Impact of creatinine measurement methods on eGFR and GFR category assignment.

Background: Accurate measurement of serum creatinine (SCr) is critical in estimating glomerular filtration rate (eGFR) and classifying kidney function. This study evaluated the analytical differences between the enzymatic and Jaffe methods for SCr measurement and their impact on eGFR estimation using two widely applied equations: CKD-EPI and EKFC. Methods: The study included 427 patients over 40 years old. SCr was measured using both enzymatic and Jaffe methods on the Alinity c platform. eGFR was calculated with the CKD-EPI (2009) and EKFC equations. Agreement between methods was assessed using Bland-Altman and Passing-Bablok regression. eGFR differences were analyzed using the Wilcoxon signed-rank test and multiple linear regression. Agreement in GFR category classification was evaluated using weighted kappa and Kendall's tau. Results: While the mean difference between methods was small, both systematic and proportional biases were statistically significant. eGFR values differed significantly between methods in both sexes (p < 0.01), regardless of the equation used. ΔeGFR was significantly associated with SCr values, but not with age. Although overall agreement in GFR categories was high (kappa > 0.91), method-dependent reclassification of patients was observed, which may influence CKD diagnosis and clinical decision-making. Conclusions: Even minor analytical differences between enzymatic and Jaffe SCr measurements can lead to clinically relevant discrepancies in GFR categorization. These findings highlight the need for harmonization in laboratory methods to ensure consistent reporting and patient management.