Which is the optimal approach to monitor glycemic control—hemoglobin A1c (HbA1c) or continuous glucose monitoring (CGM)?

Optimal Approach to Monitor Glycemic Control

Both HbA1c and CGM are essential and complementary—not alternatives—for monitoring glycemic control, with CGM providing real-time actionable data on hypoglycemia, hyperglycemia, and glucose variability that HbA1c cannot capture, while HbA1c remains the established predictor of long-term complications. 1

The Complementary Nature of HbA1c and CGM

What HbA1c Provides

• HbA1c reflects average glycemia over 2-3 months and has strong predictive value for diabetes complications, making it the established gold standard for assessing long-term complication risk 1
• HbA1c should be measured every 3 months in patients not meeting glycemic targets and every 6 months once targets are achieved 1
• HbA1c fails to detect hypoglycemia, glycemic variability, or daily glucose patterns—critical limitations that can lead to treatment errors 1, 2

What CGM Provides That HbA1c Cannot

• CGM detects hyperglycemia and hypoglycemia that traditional monitoring methods miss, particularly postprandial hyperglycemia and asymptomatic nocturnal hypoglycemia 1
• CGM provides actionable metrics including time in range (TIR 70-180 mg/dL, target >70%), time below range (<70 mg/dL, target <4%; <54 mg/dL, target <1%), and time above range (>180 mg/dL) 1
• CGM reveals glucose variability through coefficient of variation (CV), with CV >33% indicating excess hypoglycemia risk in Chinese populations with diabetes 1
• Real-time CGM in conjunction with intensive insulin regimens reduces HbA1c by 0.3-0.6 percentage points in adults with type 1 and type 2 diabetes 1

The Critical Disconnect Between HbA1c and CGM Metrics

Understanding the Glucose Management Indicator (GMI)

• The glucose management indicator (GMI, formerly estimated HbA1c) converts mean CGM glucose into an estimated HbA1c using a population-derived formula 1
• GMI and laboratory HbA1c differ by >0.3% in 51% of individuals and by >0.1% in up to 81% of cases 3
• When HbA1c is elevated above GMI, intensifying therapy based solely on HbA1c may increase hypoglycemia risk—a critical clinical pitfall 3

Factors Causing HbA1c-GMI Discordance

• Individual variation in red blood cell glycation rates, anemia, hemoglobinopathies, and the time period assessed (months for HbA1c vs. 2-3 weeks for GMI) create clinically significant differences 3
• HbA1c can mask severe glucose fluctuations and hypoglycemia while appearing "well-controlled" 2, 4

Algorithmic Approach to Monitoring Selection

For Type 1 Diabetes Patients

• All patients with type 1 diabetes should use CGM regardless of age or current glycemic control, as CGM consistently reduces hypoglycemia and improves time in range 5, 6
• Real-time CGM should be used as close to daily as possible for maximal benefit, with frequency of sensor use being the strongest predictor of HbA1c reduction 1
• Continue measuring HbA1c every 3 months to assess long-term complication risk 1

For Type 2 Diabetes Patients on Intensive Insulin

• Patients on ≥3 insulin injections daily or insulin pump therapy should use CGM to lower HbA1c and reduce hypoglycemia 5, 6, 7
• The DIAMOND study demonstrated 0.3 percentage point HbA1c reduction with CGM versus self-monitoring in type 2 diabetes on multiple daily injections 7
• Maintain quarterly HbA1c monitoring for long-term risk assessment 1

For Patients with Hypoglycemia Issues

• Real-time CGM is specifically indicated for hypoglycemia unawareness, frequent hypoglycemic episodes, unexplained severe hypoglycemia, or nocturnal hypoglycemia 1, 5, 6
• CGM reduces time spent in hypoglycemia by approximately 40% in studies focused on hypoglycemia prevention 1

For Patients with Unexplained HbA1c-Glucose Discordance

• When HbA1c does not correlate with self-monitored glucose levels, CGM can reveal the true glucose patterns 1
• Conditions affecting red blood cell turnover (anemia, hemoglobinopathies) make HbA1c unreliable—CGM becomes the primary monitoring tool 1, 3

Critical Prerequisites for CGM Success

• Robust diabetes education, training, and ongoing support are mandatory for optimal CGM implementation—prescribing CGM without adequate education leads to failure 1, 5, 6, 7
• Patients must be willing and able to learn device operation, as success depends on sustained use and proper data interpretation 5, 6, 7
• Near-daily CGM use is required for maximal benefit; intermittently scanned CGM must be scanned at minimum every 8 hours 5, 6

Common Clinical Pitfalls to Avoid

• Never intensify therapy based solely on elevated HbA1c when GMI is lower—this increases hypoglycemia risk without improving outcomes 3
• Do not rely on HbA1c alone in patients with glucose variability—you will miss dangerous hypoglycemic and hyperglycemic excursions 2, 4
• CGM measures interstitial glucose, which lags blood glucose by 5-15 minutes during rapid changes—verify with capillary glucose when symptoms are discordant 5, 6
• CGM accuracy is lowest in hypoglycemic ranges, a critical limitation requiring confirmatory testing 5, 6
• Never use CGM in intensive care units—edema, vasoconstrictors, hypotension, and high-dose acetaminophen impair sensor accuracy 5, 6

The Bottom Line for Clinical Practice

Use CGM for day-to-day diabetes management decisions (insulin dosing, detecting hypoglycemia, understanding glucose patterns) and HbA1c for assessing long-term complication risk and treatment efficacy over months 1. CGM metrics (particularly time in range) should be used in conjunction with HbA1c whenever possible 1. When the two metrics diverge significantly, investigate the cause rather than blindly treating the HbA1c number 3.