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.
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.