The benefits and challenges of Continuous Glucose Monitoring (CGM) will change as these products develop over time. Benefits to CGM are many. Patients may view glucose sensors as miracle tools without assessing the challenges, or without assessing their own expectations. CGM is a tool with limitations. Limitations may be inherent in the technology, in the brand of CGM or in how the patient uses the device and its data. Diabetes educators can help patients assess their expectations, their reasons for wanting to use glucose sensors and the "pros and cons" of sensor use as listed below. (This is one of many pages on CGM. View our full list of CGM pages)
Benefits of CGM
Benefits of CGM may include the following, depending on the brand as well as the use of the data by the patient, caregiver and healthcare provider.
Improved glycemic control:
- Reduced events and duration of hypoglycemia without increasing A1c
- Reduced events of severe hypoglycemia in hypoglycemia unawareness
- Reduced events and duration of hyperglycemia
- Improved A1c in type 1 and type 2 diabetes (on insulin or agents other than insulin)
- Improved neonatal outcomes in pregnant women with type 1 diabetes using CGM, likely attributed to reduced exposure to maternal hyperglycemia. (Although CGM is not currently indicated in pregnancy the authors of the CONCEPTT trial suggest CGM be offered to all pregnant women with type 1 diabetes using intensive insulin therapy. The CONCEPTT trial used rtCGM, and showed CGM use in pregnancy resulted in a lower incidence of large for gestational age, fewer neonatal intensive care admissions lasting more than 24 hr, fewer incidences of neonatal hypoglycemia and 1-day shorter length of hospital stay.)
Potential for increased self-management:
- Increased patient insight regarding food, portions, physical activity, stress, diabetes medication choices
- Increased proactive and retroactive decision-making by patients as a result of easy and timely access to more data, including post-prandial and overnight data.
- Increased ability and speed to self-assess validity of management changes which can increase sense of empowerment and self-efficacy
- Increased types of glycemic metrics available (% time in range, eA1c etc)
Potential for improved quality of life:
- Decreased fingerstick testing
- Decreased manual record keeping
- Increased ability to identify and treat hypoglycemia; increased reassurance for those fearful of hypoglycemia during sleep, sports, driving etc
- Increased ease of glucose monitoring; less conspicuous than fingerstick testing
- Increased speed of feedback could impact motivation and self-efficacy.
- Remote viewing of data by loved ones/care givers
Reduced limitations of A1c and fingerstick blood glucose tests:
- A1c is valuable in determining associations with risks for complications, but does not help individualize daily diabetes management. CGM overcomes many limitations of A1c which include: providing an “average” glucose level over 2-3 months; not identifying low and high patterns and where treatment could be changed; not identifying or alerting the patient to daily glycemic excursions; unreliable as a glycemic measure in patients with anemia, iron deficiency, hemoglobinopathies and not standardized to address ethnic differences in glycation rates.
- CGM overcomes some limitations of fingerstick blood glucose monitoring including inconvenience; physical intrusiveness of "poking" multiple times daily; limited single “point-in-time” glucose measurement; missed data (between meals, overnight, with sports or with asymptomatic hypo/hyperglycemia). Altitude, temperature and humidity may also impact some fingerstick glucose devices. The authors (Aleppo et al) of a 2017 Journal of Endocrine Society article note there are no current indications that extremes of temperature, altitude or humidity would impact CGM accuracy. (Although anecdotally, and not surprisingly, some CGM receiving devices have been reported to work unreliably when exposed to the cold.) Additionally, some fingerstick glucose meters are not accurate in renal disease dialysis. CGM has been used in research in both HD and PD. Limited research shows no impact of hematocrit on CGM results.
Challenges of Continuous Glucose Monitoring
Challenges for patients:
- Readings lag behind blood glucose: Patients may not understand or appreciate the difference in sensor interstitial glucose readings and fingerstick blood glucose readings, or the differences when glucose levels are changing quickly. Additionally, people may notitice that first day sensor performance is less accurate than subsequent days.
- Lost signal – missed data, sensor failures: Patients should contact the sensor manufacturer. Use of insertion sites not indicated by the company may result in the sensor not being replaced.
- Body image concerns:Sensors vary by size/depth between brands, visibility and suggested insertion sites (or comfortable insertion sites)
- Site Concerns
- Site selection: Certain insertions sites should be avoided. This can make it challenging for patients depending on their sleeping positions, clothing preferences and other variables. Many patients have accurate readings in sites not indicated by manufacturers, while acknowledging the sensor may not be warrantied with this usage.
- Site irritation: Some patients experience site irritation due to adhesive or perhaps site selection.
- Sensor adhesion: Many clients find additional taping or adhesion wipes are needed.
- Glucose sensor costs per month depend on the brand; visit our CGM Brand Information Page for details. According to manufacturer’s suggestions for sensor use and replacement, the unreduced cost ranges from $195 CAD to $397 a month in 2023. Each of the CGM companies offers reduced cost options via low income or subscription programs (see company websites for details). Some sensors can be reset “off label” for longer use if results remain accurate according to calibrations. When asking about coverage, patients are encouraged to ask insurance companies to check under categories for glucose meters (SMBG) and medical devices, as brands may be listed differently.
- Fingerstick Testing
- Calibration: Some CGM devices require twice daily calibration using fingerstick glucose tests to avoid results “drifting”. Sensor accuracy is dependent upon glucose meter accuracy or proper technique (clean finger etc) and timing (stable sensor reading). Unfortunately, some patients have developed DKA as a result skipping calibrations, or inserting the sensor reading (not a fingerstick blood glucose reading).
- Fingerstick testing is still required even if no calibration: Some clients may find it an inconvenience to still carry a blood glucose meter and ketone strips (if on insulin pump). All CGM devices require verification using fingerstick glucose tests if symptoms do not match sensor readings. Many educators routinely suggest fingerstick glucose testing to confirm hypoglycemia given the numerous anecdotal situations false lows or extremely extended duration of lows on sensor compared to glucose meters. There are other conditions for testing that are brand specific.
- Increased Data and Learning
- Data and alert fatigue: There is significantly more data available to interpret. Alerts and alarms about glucose readings may fatigue users. This may lead some patients to switch alerts off and underutilize their CGM system. Patients encouraged to work with their educators to set realistic ranges or options if not using the data when presented with alerts or if feeling overwhelmed.
- Increased learning: Many patients purchase and set up CGM alone as companies have excellent training videos. However, after setup patients may not take the time to adequately learn about data interpretation and subsequent management decisions. Their use of CGM may be underutilized or incorrectly used without education on lag time; alert settings; calibrations; indications and reminders for verification using blood glucose meter; data interpretation and subsequent management decisions; site maintenance. Most CGM have self-training resources for insertion.
- Increased math: Usually, more math calculations are required for bolusing decisions to prevent lows and highs according to trend data. More math calculations requires more decision making and hence more time.
- Over-correcting with insulin or food: The increased access to glucose data on CGM devices may result patients making too frequent insulin or food adjustments, potentially resulting in more unstable glucose levels.
- Acetaminophen: False high readings can occur for about 6 hours after acetaminophen for rtCGM (Dexcom, Medtronic).
- Matching Pump and CGM data: Matching CGM data to pump insulin data can be challenging if both are not from the same company. CGM with pump does not remove the need for management decisions.
- Duration of wear for benefits: A1c improvements usually require long term use of the device even though insight regarding dosing and lifestyle habits can be achieved with short term CGM use.
- Healthcare provider inexperience: Although CGM has been available for numerous years, wide spread use of CGM beyond insulin pump users is relatively newer. Not all physicians and healthcare professionals may be informed regarding interpretation of CGM results or adjusting insulin with trend arrows
Challenges for Healthcare Providers:
- Increased learning: Just like patients, healthcare providers must increase their learning of CGM. However, educators need to learn about all brands.
- Guidelines: There are no national guidelines for management decisions based on CGM data. Varying methods have been published with more agreement for methods using ROC arrows in Dexcom.
- Rate of Change (ROC) Arrows: ROC arrows are associated with differing levels of glucose change between brands. Not all brands are approved by Health Canada for insulin dose adjustments based on arrows. Educators must respect patients’ current practices while informing patients of measures to ensure their safety.
- Missed data: Patients may not use event markers (grams carb, insulin, exercise), or event markers may be limited in the device. Patients may eat extra as a proactive measure to avoid low blood sugars, or bolus extra to avoid hyperglycemia.This is excellent self-management. However, if this information is not reflected on by the patient or not recorded for the HCP to review, this can make it difficult to recognize some patterns that might benefit from a more proactive dose adjustment.
This is one of many pages on CGM. Please visit our main page, Glucose Sensors and Continuous Glucose Monitors (CGM), for a list of all our CGM pages.
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