The Pros and Cons of Continuous Glucose Monitoring (CGM)

You’ve probably seen me wearing a small device on my upper arm in a lot of the pictures on this website. That’s my CGM, a tool that’s instrumental in my daily diabetes management. 

It’s a brilliant piece of equipment, and I absolutely love it (most of the time).

Key Points:

• CGMs provide real-time and historical glucose data, improving awareness of how foods and activities affect glucose levels. These devices help reduce episodes of hypoglycemia, improve A1c levels, and increase confidence and security in diabetes management.
• CGM use significantly reduces the need for traditional finger-stick monitoring, providing greater freedom in daily activities. This can provide peace of mind during exercise and other activities. 
• Issues with CGM sensors can sometimes lead to inaccurate readings, potentially affecting diabetes management decisions, and the sensors can occasionally cause inconvenience or discomfort. Additionally, overreliance on the device can reduce personal judgment. 
• Despite the potential drawbacks, CGMs are highly recommended for people with diabetes who take insulin, as they offer valuable insights into glucose trends and overall diabetes management.
• Which CGM system to use depends on a variety of factors, including compatibility with insulin pumps, smartphone integration and data accessibility, and lifestyle needs. You should weigh these factors based on your specific diabetes management requirements.

What is CGM?

CGM stands for continuous glucose monitor. This device makes it possible for me to see my glucose readings in (almost) real-time and to track historical glucose data. 

Most CGMs include three main components: 

• A small sensor that is inserted just under the skin (where it reads glucose levels in interstitial fluid, or the fluid between cells)
• An attached transmitter, which sends the readings to a receiving device (this is the part you see on my arm)
• A receiver or smartphone that displays the readings and glucose trend data

There are currently several CGM systems available for personal use in the United States:

• Abbott FreeStyle Libre 2
• Abbott FreeStyle Libre 3
• Ascensia Diabetes Care Eversense E3 (a fully implantable option)
• Dexcom G6
• Dexcom G7
• Medtronic Guardian Connect

Most CGM devices provide glucose readings every 5 minutes, while the FreeStyle Libre “flash” monitoring systems provide readings upon scanning of the device.

My experience with CGM

I got my first Medtronic CGM in December 2012 and wore it every day for 18 months before taking a break. I then ordered my first Dexcom system in August 2016, and that has been my go-to since.

There are pros and cons to all CGM systems. Since I’ve only worn Medtronic and Dexcom devices to date, I’ll give you my opinions based on those so you can decide if a CGM might help you manage your diabetes better.

The advantages of continuous glucose monitoring

Data

If you do finger-stick blood sugar checks and know your body well, you probably think you have a pretty good idea of what your sugars are up to between measurements. Chances are good, though, that you may be wrong.

I got a big surprise when I started seeing how much my blood sugar actually varied throughout the day, not to mention during the night when I usually don’t check. 

CGM is a vital tool for providing this type of ongoing information that can show trends and patterns, as opposed to the snapshots that are provided by traditional monitoring.

All of a sudden, I could see how certain foods and activities affected my glucose levels over the next several hours. That information is golden! 

Plus, being able to spot low sugars before they hit (thanks to the alerts I set for when my sugar levels start dropping or get to a certain point) has really cut down on how often I have low blood sugar episodes. It also means I’m consuming fewer glucose tablets and less juice to deal with said lows.

Management

So, what to do with all this new information? In my case, it has not only helped me limit episodes of hypoglycemia (low blood sugar), but I have also managed to reduce my A1c level (a measure of glucose control over the previous 2 to 3 months) to around 6.0 percent. 

My blood sugar has always been pretty well-regulated but has never been this stable before.

What’s more, some current CGM devices can integrate with an insulin pump, allowing you to monitor both glucose and insulin levels at the same time. 

And certain devices combine CGM and insulin pump technology with an algorithm that automatically adjusts insulin dosing to help glucose stay within target range, in an innovation known as hybrid-closed loop (HCL) technology. 

Current HCL devices on the market include:

• Medtronic MiniMed 770G
• Medtronic MiniMed 780G
• Tandem Control-IQ with Dexcom G6
• Insulet Omnipod 5

Of course, just slapping on a CGM won’t manage your diabetes. But careful interpretation and use of the data can help immensely.

For instance, I’ve learned that if I sleep any later than 8 a.m. before taking my morning insulin, I’ll be battling high morning blood sugar levels until at least noon.

I’ve also learned what kind of activities will make my sugars drop fast (in my case, that would be walking), my true levels of insulin sensitivity throughout the day, and that I always need a minimum of a half unit of active insulin on board (IOB) before a workout.

Gaining this knowledge took time and did include a few missteps, but that’s life and that’s learning. The information has made it possible for me to optimize my training and has made it easier for me to stay in a range of 90-144 mg/dL (5.0-8.0 mmol/L) when exercising.

Confidence and security

My CGM has enabled me to understand my diabetes better. It has given me more confidence in the way I manage my condition and has helped me achieve tangible results.

It’s also comforting to know how my sugar levels are trending. I will even venture out sometimes without bringing all my diabetes gear. I also stopped monitoring 10 to 12 times a day because I no longer need to — the information is right there on the screen in front of me.

Using this device has enabled me to go for a walk or a hardcore gym session without fearing lows and to reach both my fitness and diabetes health goals more efficiently.

The disadvantages of continuous glucose monitoring

Accuracy

Anybody who has ever worn a CGM knows the frustration of doing a traditional finger-stick check and seeing that the sensor is way off. 

The reason behind this is that the technology isn’t perfect yet (although it is constantly improving) and the sensor measures your glucose levels in the interstitial fluid and not the blood. (There is generally a lag of a few minutes between your interstitial glucose levels and blood glucose levels)

Aside from the obvious issues, it can also be outright dangerous if you rely solely on the sensor readings and completely ignore what your body is telling you. For example, if the CGM alarms are going off telling you that you are at 250 mg/dL (13.9 mmol/L), it can be hard not to immediately react.

I have also had to stop workouts because the sensor incorrectly told me that my blood glucose was low or about to get low — always an annoying scenario when you’re in the midst of a good gym session.

Although some CGM devices are now highly accurate and approved by the U.S. Food and Drug Administration (FDA) for making treatment decisions, it is always wise to confirm any unexpected readings or readings that don’t match what your body is feeling with a traditional finger-stick before taking action.

(And several of the devices on the market, including the Medtronic Guardian and the Ascensia Eversense E3, require roughly two finger-stick checks a day for calibration.)

That said, it’s worth noting that the technology has improved tremendously over the years. According to a 2023 study, CGMs were shown to be 96.5 percent accurate in a real-world hospital setting.

Overreliance

This is actually the main reason why I originally stopped wearing my CGM and why I still take breaks on occasion.

This may not apply to everyone, but I have a tendency to rely on the readings and recommendations a bit too heavily. For example, if the reading said that my glucose was dropping quickly, I would trust it fully over how I actually feel. I think that’s a big no-no when it comes to diabetes management.

I have gotten a lot better at using the CGM as a data point and not necessarily as the complete truth, which has made it a far better partner in management.

Inconvenience

Having the CGM sensor attached to my body all the time can be extremely annoying. It’s not very big and people don’t really notice it, but it can leave somewhat unattractive suntan marks, and I also tend to scar easily.

If I wear it on my stomach, it tends to get in the way when working out, so I usually wear it on my upper arm. I find that I get pretty good readings with that placement. (The only issue is that it can be a little hard to get a sweaty sports bra off without ripping at the sensor.)

Wearing it on my upper arm has led to some pretty cool interactions and conversations with curious people. I don’t mind — I see it as an opportunity to educate and advocate. 

But if it bothers you, there are plenty of other areas to place the sensor where it’s not visible. Typical manufacturers’ recommendations for sensor placement include the upper arm, stomach, lower back, thigh, and buttocks, depending on the specific device you’re using. Refer to the instructions that come with your CGM.

On a related topic, many people wonder if CGMs hurt. In my experience, there is only a slight pinching sensation upon inserting the sensor, and this momentary discomfort is well worth the data the device provides. 

Plus, for the devices currently on the market, the sensors only need to be changed every seven to 10 days (with the exception of the implantable Eversense E3, which typically needs to be changed by a healthcare provider every 90 days).

Related: Here is a post outlining my best CGM tips in which I talk more about how to put on the sensor (with a video) and how to make it stay on.

Price

Depending on your health insurance coverage, CGM devices and their associated supplies can be expensive. 

Final thoughts

Would I recommend a CGM to anyone with diabetes using insulin? The answer is a resounding yes, at least until you understand your body and your diabetes better. 

The CGM will give you knowledge about your diabetes that you have never had before, and if after a while you feel like you no longer need it, you can just stop using it.

The key for me when using my CGM is to get better at listening to both the CGM and my body. It’s a fantastic device, but it’s not a substitute for my own thinking and experience. 

It’s a great tool to have in my diabetes toolbox, but it can’t stand alone.