Continuous Glucose Monitoring - Is it for me?

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Author: Dana Armstrong, RD, CDE and Jen M. Block, RN, CDE

Last Updated: Friday, April 11, 2008

Q: "My friend said there are devices that measure your glucose all the time and that Abbott Diabetes Care has a continuous monitor. I test 4-8 times a day and use an insulin pump. Is this something that could work for me?"

A: I am going to quote directly from a booklet called Is Continuous Glucose Monitoring for Me? How continuous glucose monitoring can help you manage your diabetes, by Dana Armstrong, RD, CDE, and Jen Block, RN, CDE. *

“Continuous glucose monitoring (CGM) is the newest tool that may help you understand your diabetes better. When deciding whether to use CGM it is important to weigh both the pros and the cons of its use carefully. Understanding CGM, how it works, its limitations and its potential will help you decide if this exciting new technology is for you. Just like most technology, there is more to CGM than meets the eye, as it is far from a “plug and play” device. If you are considering using CGM to help you or someone you love, you probably have questions. Hopefully this guide will provide you with the needed answers.

Understanding CGM

To understand CGM technology we need to start at the beginning—with blood glucose testing. A blood glucose (or blood sugar) test provides you with a value at a single point in time that tells you where you are at the moment you do the test. It’s like stopping your car, pulling out a map and figuring out where you are at that exact moment.

CGM is more like a GPS (global positioning system) or navigation system in your car. It not only gives you information about your location at any moment in time, just like your meter, but also has the ability to tell you much more. One aspect of CGM is that it provides a brief glucose history over the last hour up to the previous 24 hours, so you can understand exactly how you arrived at your current ‘location.’

Then the CGM system, just like a GPS, uses the information about where you have been to help predict where you may be going. Like a GPS, a CGM system can also alert you when things do not go as planned. If you miss a turn, your GPS alerts you. With CGM, if your glucose rises above or drops below a glucose level that you program into the system, it also may alert you.

Like a GPS, CGM provides a more complete picture of glucose control compared to what you would see from a glucose meter. Depending on how often you test using a glucose meter, you may see several blood sugars in one day. Even a frequent tester who may check their blood sugar 6 times in a 24-hour period cannot match the 288 to 1440 glucose values (depending on the particular system) a CGM device may provide in the same period. This detailed history can help you discover patterns in your glucose that you may not have been aware of before using CGM.

What you can learn from using CGM

How often and when your glucose is higher or lower than your target range.
What may cause your glucose to be higher or lower than expected.
How you and your health care team can change your management to keep your glucose levels closer to your goal.
How effective are the changes you may make to your diabetes management.
How well your meal and correction (bolus), and background (basal) insulin doses match your needs.
How different types and amounts of food affect your blood sugar.
Whether or not you over-estimate or under-estimate the carbohydrates in your meals and snacks.
What the best time is to give your insulin in relation to when you eat your meal.
How exercise (various types, durations and intensities) affects your blood sugar.
How long the effect of exercise lasts for you.
How your body responds to different types and amounts of carbohydrates to treat lows.
Whether you have different glucose patterns on different days of the week.
How stress and illness affect your diabetes management.

While CGM provides you with information about your past, present and possibly even future glucose levels, it is not perfect. Using CGM does NOT replace your blood glucose (BG) meter. In order to use CGM safely and effectively you must still use your meter to test your blood sugar to calibrate the system and to confirm any treatment decision, alarms or symptoms.

The CGM System and how it works

CGM systems are made up of several components.

Sensor: The electrochemical sensor is placed under the skin. The sensor measures the glucose in the interstitial fluid (ISF) which is the fluid between the cells. A chemical on the sensor produces a very small electrical signal that measures the amount of glucose present.

Transmitter: The transmitter is a small electronic device that connects to the sensor and sends the signals from the sensor to the receiver.

Receiver: A wireless receiver is used to capture and display the glucose values it receives from the transmitter.

How it works

The sensor is measuring the glucose that is in the fluid between the cells. It takes this information and sends it from the transmitter to the receiver. When a sensor is inserted under the skin, it can take some time to settle in. This is called the warm-up or acclimation period. It may take your body some time to adjust to the newly inserted sensor, and different systems have different warm-up periods.

In order to make sense of the glucose information the CGM is receiving, you need to enter the calibrations, which are meter blood sugar levels, into the system.

Inserting a sensor

A sensor is inserted just under your skin in the same kind of places you inject insulin. It is inserted with a needle, the needle is then removed and then the sensor remains under your skin for several days.

The needle you use to insert the sensor is a little bigger than the syringe you use to take insulin. Most sensors have a device that helps you to insert them under the skin. It is important to follow the instructions of your health care team and those that are specific to your CGM system on where and how to insert the sensor.

Sensor insertion sites

The decision of where to insert your sensor is a little more involved then deciding where to take insulin. It may be similar to selecting a pump site because the sensor will remain under the skin with the transmitter attached for several days. You will want to avoid placing a sensor near clothing that may rub or in a spot where the transmitter may be bumped or pulled when playing sports. In some cases, you may need to be careful not to inject insulin near the sensor. You will also need to closely follow the directions of your health care team as well as the instructions that come with your system.

How it feels to wear a CGM system

The most similar experience you can picture is that of wearing an insulin pump. CGM is certainly appropriate for many people with diabetes, not just pump users, but the feeling of wearing it on your body is similar. That is because the sensor stays under the skin for several days like a pump insertion set, except without the tubing. The transmitter communicates wirelessly with the receiver so the only change in what is actually attached to your body is the transmitter. The receiver needs to stay near the transmitter, and every system has its own requirements regarding receiver distance for device communication.

Special considerations when wearing a CGM system

The sensor and transmitter for all of the systems can be worn most of the time—including while you are sleeping, showering, bathing and swimming. Of course, sweating and water exposure may mean you need to work with your health care team to find options to help reinforce the adhesive on the sensor and transmitter to keep it from moving or falling off.

You should closely follow the instructions of your health care team and the maker of your CGM system for airline travel.

When to use CGM

You have the option to wear your CGM system when you want to wear it. While using CGM all the time can provide you with lots of information about your diabetes, you do not always have to have it on. Keep in mind if you have gotten used to wearing your CGM system and you choose not to use it for any reason, then you will not have the alarms and extra data it provides when you are not wearing it.

Should you want to use CGM on an occasional basis, you may want to plan its use carefully to maximize your benefits. Here are examples of times you may want to use CGM:

A few weeks before you see your health care team.
Anytime you are making changes to your diabetes regimen.
To help you learn more about the effects of different foods, medications, exercise, stress and illness.

Of course, the more you use it, the more information you will likely have about your diabetes.

On your journey into the world of CGM, you may have several questions:

Should everyone who has diabetes use CGM?

While this technology is exciting and has many new and different things to offer, it is important to carefully consider how it may affect you before you make the decision to use it.

CGM requires that you insert sensors under your skin, wear a transmitter on your body and keep a receiver nearby. It also provides a lot of information and not everyone is ready to see or use that information. It may increase the amount of time you spend managing your diabetes as you have a new device and more data to review and understand. It can also be expensive to use, especially if your insurance does not cover the device and required supplies. The upside is that you will have more information to make better decisions on your diabetes management and it can alleviate some of the guesswork that you do on a daily basis. And in some cases you can use the information to prevent a high or low by using the arrows and alarms.

Will my CGM glucose values always be the same as my blood glucose meter?

No— In fact, blood glucose meters and CGM systems may not always give you the same result. One reason for this is that they are measuring glucose (sugar) in different places. Blood glucose meters measure the amount of glucose in your blood. CGM systems measure the amount of glucose in the fluid between your cells. It takes time for the glucose in your blood to travel into the fluid between your cells. This lag time or delay can vary but is usually between 5 and 15 minutes. The lag time is usually longer whenever your blood glucose is rapidly rising or falling.^{1, 2}

What exactly is calibration?

When you calibrate your CGM system, you are telling it what your blood sugar is to help your CGM device match your meter reading as closely as possible. Each system requires that you test your blood sugar and enter it into the CGM device at regular intervals. The information you get from CGM is affected by both how and when you provide your blood glucose meter information. However, as simple as this may sound, calibrating your CGM can be tricky and is often one of the most frustrating parts of this wonderful technology. Given this challenge, there are several things that you can do to increase the chance your calibration will be successful:

Clean and dry your hands well prior to the test. Test on your fingers for calibration, and do not use an alternate test site such as your arm.
Follow all of the instructions that come with your blood glucose meter (or the built-in meter that may be in the CGM system). This includes making sure the right code number is used.
The value from your meter must be entered immediately into your CGM device. A delay in entering the value can affect the accuracy of the system. Some systems allow automatic calibration from the meter, while other systems allow you to manually enter your blood glucose reading.
If your car is moving, your GPS device will not let you enter any data. Very similarly, all CGM devices recommend that you calibrate the device when your blood sugar is not changing (or moving) rapidly. If possible, avoid calibrating during or after exercise, meals or insulin doses.
Once you have started using CGM, the data trends and arrows in the system can help you identify times when your glucose is generally steady and times when it is changing rapidly.

Can I just put on my CGM device and go?

No— Just like a GPS system in your car starts at an exact point, your CGM system bases the information about your glucose level based on an exact point provided by your blood glucose meter. This information starts your CGM system and calibrates it to an exact known position.

What can happen if I calibrate my CGM system when my glucose level is rapidly changing?

Calibration is unique to CGM and a very important concept to understand. To help explain a possible calibration error, look at the following example.

Example:

Your CGM glucose value is 100, but the arrow on your device shows that your glucose is rapidly dropping, possibly as much or even more than 2 points every minute. You check your meter and your blood glucose is 80.

This does not mean that the CGM value is wrong. Rather, it is measuring glucose in a different location, and because of the rapid drop, it may just be lagging behind the blood glucose by approximately 5 to 15 minutes. Because the glucose value is not steady, this would not be a good time to calibrate your CGM system with the number from your meter. Remember while it may not be a good time to calibrate your CGM system, it may still be an important time to test your blood sugar.

Can I use my CGM system to make treatment decisions, like taking insulin to correct a high glucose level?

No— While it may be tempting, your CGM system is a trending device, not a treatment device. It is important to understand this difference. Because of the possible difference between your meter blood glucose and the CGM glucose, all CGM systems require that you do a blood glucose test prior to any treatment decision as well as at other times.

Treatment decisions that require a blood glucose test may include but are not limited to:

Taking insulin or any medication that can affect your blood sugar level.
Treating a low glucose, in response to a low glucose reading or alarm.
Responding to a high glucose reading or alarm.
Deciding whether you are safe to drive.
Anytime you have symptoms of a low or high glucose, even if the system has not alerted you.

For all of these reasons, it is possible that having a CGM system will not decrease the amount that you test your blood sugar. You may find especially in the beginning that using a CGM system causes you to test more frequently because you may be more aware of your glucose changes. When you first turn on the alarms, they may start alerting you to low or high glucose values you were not aware of before you started using CGM.

With CGM, will I be able to see what is going on with my blood sugar all of the time?

This is a tricky question. CGM does NOT provide you with blood sugar readings. It provides you with the level of glucose in the fluid between your cells, which is calibrated with the blood sugar values you enter into the system. Provided that the system calibrations have been successful, the CGM device does provide you with information about your glucose control most of the time when you are wearing the system. Some systems can also be downloaded into software or data management programs that can help you identify patterns that may be occurring on a regular basis.

Will my CGM system always alert me when my glucose level is too high or too low?

Not always— While all CGM systems have alarms that can be programmed to alert you when your glucose drops below or rises above a programmed level, there are reasons why it may not work perfectly and may not alert you to all of your low or high glucoses. Even when using CGM you need to monitor yourself just as closely as when you are not using the device and test your blood glucose anytime you have symptoms of low or high blood sugar.

Many variables can have an impact on the ability of your system to alert you to a potential low or high glucose level. These include rapidly changing glucose values, lag time and any errors in calibration or problems with the system. Any of these issues may prevent the system from alerting you to a low or high glucose.

In an attempt to overcome lag time and the impact of rapidly changing glucose levels, some people may adjust the level at which they set the alarms. If you choose to set your low alert on your CGM system at 80, you are more likely to catch a low value than if you set the alarm at 60. However, there is a trade-off and you may get more alarms, even when you are not low. It is important to discuss how best to set your alarms with your health care provider.

Many people find the alarms hard to hear or feel if they are set to vibrate. This is especially true at night. This is yet another reason why it is important to plan to test anytime you have concerns about your glucose. Most CGM systems are not a very good alarm clock and should not be relied upon solely to alert you to lows or highs overnight.

You should also discuss the high alarm setting with your health care team. The increased knowledge of your glucose patterns may alert you to times when you are higher than you would like. Knowing how to respond to this is VERY important. When you are using insulin and any medications that affect your blood sugar you need to understand how they work.

For example, if you set a high alert for 200, and then one-hour after eating your meal your blood glucose is 260mg/dL: Should you take more insulin? Or should you wait until the insulin you took before your meals is working at its peak ability? You need to be aware of how much of the insulin you took to cover the meal is still at work in your body (this is known as insulin-on-board or IOB). The temptation to over-correct or take more insulin without regard to how much is still working in your body can result in low blood sugars. It is very important to develop a plan with your health care team for how you will respond to high alerts and to establish when it is safe to take additional insulin.

Will a CGM system prevent me from having high or low blood sugars?

No, but a CGM system can provide you with much more information about your glucose patterns. It can help you know when your glucose is rising or falling and how quickly. With the help of your health care team, you can use this information to try to prevent highs or lows.

Can CGM Improve My Blood Sugar Control?

Using CGM does not mean that you will have perfect diabetes control nor does it imply that it will alert you to all of your high and low glucose values as we have discussed. A CGM system is not an artificial pancreas—it does not control the delivery of insulin and other medications. You and your health care team are still responsible for your overall diabetes management. CGM systems offer many different advantages that are not available to you with a home blood glucose meter alone. Even with these advantages, you still need to play an active role.

Successful CGM relies very heavily on your interaction with the system. You need to look at the data frequently and learn how to understand and use the data to improve your control. Some people find it overwhelming to see all of the information when they first use CGM. You may learn that your glucose was often higher or lower than you had hoped. It is important if you plan to use CGM to know that this can happen and to work with your healthcare team to learn how you can use the information it can provide to help improve your control.

Will a CGM system help guide my insulin dose decisions?

This is also a tricky question. As already mentioned, you must confirm any CGM data with a blood glucose meter before you decide how much if any insulin to give. However, when used in combination with your blood glucose meter, CGM can help give you information about your glucose levels and patterns, which in turn can help you make better insulin dosing decisions.

Will my insurance pay for my CGM system?

Like many new technologies, CGM systems are not always covered by insurance. When considering the cost of CGM, keep in mind that CGM systems have several components. There is the receiver, which is usually a one-time purchase and the transmitter that needs to be replaced every 6 months – 1 year depending on the system.

There are disposable parts of the system that are only used once and need to be purchased on an ongoing basis. These include the sensors which can only be inserted once and worn up to several days. Some systems require the use of specific meters and test strips that may or may not be covered or may not be a preferred strip by your health plan. Your health insurance provider must answer the question of coverage.

To learn more about how much CGM may cost you here are some suggestions:

Contact the companies that make CGM devices. They may have a program to work with your insurance company to determine how much your insurance will pay for the CGM system.
Contact your insurance directly. If you do this, be sure you ask questions about whether they cover the cost of purchasing the device and the cost of the sensors that need to be replaced every few days. If the system requires the use of a specific test strip ask if they are covered and at what level. You may also want to learn more about how you can request coverage as well as how to appeal the decision if it is denied.
Talk with your health care team, as they may be able to help in this process.
Organizations like The Juvenile Diabetes Research Foundation (JDRF) have tips for helping with coverage for CGM on their website listed in the references at the end of this booklet.

In Summary

What CGM is:

Much more access to information about your glucose patterns than you have likely seen before.
A means of alerting you to low or high glucose values, in some cases before or soon after you have reached the level you program into the system. It may not catch every low or high value.
A glimpse into the future. The ability to add to your single point-in-time meter test information about whether your glucose is fairly stable, rising or falling and how fast it may be doing so.
More information about times of the day during which you never regularly tested. This is especially true of nighttime.

What CGM is NOT:

It is not a replacement for your blood glucose meter.
It is not an artificial pancreas.
It is not perfect.
It cannot always alert you to low or high glucose values.

What you need to use CGM effectively:

Be willing to be open to what you may see when your “naked” diabetes is revealed.
Have an understanding of how your medications work. With insulin and other medications that affect your glucose, you need to understand when it starts to work (onset), when its effects are strongest (peak) and how long it lasts (duration). This is very important! You need to work very closely with your health care team to understand the full effect of your medications.
Avoid the temptation to over-correct or take too much insulin. You may need to be patient with your insulin, as it does not always work as fast as you may think or hope.
Have the desire to work with the real-time alarms, alerts and trend information. Real-time data may help you detect or prevent lows or highs. You need to look back over time to determine if there are trends and patterns that you and your health care team need to address.
Understand that your CGM value may not be the same as your blood glucose meter value. They are measuring glucose in different places. The values are more likely to be different especially when your blood sugar is changing rapidly.
Remember these important points:
- Follow the guidelines of your health care team and the instructions for each device.
- You always need to test your blood glucose and use the meter number to confirm treatment decisions which include things like:
  - Insulin doses
  - When or how to treat a low
  - Before responding to glucose low or high alarms
  - To decide whether you are safe to drive
  - Anytime you have symptoms of low or high glucose
  - Anytime your glucose reading is different than how you feel
- Do not rely on low or high alarms to warn you of low or high glucose levels. Test your blood glucose anytime you have symptoms of low or high blood sugars and anytime you think you may be at risk for low or high blood glucose.
- Remember to be forgiving of yourself if things do not go as planned. Many things can affect your blood sugar and not all of them are in your control.

Conclusion:

Many people take photos and never print them or look at them again. There are people who shoot videos and never view them. When using any technology, like CGM, viewing the data on the screens and downloading are important components to achieving its fullest potential. You can use a CGM system to help alert you to when your glucose is high or low but if you never look back at what may have caused the high or low you are only using a small part of what the system can offer. The data can be overwhelming at first. Keep in mind there is no such thing as perfection. You cannot manage the numbers but you can use the numbers to help you manage your diabetes.”

References

Wolpert H, Use of Continuous Glucose Monitoring in the Detection and Prevention of Hypoglycemia. Journal of Diabetes Science and Technology. 2007;1(1):146-150.
Klonoff DC, Continuous Glucose Monitoring: Roadmap for the 21rst Century Diabetes Therapy. Diabetes Care. 2005;28(5):1231-1239.