An Example for Reflective Testing: Enzymatic Creatinine Determination Method in Patients with Chronic Renal Failure at Different Stages

Aim: This study aims to compare estimated Glomerular Filtration Rate (eGFR) values calculated using Jaffe and enzymatic creatinine measurement methods across different stages of chronic kidney disease (CKD), to assess the impact of these methods on CKD staging, and to evaluate the applicability of the enzymatic method as a reflective test.Materials and Methods: A total of 176 CKD patients were prospectively analyzed. Serum creatinine levels were measured using both Jaffe and enzymatic methods, and eGFR was calculated using the 2021 CKD-EPI formula. Correlation between methods was assessed using Spearman’s test, differences via Bland–Altman analysis, and staging agreement using Cohen’s Kappa.Results: A significant positive correlation was found between the two methods (Spearman’s rho = 0.986; p < 0.001). It was observed that eGFR values calculated using the Jaffe method led to misclassification in 39.4% of cases, particularly in stage G3a. Stage mismatch was detected in 27.3%, 20%, and 27.8% of cases in stages G3b, G4, and G5, respectively. Overall, 23.26% of patients were assigned to different stages.Conclusions: The creatinine measurement method is not only analytical but also a critical factor that directly influences clinical decisions. In particular, different staging using the Jaffe method is common in patients with moderate CKD (G2–G3a). The application of the enzymatic method as a reflective test may be effective in preventing these deviations. Therefore, it is recommended that reflective test algorithms be integrated into laboratory workflows and that the creatinine measurement method used be reflected in the eGFR interpretation.