Continuous Glucose Monitoring in Internal Medicine: A Practical Review for the Modern Physician

Dr Neeraj Manikath , claude.ai

Abstract

Continuous glucose monitoring (CGM) has revolutionized diabetes management, transitioning from a research tool to an essential component of comprehensive care. This review provides postgraduate internal medicine physicians with evidence-based insights, practical applications, and clinical pearls for integrating CGM into practice. We explore the technology, clinical indications, interpretation strategies, and emerging applications beyond traditional diabetes management.

Introduction

The landscape of glucose monitoring has undergone a paradigm shift. Traditional self-monitoring of blood glucose (SMBG) provides discrete snapshots, while CGM offers a continuous metabolic narrative, capturing glucose dynamics that inform therapeutic decisions with unprecedented precision. As of 2025, CGM technology has become increasingly accessible, with expanded indications and evolving reimbursement policies making it relevant to all internists.

Technology Overview

How CGM Works

CGM devices measure interstitial glucose through enzymatic electrochemical reactions, typically using glucose oxidase. Current systems sample glucose every 1-5 minutes, providing 288-1440 data points daily compared to the 4-7 from conventional SMBG. Modern devices employ either real-time CGM (rt-CGM) or intermittently scanned CGM (isCGM, also called "flash" glucose monitoring).

Pearl: Interstitial glucose lags capillary blood glucose by approximately 5-15 minutes. This physiological lag becomes clinically significant during rapid glucose changes, particularly during hypoglycemia or post-prandial periods.

Current CGM Systems

The major platforms include:

• Dexcom G7: 10-day wear, 12-hour warm-up period eliminated, integration with insulin pumps and automated insulin delivery systems
• Abbott FreeStyle Libre 3: 14-day wear, smallest sensor profile, real-time alerts in newer versions
• Medtronic Guardian 4: Integrated with Medtronic insulin pumps, predictive alerts

Hack: When patients report CGM readings inconsistent with symptoms, remember the "Rule of 15/15": If interstitial glucose changes >15 mg/dL in 15 minutes, expect 10-20% discordance with capillary measurements. Always confirm hypoglycemia <70 mg/dL with capillary glucose before treating.

Key Metrics: Beyond Mean Glucose

Time in Range (TIR)

TIR (70-180 mg/dL) has emerged as the gold standard CGM metric, correlating better with microvascular complications than HbA1c. Target TIR >70% for most patients with type 1 or type 2 diabetes.

Oyster: A patient with HbA1c 7.5% could have excellent TIR with stable glucose, or severe glycemic variability with equal time in hyper- and hypoglycemia. CGM reveals what HbA1c conceals.

Time Below Range (TBR)

• Level 1 hypoglycemia (54-69 mg/dL): Target <4% of time
• Level 2 hypoglycemia (<54 mg/dL): Target <1% of time

Pearl: Each 1% increase in TBR <54 mg/dL doubles the risk of severe hypoglycemia requiring assistance. This metric is particularly valuable in older adults and those with hypoglycemia unawareness.

Glucose Management Indicator (GMI)

GMI estimates HbA1c from mean CGM glucose. The formula: GMI = 3.31 + (0.02392 × mean glucose in mg/dL). Significant GMI-HbA1c discordance suggests hemoglobin variants, altered red cell turnover, or laboratory error.

Coefficient of Variation (CV)

CV quantifies glucose variability. CV = (standard deviation/mean glucose) × 100. Target CV ≤36% indicates stable glucose control. High CV despite acceptable mean glucose indicates problematic variability requiring intervention.

Hack: The "CV finger test": If you can fit three fingers between the highest and lowest glucose readings on the CGM tracing, CV is likely >36%, suggesting excessive variability.

Clinical Indications

Established Indications

1. Type 1 Diabetes: Class I recommendation (ADA 2024). CGM reduces HbA1c by 0.5-1.0% and severe hypoglycemia by 50-70%.

2. Type 2 Diabetes on Intensive Insulin: Multiple daily injections or insulin pump therapy. The DIAMOND study demonstrated 0.3-0.4% HbA1c reduction with improved quality of life.

3. Hypoglycemia Unawareness: CGM with predictive low-glucose alerts reduces hypoglycemia by 72%.

4. Pregnancy with Diabetes: Improves neonatal outcomes, reduces macrosomia by 33%, and lowers neonatal hypoglycemia risk.

Emerging Indications

1. Type 2 Diabetes on Basal Insulin Only: The MOBILE study (2023) showed significant HbA1c improvement and treatment satisfaction.

2. Hospitalized Patients: Point-of-care CGM correlates well with capillary glucose in non-critically ill patients, though accuracy decreases with vasopressors or significant edema.

3. Steroid-Induced Hyperglycemia: CGM reveals characteristic afternoon/evening glucose peaks, guiding timing of interventions.

Pearl: In steroid-induced hyperglycemia, glucose typically peaks 8-12 hours post-dose. NPH insulin given at the time of steroid administration effectively matches this pattern.

Interpretation Strategies

The Ambulatory Glucose Profile (AGP)

AGP standardizes CGM data visualization, showing:

• Median line: 50th percentile glucose
• Interquartile range (25th-75th percentile): The "blue zone"
• 10th-90th percentile range: Captures outliers

Hack: The "3-2-1 Rule" for AGP interpretation:

• 3 sections: Look at overnight (midnight-6am), fasting/morning (6am-noon), and afternoon/evening (noon-midnight)
• 2 questions per section: Where is the median? How wide is the blue zone?
• 1 intervention: Focus on the time period with the worst control first

Pattern Recognition

1. Dawn Phenomenon: Glucose rise between 4-8am without hypoglycemia. Managed by increasing evening basal insulin or metformin dose.

2. Somogyi Effect (Oyster within an Oyster): Rebound hyperglycemia following nocturnal hypoglycemia. Often misdiagnosed as dawn phenomenon. Look for glucose nadir 2-4am followed by rise. Requires decreasing, not increasing, evening insulin.

3. Postprandial Hyperglycemia: Glucose >180 mg/dL 1-2 hours post-meal. Address meal composition, prandial insulin timing, or insulin-to-carbohydrate ratios.

Pearl: The "90-minute window": Peak postprandial glucose typically occurs 90 minutes after meal start. If consistently >180 mg/dL at 90 minutes, prandial insulin adjustment is needed.

Practical Clinical Applications

Medication Titration

CGM enables precise medication adjustments:

For Basal Insulin: Evaluate overnight and fasting glucose. Increase by 2-4 units if fasting glucose consistently >130 mg/dL.

For GLP-1 Receptor Agonists: CGM demonstrates dramatic reduction in postprandial excursions, often before significant HbA1c reduction. This early feedback improves adherence.

Hack: The "Threshold Suspend Maneuver" for testing basal insulin adequacy: Have patients skip a meal while wearing CGM. If glucose drops >30 mg/dL, basal insulin is excessive.

Lifestyle Modification

CGM provides immediate biofeedback linking behaviors to glucose responses.

Pearl: The "carbohydrate-protein toggle": When patients see postprandial spikes, recommend increasing protein/fat relative to carbohydrates in that meal. Many patients find 1:1 carbohydrate-to-protein ratio (in grams) flattens postprandial curves.

Hypoglycemia Investigation

When patients report symptoms without documented hypoglycemia on CGM, consider:

1. Pseudohypoglycemia (symptoms at normal glucose after chronic hyperglycemia)
2. Rapid glucose decline (even if nadir >70 mg/dL)
3. Adrenergic symptoms from other causes
4. Nocturnal hypoglycemia occurring before CGM initiation with persistent fear

Special Populations

Older Adults

Individualize TIR targets (70-180 mg/dL may be too aggressive). Consider 80-200 mg/dL for frail elderly. Prioritize TBR <1% over optimal TIR.

Pearl: In nursing home residents, CGM reveals insulin administration errors and unrecognized hypoglycemia occurring in up to 20% of insulin-treated patients.

Chronic Kidney Disease

CGM accuracy maintained until GFR <15 mL/min. In dialysis patients, glucose variability increases dramatically. Target wider ranges and focus on hypoglycemia prevention.

Hospital Use

FDA-approved devices (e.g., Dexcom G7) can supplement but not replace capillary glucose monitoring in hospital settings. Particularly valuable during corticosteroid therapy, enteral nutrition, and transitions of care.

Troubleshooting Common Issues

Sensor Accuracy Issues

Compression lows: False hypoglycemia readings from sleeping on sensor. Appear as sharp drops followed by rapid recovery when pressure relieved.

Acetaminophen interference: Older sensors showed interference; most modern sensors are unaffected.

Calibration: Most current sensors are factory-calibrated. Adding unnecessary calibrations may worsen accuracy.

Hack: The "two-site rule": If sensor readings seem inaccurate, apply next sensor to different anatomic site. Avoid areas with lipohypertrophy, recent injection sites, or scars.

Adhesion Problems

• Clean skin with soap and water (not alcohol which dries skin)
• Apply liquid adhesive (Skin Tac) before sensor
• Use over-patches (e.g., Simpatch, GrifGrips) for extended wear
• Avoid lotion, sunscreen, or oils near sensor sites for 24 hours pre-application

Pearls for Practice

1. The 14-Day Rule: Obtain at least 14 days of CGM data before making therapeutic changes. Shorter periods may reflect atypical behaviors.

2. Percentage Time Targets:

   • TIR (70-180 mg/dL): >70%
   • TBR (<70 mg/dL): <4%
   • TBR (<54 mg/dL): <1%
   • TAR (>180 mg/dL): <25%
   • TAR (>250 mg/dL): <5%

3. The One-Week Snapshot: When reviewing CGM data with patients, focus on the most recent complete week. This balances adequate data capture with patient recall of events.

4. Exercise Patterns: CGM reveals aerobic exercise typically lowers glucose (during and 24 hours post-exercise), while anaerobic/resistance exercise may raise glucose acutely. This guides pre-exercise carbohydrate intake and insulin adjustments.

5. Alcohol Effects: CGM elegantly demonstrates delayed hypoglycemia 6-12 hours post-alcohol consumption, particularly with evening drinking.

Cost-Effectiveness and Access

As of 2024, Medicare covers CGM for insulin-treated diabetes (multiple daily injections or pump). Many private insurers have expanded coverage to include non-insulin-treated type 2 diabetes. Professional CGM (physician-owned devices) offers diagnostic option when personal CGM not covered.

Hack: For patients without coverage, consider professional CGM for 14 days during critical decision points (initiating insulin, unexplained hypoglycemia, pregnancy planning).

Future Directions

Emerging applications include CGM use in prediabetes, reactive hypoglycemia, and as metabolic health monitors in non-diabetic populations. Integration with artificial intelligence for predictive alerts and automated insulin delivery continues advancing. Implantable long-term sensors (>180 days) are in development.

Conclusion

CGM represents one of internal medicine's most impactful technological advances, providing clinicians and patients with actionable glucose data that improves outcomes and quality of life. Mastery of CGM interpretation and application distinguishes the modern internist. As access expands and technology evolves, CGM will become as fundamental to diabetes care as blood pressure monitoring is to hypertension management.

Key Takeaways

• Time in Range >70% is the primary CGM target, more predictive than HbA1c alone
• Interpret CGM using the 3-2-1 rule: 3 time sections, 2 questions per section, 1 focused intervention
• Prioritize prevention of hypoglycemia (TBR <1%) in vulnerable populations
• Obtain minimum 14 days data before therapeutic adjustments
• CGM reveals glycemic patterns invisible to HbA1c and SMBG
• Modern sensors are accurate, convenient, and appropriate for most patients with diabetes

References

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