Systemic Redox Biomarkers in Diabetic Retinopathy: Comparative Evaluation of Thiol–Disulfide Homeostasis, Ischemia-Modified Albumin, and Superoxide Dismutase

Background/Objectives: Oxidative stress is implicated in diabetic retinal injury, but systemic redox biomarkers in diabetic retinopathy remain incompletely characterized. This study aimed to evaluate systemic thiol–disulfide homeostasis (TDH), ischemia-modified albumin (IMA), and serum and urinary superoxide dismutase (SOD) activity in patients with diabetic retinopathy (DRP), diabetic individuals without retinopathy, and healthy controls. Methods: This cross-sectional comparative study included 31 patients with treatment-naïve DRP, 31 diabetic patients without retinopathy, and 45 healthy controls. Serum native thiol, total thiol, disulfide, IMA, and serum and urinary SOD activities were measured at a single study visit and compared among groups. Results: Serum native thiol, total thiol, and disulfide concentrations differed among the three groups (all overall p ≤ 0.001). Compared with diabetic patients without retinopathy, patients with DRP had lower native thiol concentrations after Bonferroni correction (p = 0.011); the difference in total thiol did not meet the corrected significance threshold (p = 0.029). Both diabetic groups had lower disulfide concentrations than healthy controls. IMA concentrations and serum and urinary SOD activities did not differ significantly among groups (all p > 0.05). Of the 31 patients with DRP, 15 had diabetic macular edema (DME), whereas 16 did not have DME. No statistically significant differences in the measured redox biomarkers were detected between patients with and without DME. Conclusions: Lower serum native thiol concentrations were associated with diabetic retinopathy in this cross-sectional cohort, whereas IMA and serum and urinary SOD activities showed no significant between-group differences. These findings suggest that circulating thiol status warrants further investigation in larger, longitudinal studies.