A continuous glucose monitor uses a sensor and transmitter attached to the body (left) that communicate with a handheld receiver (right) or an insulin pump.

To get a complete sense of how your blood glucose behaves throughout the day, you could test every minute, 1,440 times a day. (Ouch.) Or, you can wear a continuous glucose monitor and let the device do the work for you. Continuous glucose monitors (CGMs) use an under-the-skin sensor to measure glucose levels and transmit the results to a handheld receiver or insulin pump every one to five minutes, depending on the brand. The benefit of using a CGM is, of course, understanding exactly what your glucose is doing at all hours—especially overnight. Equipped with this knowledge, you can better adjust your treatment plan.

Compared with blood glucose testing, continuous glucose monitoring is still a relatively new technology, so there are only three models on the market. Their basic functions are similar—all provide at-a-glance readings and make it easy to track trends—but each stands out in its own way. You may be able to get a feel for how the brands differ by experimenting with sample products at your doctor’s office. (For starters, one has a built-in blood glucose meter; another is a pump-CGM combo. Both cut down on the number of devices you have to carry around daily.) Check with your insurer to see whether a specific device is covered before you start shopping around. Once you choose a CGM, the medical device company’s experts will walk you through the insurance approval process.

Before you decide on a product, you can learn about CGMs’ basic functions, explained below. Then look at the side-by-side comparison of the three available models.

Nonstop Testing 
How often a sensor sends glucose readings to a receiver differs among devices—one transmits every minute, others every five—but the difference is so slight that it shouldn’t affect your decision making, says Howard Wolpert, MD, a physician in the Joslin Clinic Section on Adult Diabetes and the director of the Insulin Pump and CGM Program at Joslin Diabetes Center. The bottom line: Your CGM will record glucose levels much more often than you would on your own.

Necessary Adjustments
To keep your CGM working properly, you’ll have to calibrate it on a regular basis by testing your blood glucose with a standard meter and entering the result into your CGM receiver. How often you’re required to calibrate—say, every 12 hours—varies somewhat among devices.

Data at a Glance
Take a quick look at your CGM’s home screen, and you’ll see your current glucose level and if it’s changing. All models feature arrows that indicate whether your glucose is rising or falling—and how fast.

Trend Tracker 
How have your glucose levels behaved during the past three hours? How about over the past day? If you use a CGM, you can graph glucose trends with a few button pushes. Though each model computes a different set of hourly trends—one averages the past two hours, another averages the past three—all make it easy to see the spikes and dips you’ve experienced throughout the day.

Event Markers 
Knowing how your glucose behaves can help you alter your diabetes treatment plan. Recognizing why your glucose acts in a certain way makes an even bigger difference. That’s why all CGMs allow you to note when you eat, exercise, get sick, and inject insulin.

Warning Bells
So that you don’t have to spend the day staring at your CGM’s screen, each device lets you program high- or low-glucose alarms. A couple of models will alert you if your glucose is moving very quickly in either direction, and others will sound an alarm if your glucose is drawing near to your customized upper or lower limit.

Continuous glucose monitors can help keep your glucose in line, but the devices aren’t for everyone. Many insurance plans don’t cover CGMs for people with type 2 diabetes, so it’s important to talk to your insurer about your options. Certain people find the extra handheld device inconvenient to carry. Those on insulin pumps already wear an under-the-skin cannula and may find it bothersome to maintain a second site for the CGM sensor. Plus, plenty of people with type 1 are happy enough with finger sticks, thank you very much. “The big issue, particularly among women, is the extra hardware,” says Wolpert. “It may make diabetes management easier in some ways, but it does present other demands.” In the end, whether you find the device fussy or functional is a matter of personal choice.