Fasting glucose-to-HbA1c ratio is a good indicator of G6PD deficiency, but not thalassemia, in patients with type 2 diabetes mellitus

•G6PD-deficient patients have a shorter RBC lifespan, HbA1c is underestimated.•DM patients with G6PD deficiency had significantly higher FG/HbA1c ratios.•The AUC value indicated excellent reliability for predicting the FG/HbA1c ratio. Patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency or thalassemia have a shorter red blood cell lifespan; therefore, HbA1c is underestimated in these patients. To address these issues, we sought an early indicator for G6PD deficiency or thalassemia in DM patients. A total of 4908 patients with DM and 1848 subjects without DM were included in this study. Fasting glucose (FG) levels, HbA1c levels, hemogram profiles and G6PD activities were measured. Genotypic analyses of G6PD deficiency and thalassemia were performed. DM patients with G6PD deficiency had significantly higher FG/HbA1c ratios than did those without G6PD deficiency (26.54 vs. 18.36; p