Continuous Glucose Monitoring: Weighing the Pros and Cons

Woman wearing a CGM and putting her hair up

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Continuous glucose monitoring (CGM) devices provide real-time glucose readings 24 hours a day, thereby allowing patients with type 1 or type 2 diabetes to closely track blood sugar levels and trends. Typically, CGMs report blood sugar levels every five minutes, thereby providing 288 glucose readings per day. CGMs can help patients make better-informed blood sugar decisions by alleviating much of the guesswork about daily patterns and fluctuations.

How They Work

Traditionally, CGM devices consist of three parts:

  • A tiny sensor wire that is inserted under the skin using an automatic applicator to measure glucose levels. The sensor wire is held in place by an adhesive patch.
  • A transmitter attached to the sensor, which sends glucose level data wirelessly to a receiver.
  • A receiver, which is attached to a monitor that displays glucose levels and trends.

Some devices use Bluetooth technology to submit blood glucose information from the transmitter under the skin directly to compatible smartphones, bypassing the need for a separate receiver.

Additionally, some 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 5 selected followers. This information can then be easily accessed at the doctor’s office, facilitating the tracking and analysis of patterns and displaying trends in graphical form.

Benefits of CGM Devices

Benefits of using a CGM include the following:

  • CGMs help detect trends in blood glucose levels, even if the measurements are not entirely precise. For example, CGMs can show early morning spikes or dips in blood glucose at times when most people aren’t checking sugars frequently.
  • CGMs may greatly improve the quality of life for many people with diabetes, thanks to a reduced number of required fingerpricks, which may be painful and difficult to manage frequently.
  • CGMs may help clarify the effect of diet and exercise on blood sugar levels.
  • Users can set alarms to alert them when glucose levels are too low or too high.
  • You may also be able to input meals, medicine, and physical activity into your CGM device, which can help you better track glucose trends.
  • In some large trials, CGMs lowered hemoglobin A1c anywhere from 0.3 to 1 percent when compared to traditional fingerstick glucose checks.
  • Studies have also shown that CGMs may reduce the risks of hypoglycemia.

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 both glucose and insulin levels simultaneously. In a new development, researchers have successfully created the first closed-loop insulin delivery system, which combines CGM technology with an insulin pump and a special algorithm that gives the ability for the insulin pump and CGM to use glucose readings to calibrate insulin levels accordingly and then immediately deliver a specified amount automatically.

Limitations of CGM Devices

The use of CGM is still somewhat controversial, with various potential disadvantages outlined below. 

  • Although CGM delivers blood glucose readings automatically at short intervals, twice-daily fingersticks are usually still required in order to calibrate the CGM.
  • The CGM machines often yield lower glucose readings when compared to venous blood glucose.
  • Many argue that the true benefit of CGM technology lies in the detection of hypoglycemic episodes. However, data suggest that CGM may become increasingly inaccurate at low glucose ranges.
  • CGM machines are expensive, with initial costs ranging between $1,000 and $2,000, and are available by prescription only. In addition, supplies cost between $300 and $450 per month, including sensors which should be replaced every 3 to 7 days.

Available CGM Devices

Dexcom G5: The first FDA-approved CGM device for patients age 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 to help you track your data. It does require twice-daily fingersticks for accurate calibration.

Dexcom G6: The Dexcom G6 is the first model approved for medical treatment decisions, meaning that your care provider can make changes to your diabetes treatment plan based on your CGM results alone—no fingersticks required to confirm. Users employing other models are required to confirm CGM results with a fingerstick blood glucose test before changing treatment recommendations. The G6 can predict both hypoglycemic and hyperglycemic trends. Sensors last 10 days before they must be changed.

Freestyle Libre: The first FDA-approved CGM device for patients over age 18, the Freestyle Libre eliminates the need for regular daily fingersticks except for the occasional device calibration. Users must scan the sensor at least once every eight hours—it's worth noting that the device will not automatically alert you if your levels are too low or too high—it must be scanned in order to do so. Sensors last 14 days before they must be changed

Medtronic Guardian Connect: The Medtronic Guardian Connect sensor may be worn up to seven days, though the transmitter lasts up to one year—which differs from other models that may need transmitters replaced as many 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.

Who Should Use CGM

Patient selection is a key component to successful CGM use. The Endocrine Society Guidelines recommend CGM for adult patients with type 1 diabetes with A1c levels above 7 percent who have shown that they can use these devices nearly every day.

In addition, it is crucial that selected patients demonstrate an understanding of CGM technology in order to reap the maximum benefit. The intermittent use of CGM may be appropriate for patients with overnight hypoglycemic or hypoglycemic unawareness (inability to self-detect hypoglycemia). It may also be beneficial for patients with type 2 diabetes who have overnight hypoglycemia or frequent episodes or hypoglycemic unawareness.

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