Is Continuous Glucose Monitoring Worth It?

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Continuous glucose monitoring systems (CGMS) may not make life with diabetes any easier. But they can definitely improve health, if you can deal with the hassle and expense. So how do you know if such a system is right for you?

As many readers already know, CGMS give a nearly continuous readout of glucose levels in tissue fluid, the wet stuff that oozes out when you have a scrape or a burn. To read these levels, you insert a long-lasting sensor under your skin, a process that feels similar to a needle stick. The sensor is made of material like the filters used in dialysis. It measures glucose levels and radios the results, via a connected transmitting device, to a small receiving device about the size of a pager.

This sounds nice — much more information without all the needle sticks. Unfortunately, you still have to do fingertip blood checks 2–4 times a day to keep the monitor calibrated. And the information you get from the meter is only valuable if you know how to use it.

Originally, CGMS was for your doctor. You got a continuous 72-hour readout of blood sugar levels, with a nice graph to go with it. If you conscientiously wrote down what you ate, your exercise, and medicines, your doctor would learn a lot about your body’s use of food and insulin. The doc could adjust insulin dosages and other aspects of your care. Then you gave the monitor back.

Studies showed this treatment reduced A1C levels by 0.4% to 1.0% or so. Many people with diabetes wanted this capability for themselves, so they could regularly adjust their own treatment and self-management. Now thousands of people use CGMS continuously. But how well do they work?

According to manufacturers’ data, “You can easily and discreetlyview your current glucose values continuously throughout the day,” without having to do a finger stick. It’s easy and discreet. The monitors have “trend arrows” that show you if your level is rising or falling quickly, so you can prevent highs and lows.

Most CGMS have alarm systems that tell you when you are getting close to a preset high or low limit. This hypoglycemia prevention alarm enables insulin users to keep much tighter control, as they don’t have to fear going low as much. An ICU physician named Kristin commented on a Diabetes Self-Management blog entry that she could now work a busy ICU shift without fearing lows, because of the CGMS.

According to this WebMD article, continuous glucose monitoring can help “identify fluctuations and trends that would otherwise go unnoticed with standard HbA1c tests and intermittent fingerstick measurements.”

“For example, the device can capture dangerously low overnight blood sugar levels which often go undetected, reveal high blood sugar levels between meals, show early morning spikes in blood sugar, evaluate how diet and exercise affect blood sugars, and provide up to a 72-hour complete review of the effects of changes made to your therapy by your health-care team.”

If you are a dedicated investigator, you can use CGMS to understand what’s happening with your blood sugar levels. Diabetes Self-Management reader Jim Snell used it to identify that his sugars were shooting around wildly because of his liver dumping glucose into his blood. Watching the CGMS, he learned what provoked these “dumps” and when they happened. The CGMS info enabled him to drastically change his medications, and his A1C has come down dramatically.

Using a CGMS, Diabetes Self-Management blogger Jan Chait discovered that her insulin was working on her much faster than she thought, so she was able to change the timing of her insulin to match her intake better. She says her CGMS has also helped her prevent hypos.

So What’s the Downside?
With any medical equipment, problems are possible. Although studies find that the sensor readings usually are close to fingerstick numbers, there can be significant differences (up to 15%). It takes glucose around 5–10 minutes to move from blood into tissue fluid, or back, so the CGMS measures lag behind what’s really happening in your blood if things are changing rapidly.

The CGMS doesn’t replace fingersticks. As I noted above, you still have to check your blood glucose level 2 to 4 times a day to keep the CGMS calibrated. Calibration is an ongoing job with CGMS. They also say you should not make “treatment decisions” (like taking extra insulin) based on a CGMS reading without taking a conventional blood glucose reading first.

The sensors are only good for about 3–7 days, since they are made of really thin stuff. They’re not that hard to insert, but they are expensive — $35 to $100 apiece. Some users, however, leave the sensors in a few days past their expiration time to save trouble and money, and they say they work fine.

New kinds of sensors are being developed that are sturdier and can stay in place for about 140 days, but we don’t know when they will be available on the market.

The monitors themselves cost from about $1000 to $1400. Some you can buy online with a prescription. CGMS need a transmitter battery so the sensors can communicate with the monitor. Depending on the model, these may need to be changed once a year or so and can cost around $500. provides a breakdown on available products and their costs at this Web page.

Although insurance will often pay for the CGMS, and sometimes for the sensors, some people find the costs and hassles too much. Diabetes Self-Management blogger Eric Lagergren thought the information he was getting wasn’t valuable enough to justify carrying around another attached box (along with his pump.) Other users complained of too many calibration errors.

I think anyone with any type of diabetes could benefit from using a CGMS for at least a week, if he were willing to record what he eats and does, and his medicines, and try to make sense out of it. Hopefully, he would have a doctor who was willing to work with him.

To get up to speed on noticing your own patterns and making adjustments, one week will probably not be enough. You would probably need a month or more. Some people use them for years as a safety net and guide. But if your A1Cs are good, and you don’t have a problem with hypos, you probably don’t need one.

Of course, most people probably won’t spend $1000+ on a CGMS just to use for a week. The best thing would be if people could borrow them, as from a library, or rent them for 1–4 weeks, and just buy the sensors. Some places do rent regular meters, but I’m not sure about the CGMS. If you know of anyplace like this, please let us know.

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