose readings. An attempt to recalibrate a CGM system after you have taken either acetaminophen or vitamin C will not help; rather, it will cause sensor errors because of the great discrepancy between the actual blood glucose reading and the sensor glucose value.

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