Accurate determination of the glomerular filtration rate is essential for the detection, prognosis, and treatment of chronic kidney disease. Based on a review of the relevant literature and a retrospective analysis of 435 patients from the Clinical Institute of Clinical Chemistry and Biochemistry in Ljubljana, we compared the results of three established equations used to estimate glomerular filtration rate to determine which provides the most accurate results for the correct diagnosis of chronic kidney disease. The equations analyzed included one based on serum creatinine, one based on serum cystatin C, and a combined equation that incorporates both serum renal function biomarkers. We assessed the differences in the categorization of glomerular filtration rate stages (G) and consequently in the classification of chronic kidney disease depending on the equation used and evaluated the statistical significance of these differences. Serum concentrations of creatinine and cystatin C were determined using established protocols of the Clinical Institute of Clinical Chemistry and Biochemistry in Ljubljana. It was found that in about ten percent of cases, patients were classified into different stages of chronic kidney disease depending on the equation used. Case-by-case analysis revealed that the combined equation provided the most appropriate estimates and enabled more reliable patient classification compared with equations based solely on the serum concentration of a single biomarker. Statistical tests showed a significant difference between the creatinine-based equation and the combined equation (p = 0.006), with a mean difference of –10.75 mL/min/1.73 m2. The effect size (R² = 0.285) indicated a moderate effect, while the correlation between the two equations was moderate (rp = 0.55). The difference in results between the combined and cystatin C-based equations was not statistically significant, but the correlation was very high (rs = 0.97), indicating strong agreement between the results. Of note, the cystatin C-based equation detected changes in kidney function earlier, which is clinically important when early kidney damage is suspected. In our study we also concluded that the combined equation is most appropriate for routine clinical use, particularly in patients with borderline glomerular filtration rate values or when creatinine is an unreliable marker. The main limitations of the study include its single-center design, a limited sample size for subgroup analyses, and the lack of data on clinical outcomes. These factors limit the generalizability of the findings to the broader chronic kidney disease population, and further studies in larger and more diverse patient cohorts, including clinical outcome parameters, are recommended to confirm the results and strengthen clinical applicability.
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