Background: The 2021 Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations, which removed the race coefficient, are recommended in clinical settings when accurate estimated glomerular filtration rate (eGFR) informs decision-making, although their clinical utility remains uncertain in the evaluation of live kidney donor candidates. 
Methods: We conducted a cross-sectional study of individuals who underwent living kidney donor evaluation from May 2016 to April 2025. Participants who completed all pre-donation kidney function assessments, including serum creatinine, cystatin C, and measured GFR (mGFR) using ⁹⁹mTc-DTPA clearance from a tertiary hospital. We assessed the agreement between pre-donation eGFR and mGFR using correlations coefficients, bias, root-mean-square error (RMSE), and proportion of estimates within 30% (P30) and 10% (P10) of mGFR, respectively. Subgroup analyses were conducted according to age, sex, and BMI categories. All statistical analyses were performed using R version 4.4.2. 
Results: The study included 807 candidates, of whom 739 were enrolled . The participants had a mean age of 50.0 ± 11.4 years, 42.2% were male, and the mean BMI was 24.5 ± 3.3 kg/m². All eGFR equations underestimated mGFR, showing a consistent negative bias. Among the three equations of the 2021 CKD-EPI, eGFRcys equation demonstrated the highest agreement with mGFR, with Pearson r = 0.577, Spearman ρ = 0.620, concordance correlation coefficient (CCC) = 0.471, bias = –8.59 mL/min/1.73 m², RMSE = 17.98, P30 = 96.9%, and P10 = 52.0%.

When an eGFR threshold of ≥80 mL/min/1.73 m² was considered sufficient for kidney donation eligibility, the number of candidates who would have been incorrectly excluded if eGFR alone had been used (i.e., false negatives) was lowest for the eGFRcys (n = 29) and highest for the eGFRcr-cys (n = 43). Subgroup analyses stratified by age and sex consistently demonstrated the superior performance of the 2021 CKD-EPI cystatin C equation across all subgroups. Notably, the correlation between eGFR and mGFR was stronger in older individuals and in women compared to younger individuals and men. 
Conclusions: The 2021 eGFRcys equation showed the best agreement with mGFR and minimized misclassification in donor eligibility, supporting its use as an initial screening tool in living donor evaluation.