Eosinophil Count Changes and Glycemic Control: A Retrospective Cohort Study in Adults with Diabetes

Glycemic control is key to prevention of complications of diabetes. Although eosinophils have been implicated in metabolic regulation, most human studies have been cross-sectional and have yielded inconsistent findings. We conducted a retrospective cohort study of adults with uncontrolled diabetes receiving primary care at Mass General Brigham. We evaluated the relationship between eosinophil count and HbA1c over one year, adjusting for demographic, socioeconomic, and clinical covariates. Among 21,447 patients included in the study, eosinophil count increased by a mean of 2.6 cells/L (SD 157.2 cells/L) and HbA1c decreased by a mean of 0.82% (SD 2.09%) during follow-up. Increases in eosinophil count were associated with reductions in HbA1c (parameter estimate -0.366 per 1,000 cells/L; 95% CI -0.526 to -0.206; P < 0.001), whereas baseline eosinophil count was not significantly associated with HbA1c changes. The association remained significant in a sensitivity analysis excluding systemic glucocorticoid users. Increases in eosinophil count were associated with improved glycemic control in adults with diabetes, whereas baseline eosinophil count was not. These findings extend prior cross-sectional observations and suggest that changes in eosinophils may represent a biomarker of glycemic improvement. Further prospective studies are needed to determine whether this relationship reflects underlying immune-metabolic mechanisms or treatment-related changes.