Fructosamine Series Over 3 Months and HbA1c at Endpoint: A Prospective Evaluation of Estimated Mean Glucose Concordance

Abstract Introduction: Glycemic monitoring relies predominantly on hemoglobin A1c (HbA1c), though its accuracy diminishes in patients with hemoglobinopathies or altered erythrocyte turnover. Fructosamine, reflecting 2-3-week glycemic exposure through glycated serum proteins, provides complementary monitoring capabilities. Converting both biomarkers to estimated average glucose (eAG) enhances clinical interpretability, yet limited prospective evidence exists regarding concordance between serial fructosamine-derived eAG and HbA1c-derived eAG measurements. Objective: To evaluate concordance between estimated mean glucose values derived from serial fructosamine measurements over three months and those calculated from endpoint HbA1c assessments in diabetic patients. Methods: This prospective observational cohort study enrolled 36 adults with diabetes mellitus at HIPERDIA, Itabuna, Brazil (July 2024-July 2025). Monthly fructosamine measurements were obtained alongside baseline and endpoint HbA1c determinations. Fructosamine-derived eAG utilized the formula: eAG(mg/dL) = (0.5157×fructosamine)-20. HbA1c-derived eAG employed Nathan's equation: eAG(mg/dL) = (28.7×HbA1c%)-46.7. Statistical analysis included Pearson correlation, Lin's concordance correlation coefficient, and Bland-Altman analysis. Results: Mean participant age was 58.4±12.7 years, with 80.6% having type 2 diabetes. Fructosamine declined from 312.4±45.8 to 305.2±44.6 μmol/L over three months. Strong correlations emerged between fructosamine and HbA1c-derived eAG values (r=0.782-0.805, p<0.001), with substantial concordance (CCC=0.745-0.776). Systematic bias revealed consistent fructosamine underestimation (-34.1 to -35.9 mg/dL), with acceptable Bland-Altman limits of agreement (-78.4 to 6.6 mg/dL). Conclusion: Serial fructosamine measurements demonstrate substantial concordance with HbA1c-derived eAG, supporting its utility as complementary glycemic monitoring, particularly when HbA1c reliability is compromised.