Continuous Glucose Monitors: In Real Time
See where your blood glucose has been—and where it’s going—with a continuous glucose monitor
A continuous glucose monitor (CGM) is a helpful addition to intensive insulin therapy: While a finger-stick measurement shows just one point in time, a CGM displays real-time glucose trends. What’s more, CGMs sound an alarm when glucose is rising or falling rapidly, when levels go beyond user-programmed high and low thresholds, and (for certain models) when glucose is approaching a preset upper or lower limit.
Here’s how it all works: A small sensor is inserted under the skin to check glucose levels in tissue fluid. Each sensor stays in place for three to seven days (depending on device brand) before it must be replaced. A small transmitter sends glucose information wirelessly every few minutes from the sensor to a receiver that displays the current level and the trend graphs. Once a sensor and transmitter are in place, they’re waterproof and wearable during bathing and swimming. The handheld receiver, however, is not.
For now, the Food and Drug Administration (FDA) has not approved CGMs to replace finger-stick blood glucose testing. That’s because measuring glucose in the tissue fluid just under the skin isn’t as accurate as measuring glucose in the blood. A finger stick is still needed to make treatment decisions, including using the glucose value to calculate mealtime insulin doses and determining whether glucose is truly high or low enough to treat. Also, finger-stick checks are needed to calibrate a CGM every 12 hours.
But the technology continues to evolve, and the accuracy of the devices is always improving. Certain CGM sensors and transmitters interact with specific insulin pumps (such as the Animas Vibe and Medtronic MiniMed Real-Time Revel), so that the pump doubles as the CGM receiver and glucose trend graphs appear on the pump’s screen. Two different body sites are still needed, one for the pump’s insulin infusion and another for the CGM’s sensor. For people who use insulin injections or an insulin pump that doesn’t communicate with a CGM, stand-alone CGMs (Dexcom’s G4 Platinum and Medtonic’s Guardian Real-Time) are options to consider. Both stand-alone CGMs and pump-CGM combos allow users to download data to a computer via special software.
Other CGM features to look for include sensor range—that is, how far away the sensor-wearer can be from the stand-alone receiver and still have it register blood glucose levels—and whether or not the CGM is approved for use in children.
One particular CGM, Medtronic’s MiniMed 530G With Enlite, includes a notable advanced feature: If the CGM readings show that the wearer’s glucose level has reached a preset low level and the user doesn’t respond to the alarm, the system will automatically stop insulin delivery for two hours, or until the user turns it back on. This safety feature is seen as a step toward the ultimate goal of developing an artificial pancreas that would automate insulin delivery with little or no user input. For that to happen, researchers will need to program the CGM and the pump with mathematical formulas that will allow the pump to respond precisely and reliably to accurate commands from the CGM. Progress is being made in that direction, so stay tuned.