Selecting A Continuous Glucose Monitoring Device

By Gary Scheiner, MS CDE

Continuous Glucose Monitors (CGMs) provide blood glucose estimates by way of a tiny sensor placed just below the skin. A small radio transmitter is attached to the sensor and sends a signal to a handheld device (or a pump), which then displays and graphs the data on-screen. The handheld device can also beep or vibrate to alert the user if the glucose concentration is approaching a high or low level.

The decision to use a continuous glucose monitoring device is a personal one. Although the accuracy is not quite on par with traditional blood glucose meters, CGMs provide good estimates of blood glucose levels. The high/low alerts serve as a useful early warning system, and the trending information is helpful for guiding daily self-management decisions.

Research has shown that consistent use of CGM helps to reduce the severity, frequency, and duration of hypoglycemia. For those with elevated A1c levels, CGMs help to produce a significant improvement. And that, in turn, reduces the risk of developing long-term diabetes complications.

Insurance coverage for CGM is improving along with system accuracy, comfort, and convenience. As a result, more and more people with diabetes are choosing to make CGM an integral part of their diabetes management program.   

The following are features that all CGM systems have in common:

•    Include a receiver/display, transmitter, and subcutaneous sensor
•    Wireless system; radio signals utilized to send data from the sensor to the display.
•    Waterproof sensors/transmitters (receiver/display not waterproof).
•    Require periodic calibration with fingerstick blood glucose readings.
•    Customizable alerts for high/low glucose values.
•    Multiple on-screen trend graphs.
•    Mechanical device used to insert sensors.
•    Sensors typically re-usable.
•    Warm-up period at beginning of sensor session; no data provided during warm-up.
•    Lag time between blood glucose (fingerstick) and subcutaneous glucose (sensor) values.
•    Minimal skin irritation, even with prolonged sensor use.

Given the time/energy (and trust) invested in a continuous glucose monitoring device, it is advisable to compare the available systems and select the one that best meets your needs. In some cases, your healthcare team may have demo units that you can try out before you place an order. The following are features to consider when selecting a system:

Accuracy – Consider the "mean absolute difference percentage" for each system. This represents the discrepancy between sensor glucose values and simultaneous blood glucose values measured by a lab. The larger the percentage, the worse the accuracy. Some systems also provide accuracy data relative to hypoglycemia:  how often readings below a specific threshold are detected by the system, or the mean absolute difference percentage when blood glucose values are below a specific value. Be sure to look at the conditions under which each company evaluates the performance of their systems. Were calibrations performed a similar number of times? Were real-life conditions or "controlled" conditions being used?

Complexity – The more work it takes to make a CGM function properly, the less likely it is to perform well and the less likely you are to benefit from it. Are there special conditions that have to be met when inserting and starting a sensor? How often must system components be charged or replaced? How many steps are needed to change a sensor and program the receiver?

Calibration requirements – Different systems require calibration (fingerstick glucose values entered into the receiver) at different intervals.  How many times per day must calibrations be performed? Will the calibration times work within your schedule? Can you calibrate at any time, or must your blood sugar be in a steady state?

Transmitter Capabilities – To paraphrase, the data is only useful if the signal goes through. What is the transmission range? A signal that only travels a few feet may result in frequent lost data. A stronger signal will produce better data continuity and may permit loved ones to monitor your glucose levels in another room or on the sidelines of a sporting event.

Sensor longevity – Changing sensors can be costly, inconvenient, and painstaking. What's more, the accuracy of most sensors actually improves with longer use. Some systems allow for repeated use of the same sensor while others do not.  Find out how long sensors tend to really last (diabetes blogs and chat rooms are a good place to learn this kind of information).

Integration – Nobody likes to wear or carry any more "stuff" than is absolutely necessary. Some sensors transmit data into insulin pumps so that the pump serves as the display for the information. Some of these integrated systems use the sensor data to automatically adjust insulin delivery for preventing hypoglycemia.   

Cost/Coverage – Although most private health insurance plans cover CGM, there are often copays and deductibles that must be met. And most public plans still do not cover CGM at all, so cost is still an issue. In addition to the cost of the CGM equipment, consider the ongoing costs associated with replacing transmitters and disposable sensors (taking into account how long each lasts).   

Receiver Features – Take a look at the clarity/contrast/resolution of the display. Can you see it clearly in a dark place  In the sunshine? Listen to/feel the alarms and alerts. Are they robust enough to wake you or have you take notice in the event of a high or low blood sugar? Are they customizable so as not to be too much of a nuisance? Check out the alert features in the receiver to make sure they can be set to meet your individual needs. Also, see how much data the receiver can display. Does it go back far enough? Does it generate statistics in addition to graphs?

Downloading – If data is generated but not acted upon, what good is it? CGMs generate a considerable amount of raw glucose data, and some also allow manual entry of information pertinent to one's daily diabetes management. Although all CGM systems are downloadable, there are differences between the programs. Find out if the system continuous glucose monitoring device can be downloaded to the type of computer and operating system that you are using. Is the software computer-based or web-based? Can the data be shared easily with healthcare providers? Does it produce the kind of reports that you and your healthcare team find useful?

Gary Scheiner is the 2014 AADE Diabetes Educator of the Year and author of "Think Like A Pancreas". He has had type-1 diabetes since 1985, and offers diabetes management consulting services worldwide via his website,

Last Modified Date: April 04, 2014

All content on is created and reviewed in compliance with our editorial policy.

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