Background <p>Monitoring test utilization is a key component of improving the quality of clinical laboratory testing. In diabetes management, glycated hemoglobin (hemoglobin A1c, HbA1c) reflects long-term glycemic control in addition to blood glucose levels; however, the combined use of these tests in routine health checkups has not been well evaluated in Korea.</p> Methods <p>We retrospectively analyzed data from routine health checkups conducted between 2018 and 2023 that included concurrent serum glucose and HbA1c measurements. Diabetes and prediabetes were defined using glucose-only, HbA1c-only, and combined (glucose or HbA1c) diagnostic criteria. Prevalence estimates across diagnostic definitions were compared, and factors associated with diabetes and prediabetes were evaluated using multivariable logistic regression adjusted for sex, age, and year of testing.</p> Results <p>Over the 6-year study period, test results from 113,790 individuals (56,021 men and 57,769 women) were analyzed, with a mean age of 55.4 years (SD, 13.3). The number of individuals undergoing simultaneous testing increased from 5,922 in 2018 to 36,814 in 2023, indicating increased utilization of HbA1c testing. The combined diagnostic criteria identified significantly more individuals with diabetes and prediabetes than either glucose-only or HbA1c-only definitions (<i>p</i> &lt; 0.001). In multivariable analyses, male sex and older age were independently associated with higher odds of diabetes, whereas the prevalence of prediabetes increased in more recent years of testing.</p> Conclusions <p>Simultaneous evaluation using serum glucose and HbA1c significantly improves the detection of diabetes and prediabetes compared with single-marker strategies. These findings underscore the importance of optimized test utilization in routine health checkups to enhance the early identification of dysglycemia.</p> Clinical trial number <p>Not applicable.</p>