Continuous Glucose Monitoring

July 7, 2009 10:49 am

The newest way to monitor blood glucose levels is with a continuous glucose monitoring (CGM) device. These devices don’t directly measure the concentration of glucose in blood. Instead, they use a sensor placed just under the skin to measure the concentration of glucose in the fluid between cells, called interstitial fluid. The sensor reading is transmitted to a receiving device (similar in size to a cell phone), which displays the glucose reading and can also sound an alarm if the level is higher or lower than levels preset by the user. The currently available CGM devices can also show whether glucose levels are rising or falling, and they can display graphs showing glucose levels over a certain number of hours.

The three CGM devices now on the market are the following:


The Medtronic Guardian REAL-Time System (which can be used alone or as an integrated part of the MiniMed Paradigm 522 or 722 insulin pump) is approved for ages seven and up. Its sensors can be worn for up to 3 days. After a new sensor is placed beneath the skin, there is a 2-hour start-up period during which the sensor becomes “acclimated” and will not give accurate results. After the 2-hour start-up period, glucose readings are displayed every 5 minutes. The cost of the sensors is $35.00 apiece, and the transmitter costs approximately $1000.00.

The DexCom Seven System is approved for ages 18 and up. Its sensors can be worn for 7 days, and they require a 2-hour start-up period. After the 2 hours, readings are displayed every 5 minutes. The cost of sensors is $60.00 apiece, and the list price for the starter kit including the transmitter, receiver charge cable, calibration cable, receiver case, and DM2 software is $800.00.

The FreeStyle Navigator, made by Abbott Diabetes Care, is approved for ages 18 and up. Its sensors can be worn for 5 days, and they require a 10-hour start-up period, after which glucose readings are displayed every minute. The starter kit costs $960–$1050, and the sensors cost $65.00 apiece.

For accuracy, all CGM devices must be calibrated periodically with fingerstick blood glucose monitoring results. This enables the device to report interstitial fluid glucose levels that are as close as possible to actual blood glucose levels. It’s important to perform the fingerstick checks at times when your blood glucose level is unlikely to be changing rapidly. Typically, this is before meals or several hours after meals, but it takes some experimentation to figure out the best times to calibrate for each individual.

Even with good calibration, there can be a 15% to 20% difference between blood glucose readings and sensor readings. This is because of the way glucose moves through the body: After food is eaten, glucose is absorbed through the small intestine. From there it enters the bloodstream. And from the bloodstream, it is moved to the various cells of the body and to the interstitial fluid. Consequently, the glucose level of interstitial fluid rises about 10–15 minutes after blood glucose level rises, and it similarly falls about 10–15 minutes later. This is commonly referred to as the “lag time.”

Because of this lag, it’s common for CGM users to set their hypoglycemia alarm higher than the blood glucose level at which they normally treat for hypoglycemia: If the CGM receiver reads 80 mg/dl, the blood glucose level may actually be below 70 mg/dl. Any symptoms of hypoglycemia should be checked out with a conventional blood glucose meter.

Getting the most out of a CGM takes training and practice. Some people decide it’s too much information, while others never want to be without one. If you’re interested in trying a CGM, speak to your health-care provider. Some clinics own loaner devices that can be worn for several days. This can be useful both to see what it’s like to wear one and also for the data the device captures: At the end of the trial period, your glucose levels over that period can be uploaded to a computer and displayed in a graphic format so you can see your ups and downs over the day and night.

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