Determinants of Discordance between Estimated and Measured HbA1c: The Role of Active Time in 14-Day Continuous Glucose Monitoring Data

Background: We aimed to investigate factors contributing to discordance between estimated glycosylated hemoglobin (eHbA1c) and measured HbA1c (mHbA1c) from continuous glucose monitoring (CGM) using both 14- and 90-day data in a large Asian cohort. Methods: We retrospectively analyzed 3,724 14-day and 2,664 90-day glycemic profiles with ≥70% active CGM time (AT) from 1,028 individuals with type 1 diabetes mellitus (T1DM). eHbA1c was calculated using a regression model derived from our study population. Discordance was defined as eHbA1c-mHbA1c (overestimation [eHbA1c ≥ mHbA1c] or underestimation [eHbA1c < mHbA1c]). Similarity was defined as a difference within ±0.1%. Results: The 14-day eHbA1c model explained 74.7% of the variance in mHbA1c, compared with 88.4% for the 90-day eHbA1c model (conditional R2). Absolute discordance was significantly lower in real-time CGM (rtCGM) than in intermittently scanned CGM (isCGM) (P=0.004), and in those with higher time in range (TIR) and AT (all P<0.001). Overestimation was associated with rtCGM (P=0.016) and lower HbA1c (P<0.001). Underestimation was greater in isCGM users and in those with higher HbA1c and lower body mass index (all P≤0.001). In 14-day profiles, ≥78% AT was required for eHbA1c-mHbA1c similarity, whereas 90-day profiles maintained similarity regardless of AT. A prediction model incorporating AT, TIR, and sensor type improved accuracy (R2: 14-day, 0.774; 90-day, 0.888), particularly for 14-day data. Conclusion: Factors influencing eHbA1c-mHbA1c concordance differ by CGM duration. While 14-day data require higher AT (≥78%) for accuracy, 90-day data provide more stable estimates. Clinical interpretation must account for mHbA1c, TIR, and sensor type, especially when using shorter-term CGM profiles in Asian individuals with T1DM.