A-107 Estimating Glomerular Filtration Rate by the Serum Creatinine and Cystatin C-plus-Creatinine Equations: Concordance and Effect on GFR Categorization
Authors: Faught R C , McGill M R , Hagrass H 
Published: September 2023

Abstract

Background
Estimating the glomerular filtration rate (GFR) is essential for evaluating kidney function, which is part of routine medical care. Most clinical laboratories report an estimated glomerular filtration rate (eGFR) based on serum creatinine, but cystatin C is a potential alternative marker. Evidence suggests that using the creatinine-plus-cystatin C (the 2021 CKD-EPI creatinine-cystatin C equation) eGFR gives more accurate results than the eGFR based on creatinine alone. Here we compared two 2021 CKD-EPI equations for estimating GFR—one using standardized creatinine alone (Cre) and the other using cystatin C combined with standardized creatinine (Cre-Cys). We also explored the effect of the equations on the GFR category. Methods: We retrospectively analyzed 777 simultaneous values for eGFR calculated by the two 2021 CKD-EPI equations over a period of nine months, from the time our laboratory initiated the Cre-Cys equation to the present.Cystatin C was measured turbidometrically on the Optilite (Binding Site, Birmingham, UK) and the creatinine was measured by the DxC method (Beckman Coulter, California, USA). We excluded any patient with Cre eGFR &gt;90 mL/min/1.73 m2 because we do not result any value above 90 for that parameter.

Results
Out of 777 patients (52% female and 48% male), we excluded 82 results with Cre GFR &gt; 90. The mean ± SD for Cre-Cys eGFR and Cre eGFR were 37.3 ± 23.8 and 37.1 ± 23.1, respectively, with a non-statistical difference between both groups using the paired T-Test (t = −0.8249, P-value = 0.4097). The mean difference (Cre-Cys eGFR minus Cre eGFR) was 0.28, with a standard deviation of 9.027. The correlation coefficient between Cre eGFR and Cre-Cys eGFR was r = 0.93. Two hundred and seventy-four (35%) samples had a difference between the numerical value of the two eGFR equations significant enough to change the GFR classification. 128 (34.5%) of the males had different categories when both equations calculated the eGFR vs 146 (36%) females. 36.3% of the white and 34% of the black patients had a discrepancy. The calculations matched for the other 502 patients. Of the 274 samples with discordant category classifications, the Cre-Cys equation gave a lower result and a higher (worse) category in 144 (53%), and a higher result and lower (better) category in 130 (47%).

Conclusion
The use of Cystatin C is growing and has moved beyond the nephrology community. Furthermore, the 2012 KDIGO guidelines suggest using additional tests (such as cystatin C or a clearance measurement) for confirmatory testing in specific circumstances when eGFR based on serum creatinine is less accurate. Currently, cystatin C testing is only available M-F, 8 AM–4 PM at our institute, but increasing availability and decreasing cost will likely expand Cre-Cys eGFR testing in the future. However, understanding the relationships between eGFR values from the different equations and how they will affect patient management can help clinicians optimize their use for kidney function evaluation.