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Keys to Choosing a CGM

What to know before you buy a continuous glucose monitor

By David G. Marrero, PhD , , ,

Eric Hinders/Mittera

As a diabetes researcher who’s lived with type 1 for over 40 years, I’ve had the opportunity to use and evaluate the evolving glucose monitoring technologies. In fact, I wrote my first editorial for Diabetes Forecast as an associate editor, touting the virtues of home glucose monitoring back in the 1980s.

Today, we have a lot of options, which can be customized to meet our individual needs and lifestyles. Because of my history, I am frequently asked my personal opinion about these products. Here, I’ll share some of my experiences and give you my take on the newest technologies in continuous glucose monitoring (CGM). 

In the Beginning

When I was diagnosed with type 1, the only way to know your glucose level was through blood draws at the lab or urine glucose checks. The problem with urine checks is that glucose hits the blood first, much quicker than it reaches the urine. (Glucose doesn’t spill into the urine until blood glucose rises above 180 to 200 mg/dl.) Because of that, urine glucose readings don’t provide an accurate picture of what your glucose levels are at a given point in time.

The first big change for me came when home blood glucose monitoring was developed. This allowed me to see what my glucose level was while I was sitting there. It was a big transformation for all of us with diabetes: We could now adjust insulin and carbohydrates ourselves.

The early home monitoring systems weren’t perfect—far from it. The technology required you to measure the length of time a blood sample sat on the test strip and wipe it off before putting the strip in the machine. This often affected the reading. Fortunately, meters got better, with no need to wipe the test strip and a simpler testing procedure.

However, there were still limitations. For example, a meter check simply provided a snapshot of my glucose at a single point in time. It didn’t tell me where my glucose was going. I had to check dozens of times a day to see a pattern.

The Rise of CGMS

The first time I used a CGM was a watershed experience. I could now see trends as they emerged. Another feature was much more profound: Arrows indicated the direction my glucose was heading in the next 30 to 60 minutes. Did I take too much insulin or get my carb count wrong? Was I going to be in trouble soon? Suddenly, I was able to see my diabetes management in ways I never had before.

Understanding the Differences

As with all technologies, continuous glucose monitoring has evolved. We’ve seen significant improvements in accuracy, reliability, and ease of use. And we’ve also seen a growing number of options. Today, we can choose between real-time and intermittently scanned CGMs. Both provide data about your current glucose level. (Keep in mind: CGM readings show what your blood glucose level was during the previous 15 to 20 minutes, compared with meter readings, due to greater blood flow at the fingertip.) And both offer trends and predictive information about changing glucose levels. However, there are some specific differences between these approaches that you may want to consider when choosing a system that’s right for you. Here’s what you need to know:

Real-time CGM

These systems are made up of three components: the sensor, a small wire catheter that is placed under the skin on your arm or abdomen; a transmitter that attaches to the sensor; and a handheld receiver and/or smartphone that displays your glucose data. Currently, there are four stand-alone real-time devices on the market: the Dexcom G5 Mobile and G6 systems, Medtronic’s Guardian Connect, and Senseonics’ Eversense. Medtronic’s combo pump-CGMs, the 630G and 670G, are also real-time systems. All systems automatically transfer real-time data to a receiver or mobile device and feature alarms that warn you about immediate and impending low and high glucose. And all offer the ability to share real-time data with family members, caregivers, and health care providers.

The advantage with real-time CGM systems is that I’m always informed about my diabetes management. They take over the job of constantly checking my glucose, offer around-the-clock vigilance, and provide data at times when I can use it. They say, “Hey, Dave, this is what’s happening, this is where your glucose is going, and this is a problem you want to deal with.”

Real-Time CGM Upsides

1. Offers Alerts

To me, the most significant benefit of all real-time CGM systems is having audible alarms that can warn me if my glucose is headed to a place I don’t want it to be. This gives me enough time to make adjustments that could blunt the impact of high or low blood glucose or avoid it altogether. Most important, as I get older, I have discovered that I don’t detect changes in my glucose as well as I used to. When I was younger, I was able to sense I was going low well before I would be in real trouble, giving me plenty of time to make a correction. These days, I find that I can get pretty low before I’m fully aware that I am getting into trouble. In this regard, my real-time CGM is an exceptionally valuable and potentially lifesaving tool, especially when I’m sleeping.

2. Transmits Data Ccontinuously

With real-time CGM devices, data is constantly pushed to my receiver or smartphone. The data are always right in front of me; I don’t have to perform another task, such as a finger stick, just to know my reading—something I might not have the time or ability to do.

3. Shares Data

The ability to share data with my wife and up to four other family members and friends is another important feature. It acts as a safety net, especially when I’m traveling. For example, if I don’t wake up to a low glucose alarm during the night, I’ll get a call from my wife, telling me to get my act together and do something about it (so much for compassionate spouses).

4. Eliminates Finger Sticks

While not all real-time CGMs offer this benefit, Dexcom’s G5 Mobile and G6 systems allow you to make treatment decisions—how much insulin to dose, for instance—without the need for finger-stick confirmation. And the G6 is factory calibrated, so there’s no need for twice-daily finger-stick calibrations. It also gives the option to recalibrate if my sensor values don’t match my symptoms or a confirmatory finger-stick check. This improves my confidence in using the CGM data.

Real-Time CGM Downsides

1. Requires Setup

To use the alarm and alert features, you have to program your settings, such as your low glucose threshold and target ranges. This can get a little complicated, especially if you don’t read the instructions. However, the manufacturers offer online video tutorials to guide you through the process. Your diabetes care team, particularly your diabetes educator, can help you learn to use your device.

2. Alerts Can Be Tiresome

Some people complain about the repeated alarms (real or false). It can become so bothersome—or embarrassing, depending on where you are or who you’re with—that they simply turn off the alarms altogether. However, this is mainly a problem with the older CGM systems, which were not as accurate as the newer ones.

3. Devices Can Be Expensive

Although they are covered by most insurance companies and Medicare, they may not be affordable if you have to pay out of pocket.

Intermittently Scanned CGM

Abbott’s FreeStyle Libre takes another approach. Known as an intermittently scanned CGM, the system requires you to scan the device to obtain your glucose data. It uses two components: a combined glucose sensor/transmitter (inserted in your upper arm) and a separate touchscreen reader device. The FreeStyle Libre sensor continuously samples and measures glucose levels, generates a new glucose value every minute, and records the reading every 15 minutes for 10 to 14 days of sensor wear time, depending on the model. The FreeStyle Libre provides the same glucose data as real-time CGM systems, but it doesn’t automatically “push” data to the receiver or mobile device. To get your glucose trends, you’ll need to wave the reader over the transmitter.

I think of the FreeStyle Libre system as a really sophisticated home glucose meter—it’s self-monitoring of blood glucose on steroids. It gives me both snapshots and trends when I scan the sensor. And I can do it fairly freely without having to stick my finger every time I want to check.

Intermittently Scanned CGM Upsides

1. Eliminates Finger Sticks

As with the Dexcom G5 Mobile and G6, glucose data from the FreeStyle Libre can be used to make treatment adjustments without the need for finger-stick confirmation. Plus, it’s factory calibrated, eliminating the hassle and pain of daily calibration with finger sticks.

2. It’s Convenient and Easy to Use

The sensor, which is about the size of two stacked quarters, is painless to apply, comfortable to wear, and easy to use. And you can scan the transmitter through your clothes, a real benefit when you want to be discreet.

3. It’s Affordable

The FreeStyle Libre is much less expensive than real-time CGM devices, and it is covered by most insurance companies and Medicare.

Intermittently Scanned CGM Downsides

1. Offers No Alerts

For me, the biggest drawback to an intermittently scanned CGM is the lack of alerts to warn about current or impending glucose problems. Without the chime of a CGM alarm, people with hypoglycemia unawareness (like me) may not know they’re low until their judgment is impaired. This also means you won’t be woken by a low-glucose alarm—a downside for anyone concerned about overnight lows.

2. Requires Intent

I need to remember to wave the reader over the transmitter, which might not happen on days when I’m busy or distracted, or at night. And there’s a price for forgetting: The sensor can provide glucose values for 10 to 14 days (depending on the model) if you scan at least every eight hours. If not, the glucose information from the previous eight hours will be overwritten and not available for decision-making or download.

3. Real-Time Data Isn’t Shared

Unlike with other CGMs, your glucose values can’t be shared in real time with family, caregivers, or health care providers. This is a major drawback for me because it doesn’t give me a safety net if I’m going low when I’m traveling or distracted by other things.

4. One Version Takes 12 Hours to Warm Up

When a new sensor is inserted, the 10-day FreeStyle Libre will not show any glucose data for the first 12 hours. During this time, you will need to do finger-stick checks. Thankfully, the FreeStyle Libre’s 14-day model takes only an hour to warm up.

5. Offers No Option to Calibrate

The FreeStyle Libre does not allow you to recalibrate the sensor when glucose values don’t match your finger-stick results. When your sensor and finger-stick glucose readings don’t match, it may mean your meter is inaccurate. Or it could indicate that your blood glucose is rapidly rising or falling. But it could also point to an inaccurate CGM sensor. Without the ability to recalibrate the sensor, you may need to insert a new sensor long before its indicated wear time has expired.

The Bottom Line

Whether real-time or intermittently scanned, a CGM is a major technological breakthrough in diabetes self-management. Both approaches offer opportunities to improve your diabetes management. If you’re thinking about using a CGM, it’s important to consider the issues you’re trying to address. The overriding issue should always be safety.

If you’re taking any medication that puts you at risk for hypoglycemia (such as insulin or sulfonylureas), then a real-time CGM is probably the best choice because it will alert you when your glucose is dropping so you can prevent or stop a low before it becomes critical. It’s particularly important for children and adolescents who would benefit from remote monitoring through the data-sharing feature.

But if you tell me, “Dave, I have type 2 diabetes. I don’t really get hypoglycemic because I’m only taking metformin. I’m more worried about high glucose, and I’m not always sure how the foods I’m eating are impacting me,” then an intermittently scanned system makes a lot of sense. It’s basically a very rapid response glucose meter that gets you the information you need without the hassle and mess that goes along with finger sticks and with the ability to continuously record glucose values.

Take the time to investigate both options. Talk to people who are using a CGM and ask how it’s working for them. And talk to your doctor and diabetes educator, who can provide valuable guidance and insights about the type of CGM system that may be right for you. They can help you make the transition to a CGM and provide training to help you learn how to interpret and use your data to make appropriate treatment decisions and achieve your blood glucose goals.

Regardless of what type of system you may choose, I am a total advocate for continuous glucose monitoring. It certainly changed my life!

David G. Marrero, PhD, is the director of The Center for Border Health Disparities and a professor of endocrinology and public health at the University of Arizona Health Sciences. He’s also an associate editor of Diabetes Forecast.

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