By Linda Mackowiak, M.S., R.N., C.D.E.
Even with regular blood glucose monitoring, one of the big unknowns in diabetes self-management is what happens to glucose levels between blood glucose checks. In recent years, however, the development of continuous glucose monitors that check glucose levels in the fluid just under the skin have started to change all that. The newest continuous glucose monitors measure glucose levels continually around the clock and display results every few minutes.
The first continuous glucose monitoring systems simply collected data, which had to be uploaded to a computer for analysis. When the numbers were plotted on a graph, it became possible to see, for example, whether glucose levels were rising after meals or getting too low overnight.
The newer systems display glucose readings on a screen, so the user can see, in real time, what his glucose level is and whether it is rising or falling. The newer systems can also be programmed to sound an alarm when the user’s glucose reaches high or low levels. Some systems are able to display graphs showing glucose levels over a certain number of hours on the display screen. And the data collected by all the systems can still be uploaded to a computer for graphing and analysis, if desired. (See “Devices on the Market” to learn more about currently available continuous glucose monitoring systems.)
For all the information they provide, however, continuous monitoring devices do not replace regular blood glucose monitoring. Continuous monitoring systems must be periodically calibrated with conventional blood glucose meters using fingerstick blood samples for accurate readings. In addition, users are advised to do a conventional blood glucose check before making any changes in their diabetes care regimen.
How glucose sensors work
The glucose sensors used in continuous glucose monitors are small electrodes that the user places under the skin using an introducer needle and a spring insertion device that is similar to a lancing device or insulin pump infusion set inserter. Once the sensor is inserted, the introducer needle is removed, and only the sensor remains under the skin, held in place with an adhesive patch. The sensor must be changed every few days, according to the manufacturer’s recommendation. Inserting a glucose sensor should not be painful, and wearing a sensor should be comfortable.
The sensor produces a very small electrical current based on the amount of glucose in the body fluid (called interstitial fluid) around the sensor. When the glucose level rises, the current rises. When the glucose level goes down, the current goes down.
The sensors presently on the market need a number of hours to settle in to the body before they can start giving accurate glucose information. When the sensor is ready, the user calibrates the device by doing a fingerstick blood glucose reading and inputting the reading into the device (different systems have different methods for inputting information). The continuous monitoring device uses the reading to determine what the measured electrical current means in terms of glucose concentration. Additional calibrations are performed while wearing the sensor, according to the manufacturer’s recommendations.
To get the most accurate readings from a continuous glucose monitor, it is important to calibrate carefully and correctly. You must use your best blood glucose monitoring technique, including checking on clean, dry fingers, using strips that have been stored properly, and coding and using your meter correctly. Remember: The sensor glucose information you get is only as accurate as the quality of the calibration you do.
Even with the best calibration possible, there can sometimes be differences in the level of glucose in your blood and the level of glucose in your interstitial fluid because of the way the body uses glucose. A noticeable difference is particularly likely if the glucose level in your blood is changing rapidly, which can occur just after eating or taking insulin or during exercise, but there can be differences at other times, too. It is best not to calibrate your continuous monitoring device when your blood glucose level is likely to be changing rapidly. Some people find the best time to calibrate is just before a meal.
With time and experience, you will learn when the difference between the sensor readings and your blood glucose monitoring readings is normal and when there may be a technical problem that needs addressing, such as a sensor that needs changing. If you’re concerned about the accuracy of your readings, however, speak to your health-care provider, or call your monitor’s manufacturer for help.
It may take some time to get the most from continuous glucose monitoring. Start by learning as much as you can about the technical aspects of your new device, such as how to insert the sensor properly, how to calibrate the device with fingerstick blood glucose readings, how to set the alarms, how to put the alarms on vibrate mode, and how to transfer your monitoring data to a computer. Once you are up to speed on operating the system, you can work on learning to analyze and respond to the information you’re getting from it.
Don’t be surprised if you have an emotional response to seeing all of the glucose changes you experience over the course of a day. For some people, all this new information can be confusing or even frightening. If you’re feeling overwhelmed, try not to tune out, but instead take the challenge to see how you can use this technology to help your diabetes care. Although there is a lot of information, you will quickly learn how to focus on the important trends and worry less about each individual number.
Keep in mind that even though you are using a continuous monitor, there are still times you should use your blood glucose meter. For example, you should still check your blood glucose level with your meter if you have symptoms of low blood glucose (hypoglycemia) and before making any diabetes care changes, including taking a “correction” dose of insulin for high blood glucose. (Also remember to consider how much insulin is already active in your system before you take more.) You should also check your blood glucose level before driving or doing anything that could be dangerous if you had low blood glucose.
Responding to your readings
Continuous glucose monitoring can offer information that can be of use immediately, over the intermediate term, and over the longer term.
Immediate information. Your blood glucose meter gives you point-in-time blood glucose levels, so you know where your blood glucose is, but not necessarily where it’s going. Your continuous monitor, on the other hand, reports glucose levels every few minutes, so you can see not only where it is but also which direction it’s headed. With this added information, you may decide to respond differently to the same number, depending on whether your glucose is rising or falling. Being able to see trends in your glucose levels may enable you to take preventive action–by eating a snack, for example–before your glucose reaches a level considered problematic.
As you use this new technology, you may discover that you don’t always know how to respond to the glucose trends you’re seeing. Should you take more insulin? Less insulin? Delay your meal? Do something different before eating a similar meal in the future?
If you’re not sure how to use the information you’re collecting, speak to your health-care provider.
Intermediate term. If you develop a problem requiring immediate care, such as an episode of hypoglycemia, your monitoring data may be able to help you determine when the problem started and what may have caused it. Your data can also help you evaluate how well your response to the problem worked. Did your treatment raise your blood glucose level fast enough? Did it raise it too much? Best of all, your data may give you some ideas on preventing the same problem from happening again.
Longer term. Looking at the glucose information collected overnight, over some portion of the day, or over the course of several days can help you to see the big picture. Depending on which device you’re using, you may need to upload your data to a computer to make a graph showing glucose ups and downs, or you may be able to display graphs covering certain time periods on the monitor itself.
You may notice that some days look much different from others. Why is that? You may discover that exercise, school, work, or dining out can have a big effect on your glucose pattern for the day.
What usually happens after eating? Does it vary depending on the time of day, foods eaten, or timing of your premeal insulin dose?
When does hypoglycemia happen? Do you ever get delayed hypoglycemia after exercising?
If you use an insulin pump, does the length of time between infusion set changes appear to have any effect on your glucose pattern?
It may help to keep detailed written records of your daily routine, food intake, exercise, and stress level for a few days to look at alongside a graph of your glucose levels. Bring your records along with your continuous glucose monitoring information to your next appointment with your diabetes care provider. Together you can evaluate how well your diabetes regimen is keeping your blood glucose levels in target range and what changes you might make for improvement.
A word on alarms
Finding the right alarm settings may take some time. You want the alarms to alert you to situations that require action, but you don’t want the alarms to overwhelm you or interrupt your day or your sleep too often. The best setting for an alarm may not always be the actual glucose value at which you wish to take action. You may have to move the settings up or down and reassess until you find the best setting. Also, different times of day (or night) and different situations, such as work or recreational activities, may require different settings. Expect to have to change your settings occasionally; you may even choose to silence the alarms in some circumstances.
Setting the low alarm. The higher you set your low glucose alarm, the more low alarms you are likely to get. In fact, if you set your low alarm fairly high, you may get alarms when your blood glucose level is simply at the low end of normal. One reason this may happen is that the rise in interstitial fluid glucose level lags behind the rise in blood glucose level after meals. While some people may find “low normal” alerts helpful, others may consider them a nuisance. You can avoid such alerts by setting your low glucose alarm lower, but if you do, you may not always get enough warning for real lows.
If you have hypoglycemia unawareness, meaning you do not sense the early signs of hypoglycemia, you may decide to set your low alarm on the high side to help you to prevent episodes of hypoglycemia and keep your blood glucose level in a safe range.
Setting the high alarm. The high glucose alarm on your continuous monitor can be useful in a number of situations. For one thing, it can alert you to a missed insulin dose or a problem with insulin delivery from a pump. (In both cases, the likely result is glucose that is higher than normal.) It can also be useful to evaluate the effect of food choices on your glucose level or, for those who use rapid-acting insulin, the amount and timing of premeal insulin doses.
As mentioned earlier, however, because the rise in interstitial glucose level lags behind the rise in blood glucose level just after eating, if you put the high alarm setting too high, you may not get an early warning that your glucose level is out of range. On the other hand, if you set the alarm too low, it may go off every time you eat.
Some health-care providers may suggest that you turn off the high and low glucose alarms for the first few days of using a continuous monitor (or that you set the low alarm very low and the high alarm very high), so that you can get used to wearing the device and to getting a lot more information about your glucose level than you are used to. If you decide not to use the alarms at first, keep in mind that you do not have the protection of this safety net.
Skin and tape issues
If you experience irritation or redness under the tape holding your sensor in place, or if the tape isn’t sticking well, talk to your health-care provider about it, and call the monitor manufacturer as well to report any problems. There are other tapes and skin wipes that you can try that may keep your sensor secure without bothering your skin. (The products used to hold glucose sensors in place are the same as those used to hold insulin pump infusion sets in place.) Don’t ignore the problem; it can make the difference between being willing to use continuous glucose monitoring and not being willing to use it.
With all of the new glucose information you’ll be getting from a continuous glucose monitor, it’s easy to feel overwhelmed, but try to keep things in perspective.
Aim for improvement, not perfection, and don’t try to fix everything at once. If you’re not sure what to do first, try learning about and preventing some lows. Some people find that their overall diabetes control improves when they do this. Work with your health-care provider to make small changes, reassess, and make some more small changes. In many cases, cumulative small changes can make a big difference.
Remember to keep an eye on glucose trends, not just individual glucose numbers. Focus on prevention and catching potential problems early.
If your blood glucose control improves as a result of using a continuous monitor, you may find you are gaining weight. This can happen as your body becomes able to store glucose it was previously losing in urine. You will need to rebalance your food, exercise, and insulin to stop the weight gain and stay at a healthy weight. Talk to your health-care provider about how to do this safely.
Whatever you do, try not to get discouraged. Speak to your health-care provider if your attempts at improved diabetes control aren’t having the effects you intended.
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