What are the indications, cautions, and pitfalls of Continuous Glucose Monitoring (CGM) in patients with diabetes, particularly those with type 1 diabetes or insulin-requiring type 2 diabetes?

Continuous Glucose Monitoring: Indications, Cautions, and Pitfalls

Primary Indications

Real-time CGM should be used in all teens and adults with type 1 diabetes who are on insulin therapy, particularly those not meeting glycemic targets, experiencing hypoglycemia unawareness, or having recurrent hypoglycemic episodes. 1, 2

Type 1 Diabetes (Strongest Evidence)

• All patients with type 1 diabetes should use CGM regardless of age or current glycemic control, as this technology consistently reduces hypoglycemia and improves time in range without increasing adverse events 2
• Real-time CGM demonstrates robust HbA1c reductions of 0.43-0.6% in adults with type 1 diabetes, with the greatest benefit when baseline HbA1c is elevated 1
• Significant reductions in time spent in hypoglycemic range occur in adults with type 1 diabetes, with one study in patients over age 60 showing reductions in both hypoglycemic time and severe hypoglycemic events 1
• All children and adolescents with type 1 diabetes should be offered CGM, whether using multiple daily injections or insulin pumps, as benefits correlate directly with adherence to ongoing device use 2

Type 2 Diabetes on Insulin

• Real-time or intermittently scanned CGM should be used in adults with type 2 diabetes who are using insulin (≥3 injections daily or pump therapy) and not meeting glycemic targets 1, 2
• Randomized trials in type 2 diabetes patients on insulin show HbA1c reductions with no significant increase in hypoglycemia 1
• Type 2 diabetes patients using hypoglycemic therapy who experience unexplainable severe hypoglycemia, recurrent hypoglycemia, asymptomatic hypoglycemia, or nocturnal hypoglycemia should use CGM 2

Pregnancy

• Real-time CGM should be used in pregnant women with type 1 diabetes to improve HbA1c levels, time in range, and neonatal outcomes 1, 3
• CGM in pregnancy reduces macrosomia, neonatal hypoglycemia, and hospital length of stay 3
• Gestational diabetes patients and women with pre-existing diabetes during pregnancy should use CGM as an adjunct to pre- and postprandial blood glucose monitoring 2, 3

Additional Clinical Scenarios

• Hospitalized type 2 diabetes patients on insulin therapy in non-ICU settings benefit from real-time CGM to reduce glucose fluctuations 2
• Patients with unexplainable hyperglycemia (especially fasting), dramatic glycemic variability despite self-monitoring, or HbA1c above target despite multidrug therapy should use CGM 2
• Diabetes patients with gastroparesis or special types of diabetes with dramatic glycemic variability should use CGM 2

Critical Prerequisites and Usage Requirements

CGM requires robust diabetes education, training, and ongoing support for optimal implementation—patients must be willing and able to learn the basic mechanical skills of the equipment. 1, 2

Essential Patient Capabilities

• Patients must possess ability to perform self-monitoring of blood glucose for calibration (device-dependent) and verification of readings when discordant from symptoms 1, 2
• Users need understanding of glucose changes from diet, exercise, alcohol, stress, sleep, and medications to maximize benefit 2
• Programs involving training and support improve outcomes in both adults and children using intermittently scanned CGM 1

Frequency Requirements for Effectiveness

• Real-time CGM devices should be used as close to daily as possible for maximal benefit—consistency of CGM use is highly correlated with lower HbA1c 1, 2
• Intermittently scanned CGM devices must be scanned at minimum once every 8 hours 2
• In one large trial, children and adolescents who wore CGM less frequently than adults did not achieve HbA1c improvements, demonstrating the critical importance of adherence 1

Major Cautions and Limitations

Technical Accuracy Issues

• CGM measures interstitial fluid glucose, which lags behind blood glucose by 5-15 minutes during rapid changes—this is a fundamental limitation of the technology 2
• Accuracy is lowest in hypoglycemic ranges, a critical limitation for patients with problematic hypoglycemia 2
• Sensor accuracy is affected by medications (particularly acetaminophen in therapeutic or supratherapeutic doses in older systems), temperature, humidity, and altitude 1, 2
• Mean absolute relative deviation (MARD) for current devices ranges from 8.1% to 12%, representing improved but still imperfect accuracy 1

Device Interference and Safety

• Avoid exposure to strong magnetic fields, MRI, and in some devices conventional X-ray and CT scanning 2
• Acetaminophen in therapeutic doses caused positive bias in several older CGM systems, though current systems show variable effects 1
• Skin reactions (irritation or allergy) should be assessed and addressed to aid successful device use 2

Clinical Setting Limitations

• Intensive care units are not suitable for CGM due to skin edema, vasoconstrictor drugs, hypotension, hypoxemia, and high-dose acetaminophen which adversely affect sensor accuracy 2, 3
• CGM systems require at least 12 hours of wear before data becomes reliable, and calibration should show <15% difference between CGM and meter glucose values 3

Common Pitfalls to Avoid

Patient Selection Errors

• CGM is not suitable for patients unwilling to learn device operation, as success depends on sustained use and proper data interpretation 2
• Insufficient evidence exists for screening or diagnosis of prediabetes or diabetes using CGM 2
• The evidence for real-time CGM use in young children (less than age 8 years) with type 1 diabetes is limited—one RCT showed no impact on HbA1c, though registry studies suggest benefit 1

Implementation Mistakes

• Failure to provide adequate education and support—studies showing benefit typically included substantial patient education and ongoing support 1
• Not training patients on backup self-monitoring of blood glucose for device failures or when CGM values seem incongruent with symptoms 1
• In pregnancy, over-reliance on CGM metrics alone without maintaining traditional self-monitoring targeting fasting <95 mg/dL and 1-hour postprandial <140 mg/dL 3

Data Interpretation Errors

• Testing without action—CGM does not itself lower glucose levels; data must be integrated into treatment adjustments to be effective 4
• In type 2 diabetes, mixed results exist for intermittently scanned CGM, with some studies showing benefit and others not 1
• Professional (blinded) CGM data, when coupled with diabetes self-management education and medication dose adjustment, helps identify and correct patterns of hyper- and hypoglycemia 2

Cost-Effectiveness Considerations

• Periodic use of real-time, intermittently scanned, or professional CGM can be helpful when continuous use is not appropriate, desired, or available 2
• Tailoring the application of CGM for daily or intermittent use, depending on patient profile and needs, ensures cost-effectiveness in each setting 5