The mean difference between the CKD-EPI formula and the EKFC formula was 6.2 (95% CI: 5.2–7.0) ml/min/1.73 m2 for the creatinine-based formula, 2.4 (95% CI: 1.1–3.7) ml/min/1.73 m2 for the cystatin C-based formula and 0.4 (95% CI: − 0.8–1.5) ml/min/1.73 m2 for the combined formula. Here, CST3 is linked to chronic kidney disease.