What are the indications for continuous glucose monitoring and how is it implemented (device, calibration, sensor replacement, training, and insurance considerations)?

Continuous Glucose Monitoring (CGM): Indications and Implementation

Primary Indications

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. 1, 2

Type 1 Diabetes

• All adults with type 1 diabetes capable of daily CGM use should receive it, particularly when not meeting glycemic targets, experiencing hypoglycemia unawareness, or having recurrent hypoglycemic episodes 3, 1, 2
• All children and adolescents with type 1 diabetes should be offered CGM, whether using multiple daily injections or insulin pumps 3, 1, 2
• Real-time CGM is approved for nonadjunctive use (treatment decisions without confirmatory fingersticks) in children aged ≥2 years 2
• Pregnant women with type 1 diabetes should use real-time CGM to improve A1C levels, time in range, and neonatal outcomes 3, 1, 2

Type 2 Diabetes

• Patients on intensive insulin therapy (≥3 injections daily or insulin pump) should use CGM to lower A1C and reduce hypoglycemia 3, 1, 2
• Patients experiencing unexplainable severe hypoglycemia, recurrent hypoglycemia, asymptomatic hypoglycemia, or nocturnal hypoglycemia require CGM 3, 1, 2
• Patients with unexplainable hyperglycemia, especially fasting hyperglycemia 3, 1
• Patients with dramatic glycemic variability despite self-monitoring of blood glucose 3, 1, 2
• Patients who deliberately maintain high blood glucose due to fear of hypoglycemia 3, 1
• HbA1c above target despite multidrug oral and/or non-insulin injectable therapies 3, 1, 2
• Hospitalized type 2 diabetes patients on insulin therapy in non-ICU settings benefit from real-time CGM to reduce glucose fluctuations 1, 2
• Perioperative glycemic control in type 2 diabetes patients is improved with real-time CGM 1, 2

Pregnancy-Related Indications

• Gestational diabetes patients should use CGM as an adjunct to pre- and postprandial blood glucose monitoring to achieve A1C targets and improve neonatal outcomes 3, 1, 2
• Women with pre-existing diabetes during pregnancy should use CGM 3, 1, 2

Special Clinical Situations

• Diabetes patients with gastroparesis 3, 1, 2
• Special types of diabetes with dramatic glycemic variability 3, 1, 2
• Endocrine disorders accompanied by dramatic glycemic variability 3, 1, 2

Educational and Behavioral Indications

• CGM facilitates understanding of glucose changes from diet, exercise, alcohol, stress, sleep, and medications, motivating patients to establish healthy lifestyles and improving treatment adherence 3, 1

Device Selection and Types

Real-Time CGM (rtCGM)

• Provides continuous glucose display with real-time readings 3, 4
• Equipped with high and low glucose alerts 3, 4
• Can integrate with insulin pumps for automated insulin delivery 3, 4
• The Dexcom G6/G7 is FDA-approved for treatment decisions without confirmatory blood glucose checks and integrates with automated insulin delivery systems 4
• The Senseonics Eversense is an implantable device providing real-time CGM for 5-6 months but requires minor surgical procedure 4

Intermittently Scanned CGM (isCGM)

• Measures glucose continuously but requires scanning for visualization 3, 4
• Newer FreeStyle Libre systems include alarms 4
• Approved for children aged ≥4 years 2
• Designed to be worn for up to 14 days without calibration required 3
• Must be scanned at minimum once every 8 hours to avoid data gaps 3, 2, 4

Retrospective (Professional/Blinded) CGM

• Blind measurement with retrospective analysis after data download 3
• Wide scope of application in scientific research 3
• Lacks real-time alerts but useful for identifying patterns 3
• When coupled with diabetes self-management education and medication dose adjustment, helps identify and correct patterns of hyper- and hypoglycemia 3, 2

Calibration Requirements

Modern flash glucose monitoring systems (FreeStyle Libre) do not require calibration during their 14-day wear period. 3

• Older real-time CGM systems require calibration with self-monitoring of blood glucose 3
• Users need ability to perform self-monitoring of blood glucose for calibration (device-dependent) and verification of readings when discordant from symptoms 3, 1, 2
• CGM accuracy should be assessed using mean absolute relative difference (MARD), Clarke error grid analysis, and consensus error grid analysis 3

Sensor Replacement

• Most CGM sensors are worn for 7-14 days depending on the device 3
• Flash glucose monitoring systems are designed for up to 14 days of continuous wear 3
• Implantable CGM (Eversense E3) provides 5-6 months of monitoring 4

Training and Education Requirements

Robust diabetes education, training, and ongoing support are required for optimal CGM implementation and ongoing use. 3, 1, 2

Patient Prerequisites

• Patients must be willing and able to learn the basic mechanical skills of the equipment 1, 2
• Good patient adherence is essential 3
• Ability to intervene in rapid blood glucose fluctuations and manage extreme high and low glucose levels in a timely manner based on real-time glucose data 3
• Ability to manage high/low glucose alerts 3
• Ability to record life events related to blood glucose fluctuations 3

Specific Training Components

• Understanding how to respond to real-time glucose data 3
• For patients using real-time CGM and insulin pump integration, if they experience large glucose fluctuation or high/low glucose alerts, the hypoglycemic regimen should be adjusted under guidance of a clinician after rechecking finger-stick glucose level 3
• Regular and intermittent use with constant outpatient follow-up and active communication with clinicians 3

Usage Frequency for Effectiveness

Real-time CGM devices should be used as close to daily as possible for maximal benefit, as the greatest predictor of HbA1c lowering with CGM for all age groups is frequency of sensor use. 3, 1, 2

• Intermittently scanned CGM devices must be scanned at minimum once every 8 hours 3, 2, 4
• Benefits of CGM correlate directly with adherence to ongoing device use 3, 1, 2
• Most patients receiving intensive insulin regimens should self-monitor blood glucose before meals and snacks, at bedtime, occasionally after meals, when they suspect low blood glucose, after treating low blood glucose until normoglycemic, and before exercise and critical tasks like driving 2

Critical Contraindications and Limitations

Absolute Contraindications

• 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 1, 2
• Patients unwilling to learn device operation, as success depends on sustained use and proper data interpretation 1, 2

Technical Limitations

• CGM measures interstitial fluid glucose, which lags behind blood glucose by 5-15 minutes during rapid changes 1, 2
• Accuracy is lowest in hypoglycemic ranges, a critical limitation for patients with problematic hypoglycemia 1, 2
• High-dose acetaminophen (at higher than therapeutic doses) can interfere with glucose meter accuracy 3, 2, 4
• Ascorbic acid (vitamin C) can affect CGM readings 2, 4
• Salicylic acid can interfere with readings 4
• Uraemic toxins in patients with kidney disease may interfere with readings 2
• Certain medications like tetracycline and mannitol (for implantable sensors) can affect accuracy 2

Device-Specific Precautions

• Avoid exposure to strong magnetic fields, MRI, and in some devices conventional X-ray and CT scanning 1, 2
• Skin reactions (irritation or allergy) should be assessed and addressed to aid successful device use 3, 1, 2, 4

Patient-Specific Limitations

• Real-time CGM not applicable for diabetes patients with depression or anxiety 3
• Requires educational background for optimal use 3

Insurance and Reimbursement Considerations

• One CGM device with accuracy (MARD) of approximately 10% has been approved for reimbursement for therapeutic use in the United States 5
• People who have been using continuous glucose monitors should have continued access across third-party payers 3
• Cost-effectiveness analyses have indicated that CGM is a cost-effective adjunct to type 1 diabetes management that is associated with reduced diabetes-related complications and hospitalization 6
• The FreeStyle Libre systems have advantages including lower cost compared to real-time CGM systems 4

Alternative Use Patterns

• Periodic use of real-time, intermittently scanned, or professional CGM can be helpful when continuous use is not appropriate, desired, or available 1, 2
• Retrospective CGM requires regular and intermittent use with constant outpatient follow-up and active communication with clinicians 3

Common Pitfalls to Avoid

• Do not rely on CGM readings during rapid glucose changes without confirmatory fingerstick testing 1, 2
• Do not use CGM in ICU settings where accuracy is compromised 1, 2
• Do not prescribe CGM without ensuring comprehensive patient education and ongoing support 3, 1, 2
• Do not ignore skin reactions, as they can lead to device abandonment 3, 1, 2, 4
• Do not assume all CGM devices have the same capabilities—verify FDA approval for nonadjunctive use and automated insulin delivery compatibility 4