Continuous Glucose Monitoring
When you have diabetes and require several self-checks a day to monitor blood glucose levels and adjust insulin doses; when you and your healthcare provider want to evaluate changes in your glucose levels over specified periods of time; when you have frequent low blood glucose (hypoglycemia), especially when you do not experience distinct symptoms (hypoglycemia unawareness)
A continuous glucose monitoring (CGM) device includes a small sensor wire that is inserted beneath the skin of the abdomen or the upper arm and held in place with an adhesive patch. The sensor measures glucose in the space around cells (interstitial space). CGM measures glucose at frequent intervals and sends the results wirelessly to a device that is attached to your clothing or in some cases to a smart phone. These digital readouts let you know your equivalent blood glucose level in real time.
No test preparation is required, but some CGM devices require that twice daily blood glucose self-checks also be performed.
Continuous glucose monitoring (CGM) measures the glucose in the space surrounding cells (interstitial space). CGM measures glucose every few minutes to track changes in glucose levels in real time. Blood and interstitial fluid glucose levels are not exactly the same, but they mirror each other closely.
Glucose is the primary energy source for the body's cells and the only short-term energy source for the brain and nervous system. A steady supply must be available for use, and a relatively constant level of glucose must be maintained in the blood.
During digestion, the carbohydrates that you eat are broken down into glucose (and other nutrients). They are absorbed by the digestive tract, move into the blood and circulate throughout the body. Normally, blood glucose rises slightly after a meal and the hormone insulin is released by the pancreas into the blood in response. The amount of insulin released corresponds to the size and content of the meal. Insulin helps transport glucose into the body's cells, where it is used for energy. As glucose moves into the cells and is broken down (metabolized), the blood glucose level drops and the pancreas responds by decreasing the release of insulin.
If the glucose/insulin blood feedback system is working properly, the amount of glucose in the blood remains fairly stable. If the feedback system is disrupted and the glucose level in the blood rises, then the body tries to restore the balance by increasing insulin production.
Diabetes is the most common disease resulting from an imbalance between glucose and insulin. Type 1 diabetes results when the body is not able to produce sufficient insulin to control blood glucose levels. Usually in type 1 diabetes, the cells that produce insulin (beta cells) have been destroyed by the person's own immune system. Type 2 diabetes results from a combination of insulin resistance (the body does not react normally to insulin) and a relative decline in insulin production.
People with diabetes who use insulin to help control blood glucose levels must self-check their blood glucose several times a day. Traditionally, this has been done using fingersticks, and then adjusting their insulin doses based upon the glucose readings. This can add up to many needle sticks a day. Also, this type of monitoring provides glucose readings only at the time of testing and not continuously.
CGM has been developed to give you and your healthcare practitioner a better understanding of how your glucose levels vary throughout the day. It gives you an opportunity to gain better control of your blood glucose, with fewer incidences of high and low sugars and a more timely response to them.
Different versions of CGM devices are available and they are continuing to improve. CGM devices are intended primarily for people with type 1 diabetes but are also being used by some people with type 2 diabetes who use insulin. They are FDA approved for use in adults and teens and some are approved for use in children.
Some features of CGM devices include:
- A device with a small sensor wire that is inserted beneath the skin of the abdomen or the upper arm and then held in place by an adhesive patch. This CGM device stays in place for several days (7, 10, or 14). One device has a sensor that is implanted in the upper arm for up to 3 months with a transmitter above it on top of the skin. When the time limits have been met, the CGM devices are removed and another new device is attached/implanted.
- The CGM device measures interstitial glucose every few minutes and sends it to a receiver or in some cases to a smart phone or tablet. The devices can store the data, then download it. Software can be used to create reports of the data that can then be evaluated by the person and by a healthcare practitioner.
- Most CGM devices measure and display glucose "real-time" readings automatically at set intervals, while some display the last glucose result performed when they are scanned (intermittent scanning or flash).
- CGM devices may send alerts and have alarms. They may indicate when glucose is high or low, or give alerts based on trend data – such as the glucose is decreasing and likely to be low within the next 60 minutes.
- Personal CGM devices provide the results to the person wearing the device and may also be set up to provide the results to several other people at the same time, such as sending alerts and results to a parent or healthcare practitioner as text messages.
- Professional CGM devices may be masked, in which case they collect the data but do not show it real-time to the person wearing the device. Data is downloaded by the healthcare practitioner, then discussed with the patient. Unmasked versions collect this data but also have a display that the person can read.
One use of CGM is in conjunction with an insulin pump. When you eat and your glucose level rises, your CGM device measures glucose levels. Controls in the insulin pump react to this result to deliver insulin. Originally approved in adults and teens, these systems can now be used in children as young as 7 and are being studied in younger children.
How is the test used?
CGM is not used by a majority of people with type 1 diabetes, but as CGM devices become increasingly accurate and user-friendly, their use is increasing.
CGM is primarily intended for adults, teens, and some children with type 1 diabetes and may also be useful for those with type 2 diabetes, especially those using insulin who are self-checking their glucose multiple times a day.
CGM is used to help maintain good glucose control on a daily basis and meet glycemic (hemoglobin A1c) goals. It is also used to anticipate and prevent episodes of hypoglycemia, especially if you experience them frequently and/or have no distinct warning symptoms (hypoglycemia unawareness).
Recent recommendations on CGM from the American Diabetes Association (ADA) "Standards of Medical Care in Diabetes" include the following:
- Real-time CGM is a useful tool when used properly with intensive insulin regimens to lower hemoglobin A1c levels in type 1 diabetic adults who are not meeting their glycemic goals.
- CGM may be useful in diabetic adults with frequent hypoglycemic episodes and/or with hypoglycemia unawareness.
- Real-time CGM should be considered in children and teens with type 1 diabetes to help improve glucose control and reduce risks of hypoglycemia.
- Diabetes education, training, and support are important for ongoing CGM use.
- When used, CGM should be used as close as possible to daily for maximal benefit.
- Real-time CGM may be used to help improve A1C levels and neonatal outcomes in pregnant women with type 1 diabetes.
- Intermittently scanned CGM may be considered as a substitute for glucose self-monitoring in adults with diabetes who require frequent glucose testing. In this technology, the meter collects the glucose level measurements but does not display these measurements in real-time.
When is it ordered?
Real-time CGM may be used daily and constantly to have better glucose control when you have type 1 diabetes. This may be when you are using intensive insulin regimens [e.g., frequent injections of insulin under the skin (subcutaneous) or a continuous subcutaneous insulin infusion (CSII)] and/or when you are not meeting your hemoglobin A1c goals. In some cases, CGM may be used when you have type 2 diabetes and you are using insulin and require frequent glucose self-checks.
CGM may be used periodically when you have diabetes and your healthcare practitioner wants to collect and evaluate data on your day-to-day glucose variability and control.
What does the test result mean?
Glucose values from CGM devices generally correlate closely with blood glucose levels. Your target range for your glucose levels is determined by your healthcare practitioner and depends on several factors, such as:
- How long you have had diabetes
- Your age and life expectancy
- Other underlying conditions you may have, including heart disease
- Whether you do not experience distinct symptoms of low blood sugar (hypoglycemia unawareness)
CGM results can identify variability in glucose levels throughout the day, identify trends and anticipate when your glucose level is getting too low (hypoglycemia) or too high (hyperglycemia), sending an alert. It sends alarms when glucose levels reach the device manufacturer's established limit.
Looking at patterns of glucose data (such as data points collected over several days and graphed in a report) can help you and your healthcare practitioner evaluate variations in your glucose levels and suggest actions that may help stabilize glucose levels and improve glucose control.
Are all CGM devices the same?
No, they vary by manufacturer. They all measure glucose in the interstitial space, but the number of days of use and the intended user ages can be different. Some CGMs may trigger an alarm, and the way that they send alerts and/or report can also vary. As they continue to improve, the American Association of Clinical Endocrinologists and American College of Endocrinology (AACE/ACE) and others in the medical community would like to see some standardization with the devices, potentially including:
- Default alert levels, such as glucose less than 70 mg/dL for hypoglycemia and glucose greater than 300 mg/dL for hyperglycemia, as well as other triggers that can be programmed by the healthcare practitioner and/or the person wearing the CGM device
- Standardized reporting (which would make it easier for people and for healthcare practitioners to evaluate them)
- Displays with current glucose levels, trend arrows (i.e., is glucose going up or down), and graphs that show glucose patterns for the last day
- Insulin pump data (where applicable) that can be downloaded and viewed along with glucose data
Can I buy a CGM device over-the-counter?
Can I monitor my child’s type 1 diabetes with a CGM device?
Can I use a device for longer than indicated?
Is there anything else I should know?
At this time, most of the CGM devices still require two self-check blood glucoses a day for comparison and to ensure the CGM device is set to measure glucose correctly (calibration). Some also require you to verify a high or low CGM result with a self-check blood glucose before making a change in your treatment. These requirements are likely to change as the devices continue to improve.