What is the utility of a continuously recording glucose monitor (continuous glucose monitor, CGM) as an adjunct to fasting plasma glucose and hemoglobin A1C?

Continuous Glucose Monitoring as an Adjunct to Standard Glycemic Assessment

Continuous glucose monitoring (CGM) provides critical complementary information to HbA1c and fasting plasma glucose by capturing glycemic variability, hypoglycemic episodes, and real-time glucose patterns that these static measurements cannot detect—making it a valuable adjunct rather than a replacement for traditional metrics. 1

Why CGM Complements Rather Than Replaces Traditional Metrics

HbA1c reflects average glucose over 2-3 months but fundamentally fails to capture:

• Acute glycemic excursions (both hypo- and hyperglycemia)
• Magnitude and frequency of glucose variability
• Daily glucose patterns and timing of excursions
• Individual episodes that inform immediate therapy decisions 1

The utility of HbA1c is enhanced when used as a complement to CGM data, not replaced by it. 1 HbA1c remains the only prospectively validated tool for assessing long-term diabetes complication risk, so its importance in clinical decision-making should not be undervalued.

Specific Clinical Advantages of CGM

For Insulin-Treated Patients

CGM demonstrates clear benefit in patients on basal insulin without prandial insulin, achieving:

• HbA1c reduction of 0.4% compared to blood glucose meter monitoring (9.1% to 8.0% vs 9.0% to 8.4%)
• Time in target range (70-180 mg/dL) increased from 43% to 59%
• Time above 250 mg/dL reduced from 27% to 11%
• Mean glucose reduced by 26 mg/dL 2

For Detection of Prediabetes

When combined with HbA1c in a two-step approach (HbA1c ≥5.7% automatically categorized as prediabetes, then CGM features for those <5.7%), CGM dramatically improves sensitivity from the HbA1c-alone benchmark to 76-78% while maintaining specificity of 78-80% 3

Standardized Metrics for Clinical Use

The 2019 International Consensus established 10 core CGM metrics for clinical practice 1:

1. Number of days CGM worn (minimum 14 days with >70% active time)
2. Percentage of time CGM is active
3. Mean glucose
4. Estimated A1C
5. Glycemic variability (coefficient of variation)
6. Time >250 mg/dL
7. Time >180 mg/dL
8. Time in range 70-180 mg/dL (primary target)
9. Time <70 mg/dL
10. Time <54 mg/dL

Adequate data requires >70% CGM use over 14 days, which correlates strongly with 3-month mean glucose and time-in-range metrics. 1

When to Deploy CGM

Use CGM when pattern information can improve management, not simply because more glucose data are available. 4 Specific indications include:

• Discordance between HbA1c and suspected glycemic control (e.g., conditions affecting HbA1c accuracy: anemia, hemoglobinopathies, iron deficiency, pregnancy) 1
• Patients on basal insulin requiring safer treatment titration 2
• Identifying hidden hypoglycemia or nocturnal glucose patterns
• Clarifying fasting vs postprandial hyperglycemia to guide therapy selection 4
• Supporting lifestyle modification with real-time feedback

Important Caveats and Limitations

Accuracy Considerations

CGM measures interstitial glucose with inherent delay in registering blood glucose changes, particularly problematic during rapid glucose fluctuations 5. Current devices achieve ~10% mean absolute relative difference (MARD) 6.

Patient Factors

• Requires active engagement to be effective—passive surveillance provides no benefit 1
• May induce anxiety in some patients 1
• Potential for allergic reactions to adhesives
• Not universally available globally 1

Evidence Gaps

For non-insulin-treated type 2 diabetes, evidence supports glycemic metrics improvement but long-term outcome data remain limited 4, 7. The average HbA1c reduction across populations is modest at 0.3%, though individual patients may benefit considerably more 7.

Practical Implementation Algorithm

1. Patient Selection: Identify specific clinical problem CGM will address (not default screening)
2. Education: Train on device use, interpretation, and action thresholds before deployment
3. Data Collection: Ensure ≥14 days with >70% active time
4. Structured Interpretation: Focus on time-in-range, hypoglycemia burden, and variability patterns
5. Action: Integrate findings into medication adjustment, lifestyle counseling, and shared decision-making 4

CGM functions as structured decision support for specific clinical questions, complementing but not replacing HbA1c and fasting glucose in comprehensive diabetes assessment.