What to Know About Continuous Glucose Monitoring

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Continuous glucose monitoring (CGM) devices provide real-time blood sugar readings 24 hours a day, allowing people with type 1 or type 2 diabetes to closely track blood glucose levels and trends. Most CGMs report blood sugar levels every five minutes, for a total 288 glucose readings per day.

CGMs can help people with diabetes make informed decisions about food choices, exercise, and other aspects of diabetes management by alleviating much of the guesswork about daily patterns and fluctuations.

Woman wearing a CGM and putting her hair up
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How It Works

Most CGM devices consist of three parts:

  • A tiny sensor wire that is inserted under the skin of the arm or abdomen, which uses an automatic applicator to measure glucose levels. The sensor wire is held in place by an adhesive patch. The sensor measures interstitial glucose (glucose in the fluid between cells), rather than glucose found in blood.
  • A transmitter attached to the sensor, which sends data wirelessly every few minutes to a receiver.
  • A receiver attached to a monitor that displays glucose levels and trends.

Some devices use Bluetooth technology to submit blood glucose information directly to compatible smartphones or tablets, bypassing the need for a separate receiver. Some monitors also may be combined with an insulin pump.

Other CGM devices include user-friendly cloud-based reporting software. When blood glucose information is received by a smart device, it is then transmitted via the cloud to up to five selected followers, such as parents, caregivers, and medical providers. This information can then be easily accessed at the doctor’s office, facilitating tracking and analysis of patterns and displaying trends in graphical form.

Some CGM devices allow the user to input information about meals, snacks, medication, and physical activity to further help with tracking glucose trends.

Benefits

CGM can help improve diabetes management by:

  • Helping detect trends in blood glucose levels, even if the measurements are not entirely precise. For example, CGMs can show spikes or dips in blood glucose at times when it's inconvenient to check sugar levels frequently, such as in the wee hours of the morning.
  • Eliminating the need for numerous finger pricks, which may be painful and difficult to manage frequently.
  • Helping to clarify the effect of diet and exercise on blood sugar levels.
  • Alerting users with an alarm when glucose levels are too low or too high.

What Research Says

Major clinical trials of insulin-treated patients have included self-monitoring of blood glucose as part of interventions to demonstrate the benefit of intensive glycemic control on diabetes complications. As such, the American Diabetes Association (ADA) recognizes CGM as an integral component of effective treatment planning for many patients with diabetes taking insulin.

CGMs can also be used with insulin pump therapy. A sensor-augmented pump (SAP) combines the technology of an insulin pump with a continuous glucose sensor, allowing users to monitor glucose and insulin levels simultaneously.

Another innovation in the realm of CGM is a closed-loop insulin delivery system, which combines CGM technology with an insulin pump and a special algorithm that allows the insulin pump and CGM to use glucose readings to calibrate insulin levels and immediately deliver a specified amount automatically.

Limitations

Despite the positives, CGM has drawbacks that should be considered:

  • Although CGM delivers blood glucose readings automatically at short intervals, twice-daily finger sticks usually are necessary to calibrate the CGM for accuracy.
  • CGM devices often yield lower glucose readings when compared to venous blood glucose.
  • There is data to suggest CGM may become increasingly inaccurate at low glucose ranges—even though the technology has been touted as being especially beneficial for detecting hypoglycemic episodes.
  • CGM machines are expensive, with initial costs ranging from between $1,000 and $2,000, and require a prescription. In addition, supplies cost between $300 and $450 per month, including sensors that should be replaced every three to seven days.

The ADA notes in the latest Standards of Medical Care in Diabetes that there is no “one-size-fits-all” approach to technology use in people with diabetes. The appropriateness of CGM verus other technology options will be carefully considered by your doctor. 

Who Should Use CGM

Patient selection is a key component of successful CGM use. The Endocrine Society recommends CGM for adults with type 1 diabetes who have A1C levels above 7% and who have shown they can use these devices nearly every day. Some devices are also approved for children over 2 with a doctor's prescription.

It's crucial to understand CGM technology in order to reap the maximum benefit. The intermittent use of CGM may be appropriate for people with overnight hypoglycemia or hypoglycemic unawareness—an inability to recognize the signs of dips in blood sugar levels that can be dangerous if not dealt with immediately. It may also be beneficial for people who are on tight blood sugar control, also known as intensive insulin therapy.

Specific Devices

If you and your endocrinologist determine continuous glucose monitoring might be advantageous to your diabetes self-care, you'll have to make a choice regarding which device is best for you. Among your options:

Dexcom G5: The first CGM device approved by the Food and Drug Administration (FDA) for patients 2 and older, the Dexcom G5 works with an easy-to-use app, offers cloud storage, and syncs with both Android and iPhone operating systems. Requires twice-daily finger sticks for accurate calibration.

Dexcom G6: The Dexcom G6 is the first model approved for medical treatment decisions, meaning your healthcare provider can make changes to your diabetes treatment plan based on your CGM results alone—no finger sticks necessary. The G6 can predict hypoglycemic and hyperglycemic trends. Sensors last 10 days before they must be changed.

Freestyle Libre: FDA-approved CGM for people over 18, the Freestyle Libre eliminates the need for daily finger sticks except for the occasional device calibration. Users must scan the sensor at least once every eight hours—the device will not automatically alert you if your levels are too low or too high. Sensors last 14 days before they must be changed.

Medtronic Guardian Connect: The Medtronic Guardian Connect sensor may be worn for up to seven days, though the transmitter lasts up to one year—which differs from other models that may need transmitters replaced as often as four times per year. The Guardian Connect CGM works closely with the Sugar.IQ diabetes assistant app, helping you monitor and keep track of your levels from a simple-to-use interface.

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