My continuous glucose monitoring device worked — until it didn’t

My blood glucose readings were inaccurate when compared to a finger stick

William Ryan avatar

by William Ryan |

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One of the “pleasantries” of life with cystic fibrosis (CF) is navigating cystic fibrosis-related diabetes (CFRD). I’ve been periodically dealing with it since the summer of 2007. For the last two and a half years, though, it’s stayed with me like an unwanted memento.

As I deal with CFRD, one tool I’ve become familiar with is a continuous glucose monitoring device (CGM), which keeps track of a person’s blood glucose levels.

My experiences with CGMs have been complicated and, at times, downright frustrating. That isn’t to say that everyone will have the same results as I did, and others may find using CGMs to be incredibly beneficial.

Keeping track of blood glucose levels is key to maintaining a healthy life with diabetes. It can be harmful or dangerous when these numbers are off.

Early last year, I received my first shipment of glucose monitors from a mail-order pharmacy. I was told to swap the monitors out every 10 days. I was thrilled to receive them and looked forward to using them.

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What is going on?

For the first few months, the CGM readings closely matched the results from a finger stick or from bloodwork at the hospital. But one disappointing development was that the signal didn’t always work on one of the CGMs, so I couldn’t get a reading, sometimes for hours.

As frustrating as that was, I couldn’t complain because the readings were fairly accurate. It’s recommended (for glucose values greater than 100 mg/dL) that finger stick readings and CGM numbers be within 15% of each other, and mine were.

By late summer, though, the difference between the two was becoming more than 15%. The most notable time was when I was in the hospital last October. Providers at my hospital rely on finger sticks because it’s easier for them to monitor how I’m doing throughout the day. When the difference between the two readings exceeded 15%, my endocrinologist recommended I stop using the CGM until I could upgrade it to a newer model.

Last month, I received a new model, and unfortunately, it has been a disappointment. The difference in readings between my finger stick and the CGM still exceeded 15%. While I wasn’t expecting the new model to be perfect, I didn’t understand how it could be just as flawed as the older one.

I was even placing the CGMs on different parts of my body. I put the old one on my stomach, where I have a lot of scar tissue from surgeries as well as a feeding tube when I was an infant, and the new one on my upper arms, which were relatively untouched.

Most frighteningly, when I slept, the CGM constantly alerted me that my blood glucose level was around 40 mg/dL or less, which is dangerously low. After a few days of believing what I was reading, I noticed that I didn’t feel like I had hypoglycemia. The next night, I had my finger stick with me for when the CGM alert sounded. When that happened, it said I was below 40 mg/dL, but my finger stick read about 100 mg/dL. Not only was it higher, but the difference was also greater than 15%.

After searching online, I realized I wasn’t the only one who was having this problem. Many people expressed frustration about the same thing.

In my opinion, CGMs have benefits — when they actually work. They do a great job of monitoring how your body is dealing with glucose levels in real time. But for me, they’ve become more of an inconvenience to the point that I don’t want to wear one anymore.

The CF community, along with other chronic illness communities, deals with constantly updated technologies intended to benefit our lives. Of course, not all technology is perfect or created equally. My main concern with imperfect technologies is that when they don’t work, they might adversely affect people’s health.

If you’re thinking about using CGMs, I would suggest that you closely monitor the readings and compare them with finger sticks.

Do you use CGMs? How do they work for you? Please share in the comments below. 

Note: Cystic Fibrosis News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. The opinions expressed in this column are not those of Cystic Fibrosis News Today or its parent company, BioNews, and are intended to spark discussion about issues pertaining to cystic fibrosis.


Frances Czochanski avatar

Frances Czochanski

Sorry to hear this Will but with constantly hearing from you and all the users it should help to get this technology upgraded to be more accurate 🤞it’s a serious glitch

Helen Palmiero avatar

Helen Palmiero

Hi Will! I can certainly empathize with you in this latest column of yours. My husband has been using a continuous glucose monitoring device (CGM) for many years now. For the most part, it works well. But more often than not, his glucose readings fluctuate so much - from alarmingly low to also alarmingly high. Then there are the times when the sensor comes out of his upper arm and he doesn't know how or why. We are careful not to inject the sensor into the same spot all the time and the few times we've tried the other arm, the readings were still off. So we do keep the fingerstick equipment nearby for times like these but we thought the CGMs would be a great alternative to that. I don't know what the solution is here. I guess we just have to keep trying - hit or miss! Much love, Helen

Rusty avatar


I use the Libre 2 cgm and have learned to take the readings with a grain or two of salt. The waveform is almost more helpful than the absolute number displayed as I can see if I am up or down and whether it is moving sharply in one direction or the other or if it is fairly steady. As for the nighttime lows, you could be experiencing what is known as a compression low. This is when you have been lying on the sensor and compressing the tissue that the filament is embedded in causing the false low reading. I like having the ability to do a quick check of my glucose but there is still a lot of room for improvement on the readings. By the way, the cgm does not give a real-time reading. It is behind the actual blood glucose reading by 10 minutes, more or less.


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