B-288 Comparison of Three CKD-EPI eGFR Equations for the Assessment of Kidney Disease

Abstract

Estimated glomerular filtration rate (eGFR) is considered a reliable way to measure kidney function, but over the past few years several changes have been made to the recommendations for calculating eGFR. In 2021, the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) Task Force updated the creatinine-based eGFR calculation to remove race from the equation due to overestimation of eGFR in Black patients. This overestimation has led to disparities in access to care for kidney disease. In 2021, the Task Force also recommended that cystatin C be used routinely and combined with creatinine to provide a more accurate assessment of eGFR and to aid in clinical decision making. Additionally, the CKD-EPI launched a new eGFR equation that uses both creatinine and cystatin C. To facilitate compliance with these recommendations, Quest Diagnostics launched a test panel containing creatinine, cystatin C, and 3 methods of calculating eGFR: the 2021 CKD-EPI equation based on creatinine without race (eGFRcr_R), the 2012 equation based on cystatin C (eGFRcys), and the new CKD-EPI equation based on creatinine and cystatin C without race (eGFRcr-cys_R). The purpose of this study is to provide an understanding and context for the differences in the eGFR results between the different equations. In October 2024, Quest Diagnostics launched the new panel containing creatinine, eGFRcr_R, eGFRcys, and eGFRcr-cys_R. Utilization of the new panel was evaluated between October 2024 and December 2025. eGFR results were reviewed to determine overall difference in the eGFR calculations and classification of kidney disease, based on a target eGFR of >60 mL/min/1.73m2. Creatinine and cystatin C testing are performed at Quest Diagnostics on Beckman Coulter AU Series analyzers using Beckman Coulter and Gentian Diagnostics reagent kits, respectively. Over the study period, utilization of the combined eGFRcr_R, and eGFRcr-cys_R panel grew each quarter. During this time, demand for cystatin C and eGFRcys alone grew by 46%. However, the combined test code was only used for 4.5% of cystatin C and eGFRcys orders. The average eGFR values were 65 mL/min/1.73 m² for eGFRcr_R, 62 mL/min/1.73 m² for eGFRcr-cys_R, and 56 mL/min/1.73 m² for eGFRcys. eGFRcr_R had an average bias of 6.6% compared to eGFRcr-cys_R, and 14.4% compared to eGFRcys, while eGFRcys had a lower average bias of 10% compared to eGFRcr-cys_R. On average, patients measured with the eGFRcr_R, or eGFRcr-cys_R would be classified as having normal kidney function (in the absence of known kidney damage), while patients measured with the eGFRcys would be classified as having mild to moderately decreased kidney function. Differences between eGFR equations can result in reclassification of patients. As the popularity of cystatin C and the newer eGFRcr-cys_R equation for assessing kidney function increases, providers need to be aware of the inherent biases between the different equations and how they may impact kidney function classification.